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In many discussions of LPAM transmitter design you will hear or read references to “the Wenzel circuit” or “the Wenzel transmitter.” These refer to the clever transmitter design downloadable at www.wenzel.com/pdffiles/amxmit.pdf

During the mid 1990s and early 2000s, this was the best available AM transmitter circuit if you wanted to build something yourself for around-the-house “yardcasting.” The range is limited but the sound quality is good compared to other homebrew circuits that were published at the time.

In many discussions of LPAM transmitter design you will hear or read references to “the Wenzel circuit” or “the Wenzel transmitter.” These refer to the clever transmitter design downloadable at www.wenzel.com/pdffiles/amxmit.pdf

During the mid 1990s and early 2000s, this was the best available AM transmitter circuit if you wanted to build something yourself for around-the-house “yardcasting.” The range is limited but the sound quality is good compared to other homebrew circuits that were published at the time.

An updated, illustrated web page about this circuit, including a modified version with a different output section, is published by Mr Wenzel himself at www.techlib.com/electronics/amxmit.htm

Below are some comments that were published in various newsgroups about the original version of the circuit. It’s fascinating to see what a detailed analysis and spirited discussion this circuit received!

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Subject: Re: Micro Transmitter Project
From: Syl
Newsgroups: rec.antiques.radio+phono
Date: Sat, 30 Nov 2002 12:00 AM
Joe Bento a écrit :

Is that the little transmitter that Syl says is among the best he’s tried? If so, I am using that as a basis for my design. If an LC works in place of the crystal, I’ll certainly give it a try. — Joe

Yes it is. It’s also on my website. The only really good design so far. I use planar transistors (2N5551) to replace the 2n4401 because I had them on hands by the dozen and I can drive them with 150V…(like I need it…) I suspect any 2SCxxxx from the junk box might also work… Now that Sal sent me a crystal, I’ll replace the LC I replace it with… I think Bill M. also did this with his…

Syl

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Subject: Re: AM Transmitters.
From: Syl
Newsgroups: rec.antiques.radio+phono
Date: Tue, 10 Dec 2002 10:27 PM

John Byrns a Žcrit :

The “diff amp” type circuit favored by Syl, and BM, looks like it may have distortion problems related to antenna loading.

Tried that and it doesn’t seems to affect the quality of the ouput.

Even on the scope. At least not within it’s “useful” range with wrong antenna loading. With the proper loading (which is somewhat important for increased range), sound is excellent. If unproperly loaded, close to the transmitter the sound is excellent too but the range suffers a *lot.

It does distort outside the “useful” range but I think it is caused by the weak signal (some of my radios are better than other though)as the ouput is very clean close to the transmitter. I tune the antenna using an adjustable coil (using a ferrite) I found in my junk box.

I could also use a fixed value for the L and use an adjustable C to tune the antenna I guess.

Syl

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Subject: Re: AM Transmitters.
From: “Phil B”
Newsgroups: rec.antiques.radio+phono
Date: Thu, 12 Dec 2002 1:53 AM

“John Byrns” wrote:

The “diff amp” type circuit favored by Syl, and BM, looks like it may have distortion problems related to antenna loading.

If you look closely at the “diff amp” circuit at http://www.wenzel.com/pdffiles/amxmit.pdf you will notice a 470 ohm resistor paralleling the output tuned circuit.

This resistor is essentially the only load on the output, since the short antenna doesn’t even come close to loading the output. Without the resistor the tank just rings horribly and sort of loosely follows the modulation envelope. With the resistor the output is very clean and linear and is quite unaffected by the antenna. 99% of the “100 mw” power is absorbed in this resistor, but the waveform is linear.

The Vectronics doesn’t have a load resistor. It does have a better output matching circuit. It has a “reverse” pi network (low impedance in and high impedance out). This maximized the voltage presented to a short antenna with its very low radiation resistance to improve the loading (marginally). The Vectronics circuit would benefit with a load resistor of say 500 – 1000 ohms across C8 or a larger resistor of say 20k – 50k from the antenna terminal to ground.

This loading problem is common to all BC frequency transmitters with short antennas. You just can’t get very much of the power to go to a short antenna. Another solution is to use an antenna of several hundred feet or more and a good antenna matching network.

Phil B

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Subject: Re: AM Transmitters.
From: Syl
Newsgroups: rec.antiques.radio+phono
Date: Fri, 13 Dec 2002 11:58 PM

Bill M a Žcrit :

Thats the good one even though John Byrns has reservations about its quality of signal 🙂

Play a little with the osc. resistors to get a better sinewave. I changed a few resistors until I got an almost perfectly uniform sinewave on the scope. For some reasons, on some radios the signal got much better…

I’ve never really haggled much over the output loading inductance because I have used ‘illegal’ antenna lengths and the coverage area has been sufficient for my needs. But I wonder if a 3 foot antenna would really present much difference at 1.6 versus 1.0? Both present an extremely high impedance. I think his measure of “half” the reading is based on reflection (VSWR) showing up on the meter. I suppose ideally a tunable inductor that would go up to maybe 1.5mh

Exactly what I am using on mine. Dunno about the exact value. Right from the junk parts bin. I tuned by ear for best signal. It does improve again the signal…

Syl

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Subject: Re: AM Transmitters.
From: [email protected] (John Byrns)
Newsgroups: rec.antiques.radio+phono
Date: Fri, 13 Dec 2002 10:07 PM

In article , Bill M wrote:

Thats the good one even though John Byrns has reservations about its quality of signal 🙂

Hi Bill,

Someone else pointed out in one of these transmitter threads that circuit used in “amxmit.pdf” doesn’t suffer the loading sensitivity problem, I mentioned that type of diff amp transmitter has, because of the 470 Ohm resistor that I failed to notice in the circuit, which provides the main load for the transmitter, making the load from the antenna largely irrelevant. Think how much more power you could get out of that little transmitter, if the 470 Ohm resistor were removed, and the antenna was actually properly matched to the output of the diff amp.

As far as I am concerned it would be better to just use a simple class C stage like the Vectronics, and the WPDJ, then you wouldn’t have to worry about proper loading, or dumping most of the power into a resistor. A similar transmitter with a class C output stage could also be done with 3 transistors.

Regards, John Byrns

Surf my web pages at, http://www.enteract.com/~jbyrns/index.html

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Subject: Re: AM Transmitters.
From: Bill M
Newsgroups: rec.antiques.radio+phono
Date: Fri, 13 Dec 2002 10:33 PM

John Byrns wrote:

As far as I am concerned it would be better to just use a simple class C stage like the Vectronics, and the WPDJ, then you wouldn’t have to worry about proper loading, or dumping most of the power into a resistor. A similar transmitter with a class C output stage could also be done with 3 transistors.

Most of the power is not “dumped into the resistor”. It only serves to flatten the Q to make the circuit more easily reproducible and flexible across the BCB range with equal results.

Granted that better Q could deliver more output (at some risk of affecting modulation peaks if one designs by the book) but I think the philosophy presented by the guy is more along the lines of making something that will work for the average homebrewer without poring over particular voltages, output transistor types, antenna lengths, etc.

This is a very important aspect for published designs that can actually work properly when the user may have nothing more than a junkbox and a broken VOM to work with. To its credit, I deadbugged this circuit with junkbox equivalent parts into a ball of stray leads tacked together to see if it would work before I proceeded with a permanent mounting configuration. Many ‘idea’ circuits won’t pass such a test.

I’m powering it with a 9 volt battery and LC in place of the xtal. I bypassed the 100 ohm emitter resistor with 4.7uf or so and I think it sounds superb compared to any of the tube-type versions of a “home-phono-oscillator” that I have tried.

Your improvements on the basic design would be appreciated.

-Bill

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Subject: Re: AM Transmitters.
From: [email protected] (John Byrns)
Newsgroups: rec.antiques.radio+phono
Date: Sat, 14 Dec 2002 2:11 AM

In article , Bill M wrote:

Most of the power is not “dumped into the resistor”. It only serves to flatten the Q to make the circuit more easily reproducible and flexible across the BCB range with equal results.

Leaving aside for the moment exactly how much power is being dumped into the resistor, the resistor clearly does more than simply “flatten the Q”. The value of the resistor is an order of magnitude lower than is required simply to “flatten the Q”, in fact with the 470 Ohm resistor, the Q of the circuit is only about 2 at 1 MHz. Consider operation at 1 MHz, from the chart in the transmitter documentation, the tank circuit L should be about 35 uH. Lets say we want the audio response, without any antenna load at all, to be down 0.26 dB at 5 kHz, and down 0.97 dB at 10 kHz, then assuming no other losses in the circuit, the resistor would be 5,328 Ohms. This is more than an order of magnitude higher than the 470 Ohm resistor that is actually used in the circuit, plus there are other losses in the real circuit, making the necessary resistor value even higher.

Clearly the resistor is doing more than insuring that the Q is adequately low for good audio response, even with no antenna loading. I suspect that purpose of the resistor is to swamp the output, to insure the modulation of the diff amp is linear even when lightly loaded by the antenna.

My philosophy is to design so the adjustments are as simple as possible, and I think a class C output stage is a better way to accomplish that, than is the diff amp design, because a class C stage with high level modulation will modulate properly even if it is not fully loaded. The design of the “amxmit” still requires the builder to futz with two adjustments to the output stage, the tuning for resonance, and selecting the proper loading coil. The 470 Ohm resistor appears to have the function of swamping the antenna load to keep the modulation linear, even if the loading by the antenna is not correct. The class C stage on the other hand does not require this wasteful resistor for linear modulation, even when it is lightly loaded. With the class C stage, if one is not concerned with squeezing out every last bit of range, you can ignore the loading, and just peak the tuning. To get the same simplicity of operation with the diff amp circuit, the designer had to resort to the 470 Ohm resistor to insure that the load presented to the diff amp stage was not too great for proper modulation.

One possible advantage that the diff amp may have is that it may be less subject to the drive signal bleed through that limits the negative modulation of a class C transistor amplifier, unless the drive signal is also partially modulated. I don’t know if that is the case or not, it is a question to be answered. The diff amp may also be simpler in that it doesn’t require neutralization, while the class C amplifier may an extra buffer stage to eliminate any possible need for neutralization, which would otherwise violate the simplicity rule. Both of these effects have the same root cause. That would bring the transistor count for the class C design up to 5, or one more than the diff amp circuit uses.

Regards, John Byrns

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Subject: Re: AM Transmitters.
From: “Phil B”
Newsgroups: rec.antiques.radio+phono
Date: Sat, Dec 14, 2002 3:59 AM

“John Byrns” wrote:

design of the “amxmit” still requires the builder to futz with two adjustments to the output stage, the tuning for resonance, and selecting the proper loading coil. The 470 Ohm resistor appears to have the function of swamping the antenna load to keep the modulation linear, even if the loading by the antenna is not correct. The class C stage on the other hand does not require this wasteful resistor for linear modulation, even when it is lightly loaded. With the class C stage, if one is not concerned with squeezing out every last bit of range, you can ignore the loading, and just peak the tuning. To get the same simplicity of operation with the diff amp circuit, the designer had to resort to the 470 Ohm resistor to insure that the load presented to the diff amp stage was

John,

You are correct in your analysis. I posted spice simulation results of this circuit on the binaries. The effect of changing the 470 ohm resistor is quite dramatic. Here is what you will see on the binaries post:

This post relates to the current discussion of the same title on RAR+P.

Here are spice simulation results for the “Personal Radio Station” transmitter RF out and modulator sections showing the effect of changing the 470 ohm resistor across the output tank.

The pictures show 3 values for this resistor.
1. 470 ohms (as specified in the schematic)
2. 1000 ohms
3. Resistor deleted.

The simulation was run with:
– 1610 khz RF, 3 khz modulation
– Antenna model is: 50 pf in series with .1 ohm radiation resistance in series with 30 ohms to ground (typical ground loss)
– Waveforms taken across .1 ohm radiation resistance
– Tank L at resonance = 21 uH
– Tank C at resonance = 200 pF
– Modulation was limited to less than 100% due to a circuit limitation that can easily be fixed with some resistor value changes. (I will address this in another post).

Results clearly show that without proper resistor value, the tank circuit rings and does not faithfully follow the modulation signal.

Phil B

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Subject: Re: AM TRANSMITTERS
From: “Phil B”
Newsgroups: alt.binaries.pictures.radio
Date: Sat, 14 Dec 2002 11:28 PM

Since the “Personal Radio Station” transmitter web page seems to emphasize using a loading coil, I re-ran the spice with a loading coil. I neglected to mention in the last post that those spice results were WITHOUT a loading coil. I got to thinking that a loading coil could make a big difference in the amount of power extracted from the tank circuit.

I spent an inordinate amount of time playing with the numbers until I came up with a loading coil value and changes to the L/C tank values to get a peak. The loading coil affects the resonance of the tank. A truly practical version of this transmitter would require a tunable L inductor for the tank and a tunable loading coil to get an optimum peak. This is a fact glossed over in the author’s description.

Anyway, here are the same three pictures WITH a loading coil. Things to notice: The output voltage across the radiation resistance is almost 10 times higher (shows the value using a loading coil). The waveform is not distorted as greatly with load resistors higher than 470 ohms. But the waveform is still significantly distorted, so the resistor is still necessary.

As before:
The pictures show 3 values for this resistor.
1. 470 ohms (as specified in the schematic)
2. 1000 ohms
3. Resistor deleted.

– 1610 khz RF, 3 khz modulation
– Waveforms taken across .1 ohm radiation resistance
– Modulation was limited to less than 100% due to a circuit limitation that can easily be fixed with some resistor value changes. (I still need to address this in another post).

Things I changed: – Antenna model is: 40 pf in series with .1 ohm radiation resistance in
series with 30 ohms to ground (typical ground loss). Changed from 50 pf to be possibly more accurate representation of a 3 meter antenna in proximity to objects.
– Tank L at resonance = 9.5 uH
– Tank C at resonance = 200 pF
– Loading coil = 223 uH

Phil B

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Subject: Re: AM TRANSMITTERS
From: “Phil B”
Newsgroups: alt.binaries.pictures.radio
Date: Sun, 15 Dec 2002 2:08 AM

“Joe Bento” wrote:

I don’t claim to understand spice models – something I should probably learn. But I know what an AM envelope should look like, and your models with graphs open a lot of areas to investigate with this transmitter. It appears capable of achieving near 100% modulation, and some of you models seem to show it can do so cleanly.

Joe,

Yes indeed, this transmitter can do 100% modulation, but not without a simple bias change on the differential output transistors. This is something I discovered in doing the simulations. My posted simulations showed something like 60% or 70% modulation. I found that the differential output transistors overloaded above this level.

Since I was posting on the effect of the 470 ohm tank resistor, I didn’t want to complicate matters by introducing distortion from an unrelated source. I am working on the optimum bias resistor change and will post when I am satisfied.

BTW, when I was a kid I had a battery operated Allied 20:1 electronics kit that included a simple AM transmitter circuit. It used a single metal-diaphragm headphone as the mic. I asked my father to talk while I listened on the car radio about 30 ft. away. I was astonished at the clarity. The antenna was about 2 ft. of wire hanging off the board and there was no real ground because he was holding it in his hands. Go figure.

Phil B

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Subject: Changes to Wentzel “Personal Radio Station” At 100% Mod
From: “Phil B”
Newsgroups: alt.binaries.pictures.radio
Date: Mon, 16 Dec 2002 1:30 AM

If you choose to run this transmitter into a short antenna without a loading coil, it will distort at 100% modulation even with precisely tuned L/C tank. If you use a precisely tuned loading coil and you precisely tune the L/C tank, then it works fine at 100% modulation.

I have found with my spice modeling that it is very important to precisely tune the L/C tank and the loading coil to get a good waveform (at any modulation level). I would think that builders may choose to exclude the loading coil to reduce complexity. Without the loading coil, the RF output stage exceeds its bias levels at 100% modulation.

You may want to make the following bias resistor modifications to eliminate the problem. These changes simply improve the margins and will have no effect when a loading coil is used, but will eliminate distortion when a loading coil is not used.

The schematic to reference is at: http://www.wenzel.com/pdffiles/amxmit.pdf

Changes:
1. Change the resistors from base to ground on the two differential RF output transistors from 1.8k to 1.0k.

2. Change the audio amp emitter-to-ground resistor from 100 ohms to 82 ohms.

3. Change the audio amp base-to-ground resistor from 2.2k to 1.8k.

As a side effect there is a minor added benefit: the audio gain is increased slightly. You will now reach 100% modulation at 1.6 v p-p input instead of 2.0 v p-p.

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Subject: Re: Changes to Wentzel “Personal Radio Station” At 100% Mod
From: “Phil B”
Newsgroups: rec.antiques.radio+phono
Date: Tue, Dec 17, 2002 12:20 AM
Message-ID:

“John Byrns” wrote:

Am I correct in assuming that the 470 Ohm resistor was still in the circuit for the simulations of the modified circuit? If it is, the real trick would be getting clean modulation without the 470 Ohm resistor, and no antenna connected.

A question whose answer should be available from your simulation, with the loading coil in place, and everything properly tuned up, how much RF power is going into the 470 Ohm resistor, and how much into the combination of antenna system radiation resistance and ground resistance. In other words, what is the RF Voltage at these two points?

Yes, this still includes the 470 ohm resistor. Without the resistor, I can’t get an undistorted waveform under any tuning conditions, with or without the loading coil, and with or without my bias changes. I only suggest the bias changes for anyone that chooses to run without the loading coil. The loading coil increases the p-p voltage across the antenna radiation resistance by more than 10 times, but I have found that it must be precisely tuned to the antenna capacitance or the waveform becomes distorted. The need to tune the loading coil precisely is a PTIA as far as I’m concerned. If the range is adequate without it, I would be in favor of eliminating it and possibly going with a somewhat longer antenna instead. Tuning the L/C tank is much less critical for undistorted output because the Q is lower than the Q of the loading coil.

For the simulations I just ran for Bill M (using the 2SC1815 transistors) I have the following readings for the modified version:

WITHOUT the loading coil:
– Collector of output transistor: 20v p-p, centered at 15 v (this is the rf across the 470 ohm resistor) – At antenna terminal after .1 uf blocking cap: 20v p-p centered at 0 v.)
– At top of series connected .1 ohm radiation resistance and 30 ohm
ground resistance: .320 v p-p. (this point is after the 40pf antenna capacitance).
– Across the .1 ohm radiation resistance: 530 microvolts p-p

And WITH the loading coil:
– Collector of output transistor: 17v p-p, centered at 15 v (this is the rf across the 470 ohm resistor)
– At antenna terminal after .1 uf blocking cap: (17v p-p centered at 0 v.)
– At top of series connected .1 ohm radiation resistance and 30 ohm ground resistance: 1.9 v p-p. (this point is after the 40pf antenna capacitance).
– Across the .1 ohm radiation resistance: 6200 microvolts p-p.

Phil B

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Subject: Re: Changes to Wentzel “Personal Radio Station” At 100% Mod
From: [email protected] (John Byrns)
Newsgroups: rec.antiques.radio+phono
Date: Tue, Dec 17, 2002 12:49 PM

In article , “Phil B” wrote:

And WITH the loading coil:
– Collector of output transistor: 17v p-p, centered at 15 v (this is the rf across the 470 ohm resistor
– At antenna terminal after .1 uf blocking cap: 17v p-p centered at 0 v.
– At top of series connected .1 ohm radiation resistance and 30 ohm ground resistance: 1.9 v p-p. (this point is after the 40pf antenna capacitance).
– Across the .1 ohm radiation resistance: 6200 microvolts p-p.

With the loading coil, if I am operating my calculator correctly, this means 76.9 mW of RF power is dissipated in the 470 Ohm swamping resistor, and 14.6 mW makes it to the actual load composed of the antenna radiation resistance, and the ground system resistance. This seems to confirm my initial suspicion that most of the actual RF power is dissipated in the 470 Ohm resistor. With a class C output stage, of the same power output, we would have more than six times as much RF power going to the antenna/ground system.

Regards,

John Byrns

Surf my web pages at, http://www.enteract.com/~jbyrns/index.html

Vectronics’ VEC-1290K is a very low power AM transmitter kit costing about $30. On this page we present our review of the kit followed by some assembly suggestions and performance enhancing tips extracted from various sources.

MWA review:

The audio quality of the 1290K is very good: crisp and clean. It beats the Ramsey AM-1 by an order of magnitude. After you set the frequency and the variable capacitor stabilizes, the VFO is not too drifty, however the frequency will shift if you touch the antenna or if it sways in the breeze. Range with the 6-foot antenna is adequate for across the house “broadcasting.”

Some owners have reported they only get 20 or 30 feet of range, but a few users have reported hearing their 1290K’s at a distance of 1/4 mile.

Go figure.

Overall the VEC-1290K is a fun kit, affordable and very easy to assemble — certainly worth the price. It has gotten many favorable comments in various online forums. However, it lacks the frequency stability and range that you’d get from a more sophisticated Part 15 transmitter.

The circuit design is clever. An LC tank is connected to a 4049 CMOS inverter. The output of this oscillator is connected to the base of the 2N3904 transistor. Line-level audio input goes into an LM386; the output from the 386 goes through a choke into the collector of the 2N3904. Output is taken from the collector through jumper-selected inductors that tune the antenna to various frequency ranges. Unlike some other kits, the 1290K actually attempts to match its output stage to its antenna. This is part of the reason for the good sound quality.

Users’ reviews:

(The comments below were posted in rec.antiques.radio+phono
and other newsgroups.)

It only took me about two hours to completely put it together. Our home is only one story, so your mileage may vary, but I get fantastic sound at 50 feet, so-so at 70, and about 90% static at 100 feet range. If that’s within your tolerance, go for it. It took some interesting tweaking (make sure you have a non-conducting screwdriver!). -Curt Wiederhoeft

I’ve built and sold several of the Vectronics AM kits and they work great. The only problem I’ve had in building is some bad ceramic trimmer caps. Be careful soldering those as they can easily overheat and be ruined. -Richard Mann

I checked it cursorily with a scope and its modulation is symmetrical and at about 85%– although there does seem to be a supersonic component in the output, too. Still, sounds pretty good, and the price is very right. -Norm Lehfeldt

It used a regulated 13.8 DC power supply, but the range was limited to around 30-feet……..so I got rid of it. I want 1/2-mile range and Vectronics is simply too weak for this. ;-(. -Gaffo

My Vectronics can transmit up to 1/4 mile (tested it with my car radio). Something must be wrong with the one you have. BTW, I didn’t like the AC adapter because it caused some interference, so I use a sealed lead acid Interstate PC12180NB 18 amp-hour rechargeable battery. -r.t.

It gets downstairs alright, enough to completely close the eye tube on the Scott 800b. This radio has an usually wide AM frequency response, and the fidelity of this little transmitter is outstanding. -Banjo Power

Assembly Tips

I just built one of these and it works well. I left the 10 mfd cap out as per manual and the line out on my CD player works good. A friend pointed out to me that you can install the frequency adjust variable cap the wrong way as two of the terminals are connected together. He did and his unit was dead. He found his error and it worked fine after correcting it. -Lou deGonzague

C11 should be mounted so the two widest tabs (actually lead shoulders) on the capacitor body are closest to the board edge. Note that these are soldered to the ground foil on the bottom of the board… For C12, the two wide leads are “parallel” to C9, with the narrow lead pointing to pin 8 of U1. If you trace out the runs, the narrow lead of C9 connects to pin 7 of U1. -Uncle Peter

Tuning the trimmer caps in this kit is made much easier if you epoxy a small nylon wheel or roller to the trimmer caps. Wish I’d thought of that several kits ago! -Richard Mann

Troubleshooting: Parasitic Oscillations

Some specimens of the 1290K suffer from an audible buzz or a supersonic piggyback signal caused by parasitic oscillations in the power supply or in the oscillator section. (This is not the same thing as the 60 cycle hum that plagues many LPAM circuits.)

Writing in rec.antiques. radio+phono, Werner offered the following tips:

1. If you made your own power supply and are using a 7812 regulator, make sure you use a 0.1uf decoupling cap close to the regulator on the input and output. Without them the regulator may oscillate at very low levels and thus modulate your signal.

2. The 4049 oscilator is somewhat unstable. It tends to vary the pulse width and or have ringing on the rising edge of the square wave it produces. This is transferred directly to the base of the modulating transistor & then appears superimposed on the transmitted signal. I effecively moved this “oscillation” to a higher (inaudible) frequency by adding a 220 pf cap in parallel to R4. This makes the unit quite useable. I suspect that a different 4049 would eliminate this problem or at least provide different results.

…Using Cmos gates to make this kind of oscilator has always been regarded as a short cut and not very reliable, by nature it is dependent on operating the chips in a linear mode which is not specificaly documented by the chip manufacturer. In short it is a bit of a kluge by nature. I hope this helps anyone experiencing similar problems. -Werner

Troubleshooting: Hot Chips

A few specimens of the 1290K suffer from an overheating LM386 chip. Here are some ideas…

I’ve noticed the LM386 is quite hot to the touch…it’s uncomfortable to leave your finger on it. -Banjo Power

You could get some thermal conductive epoxy (the overclockers use it on PIIIs and P4s) and glue a small heat sink to the chip. -Mike S

The voltage on the +vcc pin, which I think is pin 6, should generally be no more than +12V. If it’s more, something is wired wrong, IMHO. If it’s wired correctly, a bypass cap of about .01uf may help. Otherwise, it may be a bad chip. -Bill Hutchinson

There are several versions of the LM386. If your is a LM386M – note the last letter – the dissipation is only .73W. A LM386N is 1.25 W. Along those lines, the best choice, if you can find it and are replacing your chip, is an LM386N-4 as it operates essentially the same, has the better dissipation, but tolerates 22v on +VCC. -Prof F J Flutter

I have the 386N-1 — what is its output? -Gaffo

325 mW. Max. Vcc of 12 volts. -Uncle Peter

If you’re using a wall wart, be aware that those things will produce much more voltage than their ratings unless you actually draw the rated current… Verify the supply voltage. It may be higher than the chips are happy at. -Robert Casey

Modification: Longer Antenna

If built according to instructions, the 1290K can work properly with an antenna up to 12 feet long. To use a longer wire than that, you would have to change the values of the components in the pi network. If you are in the US and you are not on an educational campus, the Part 15 rules limit your combined antenna and ground lead length to 3 meters (which is equivalent to 9 feet and 10 inches).

Modification: Overhaul

Untested idea: The 1290K circuit board could be used as the basis for a similar but more stable and more powerful rig. A simple 1-transistor crystal oscillator, perhaps with a 1-transistor buffer amp, would replace the CMOS chip and its LC tank. (I would prefer to have a clean sinewave carrier instead of a parasitic-infested squarewave.) The 2N3904 would be replaced by something able to handle more power, perhaps a 2N3553. Instead of an LM386N-1, the modulator would be an LM386N-3 or LM386N-4.

User Commentary

I know that this little toy has been the object of much discussion in the past. I have been using a WPRDJ for almost 3 years now, and after a lot of experimenting, have come up with a pretty good setup for it.

I am on my 2nd unit, as the first got lost in a move- I have the white and blue version that comes with the external speaker, which I removed before using the unit.

User Commentary

I know that this little toy has been the object of much discussion in the past. I have been using a WPRDJ for almost 3 years now, and after a lot of experimenting, have come up with a pretty good setup for it.

I am on my 2nd unit, as the first got lost in a move- I have the white and blue version that comes with the external speaker, which I removed before using the unit.

The WPRDJ runs around $25 or so at Toys-R-Us stores, or on Amazon’s website. Do a Google search, first, and you may find other sellers charging even less

My Audio is from mp3s compressed at 64k, run through Winamp and the Izotope Ozone plug-in (free version). Using the “extra wide sound” preset along with the other presets gives a warm, rich sound.

Using the EQ settings on Winamp approximates pre-emphasis, and the cross-fade plugin gives a professional sound with no dead air. The sound processing does a killer job of making up for the transmitter’s low modulation (estimated in other reviews to be around 65%)

I have the whip antenna connected to an alligator clip, then plugged into the [u]neutral[/u] pin of an electrical outlet, and the ground clip is attached to a wire coat hanger. This set-up eliminates hum, and allows the signal to be heard almost 300 feet away

Battery drain isn’t much of an issue unless you use the built-in tape deck- I don’t, and 4 good AA’s last about 20-30 hours before needing replacement

If you want to stick with a FCC Type-Approved transmitter, the cost on this sucker can’t be beat. Use a set-up similiar to mine, but go with what sounds good to your own ears, and you can be on-air quickly and for a small amount of money. I did, at one point, play around with a Ramsey AM TX, but believe me, the drift was a killer, and the audio left a lot to be desired

IF 1610 is clear in your area, GO with the WPDRJ !

Analysis and possible modifications of this affordable toy AM transmitter.

Note: These comments apply to the blue-purple version of the toy and revision PR6 of the circuit board. There is also a white version of the toy with an external speaker, which reportedly contains the same or a very similar circuit board.

Analysis and possible modifications of this affordable toy AM transmitter.

Note: These comments apply to the blue-purple version of the toy and revision PR6 of the circuit board. There is also a white version of the toy with an external speaker, which reportedly contains the same or a very similar circuit board.

Introduction

Wild Planet’s Radio DJ (called the WPRDJ for short) is a fun little self-contained AM radio station containing a microphone, cassette player, audio mixer and FCC certified Part 15 transmitter, all for less than $25.

Although some people have criticized its extremely weak signal, how much transmitter power do you really want to put into the hands of six year old kids?

One source says the RF output power of this unit is 1.3 milliwatt. Judging by the range of the signal, the effective isotropic radiated power (EIRP) must be much lower than this, down in the vicinity of 10 to 20 microwatts.

Here are some thoughts on improving the performance of this rig.

Antenna and ground

There’s a proverb that says “the three most important factors in real estate are location, location, and location.” With an extremely weak transmitter the three most important things to improve are antenna, antenna, and ground.

The antenna of the WPRDJ should be vertical, upright in a straight line, and as far away from any metal objects as you can get it.

The ground clamp should be connected to a real electrical ground stake hammered into moist earth, or connected to an artificial ground plane such as a set of wires radiating out from a central point. (An artificial ground plane made of wires can be hidden under the carpet in a large room.)

One experimenter took a WPRDJ to a riverside location, dropped a 40 foot ground wire into the river, and reportedly got much better range.

If you must operate indoors and you have a choice of upstairs and downstairs locations, upstairs will probably produce better results. However, your mileage may vary.

Opening up the device

Opening the case of the WPRDJ and modifying it may void your warranty and, in areas under FCC jurisdiction, may nullify your authority to operate the device.

Put a set of fresh batteries into the WPRDJ before you start this process.

To begin opening the WPRDJ, turn the unit “bottoms up” and remove the visible screws. You will also find two screws hidden under the rear rubber foot-pads. Now you can remove the bottom cover.

You will notice two adjustable components on the underside of the board: a variable capacitor near one edge, and a trimpot (variable resistor) kind of near the middle.

The trimpot (variable resistor) will adjust the speed of the cassette player. If you need to adjust this, turn the unit right side up. While playing a cassette that contains a reference tone or a song familiar to yourself, adjust the trimpot with an insulated screwdriver until the player reaches the correct speed. Then put a blob of glue or nail polish on the trimpot to help keep it set where you want it.

Also on the underside, located very close to where the antenna and ground wires are attached to the board, is a variable capacitor for tuning the antenna.

If you have some way to measure the RF power output of the unit– perferably a field strength meter– you can use an insulated tuning tool or insulated screwdriver to adjust this capacitor for maximum power output. When you make this adjustment, the WPRDJ should be grounded and its antenna should be straight up vertical.

In my opinion, for the very best results possible, you should make this adjustment with the unit attached to the antenna and ground system that you will be using all the time. The setting that works best on an electronic workbench is likely to be a little different from the setting that works best with your everyday antenna and ground system.

In a message on the Community Radio USA board, Pat Ryan wrote the following:

After removing the bottom cover, I found the antenna trimmer capacitor, (located right at the antenna lead) then while using a Radio Shack field meter and my scope I was able to adjust the trimmer for maximum output and according to the “scope” was about 4 times its output set by the factory.

v Be sure that the Radio DJ is GROUNDED while making this adjustment. The only drawback to this “mod” is that the LED will be on all of the time (apparently RF sensitive). However, I’m sure all I have to do is replace a resistor in the LED circuit.

After making these adjustments, most people should stop fooling with the transmitter. Put the bottom cover back on and enjoy the WPRDJ for what it really is– a cheap but enjoyable toy. If you must know what the top side of the circuit board looks like, just eyeball the photo at the bottom of this page.

Removing the motherboard

Hardcore tinkerers with some electronic experience may want to go farther into the circuit. Here are some tips for loosening up the main circuit board.

The knob for the microphone fader is glued to a slider pot which is soldered onto the circuit board. This makes it difficult to remove the circuit board without breaking something 🙂

In the LWCA mailing list, Les Rayburn offered the following disassembly tips: “Remove [the] cassette player drawer. There are two screws inside the tape deck that secure it to the base. Using a pair of wire snips, break off the plastic slider volume control. It is glued to the pot underneath, and if you try to force it, you’ll only break the pot. (I did!)”

Frequency change

The crystal is 6.44 MHz. The 74HC00 and 74HC74 chips oscillate at that frequency and then divide by 4 to create the 1610 kHz carrier wave. You could operate the WPRDJ on a different frequency by installing a new crystal; you could probably move to 1590, 1600, 1620 or 1630 without needing to change any other components. You would have to order a custom made microprocessor crystal from a company such as Crystek or Bomar, and that would probably cost more than you paid for the transmitter.

Modulation level increase

The collector of Q1 gets its current supply from pin 5 of an LM386 amplifier chip. This is the same modulation scheme used in the Vectronics kit and in the Poppet QRP transmitter. For some reason, the WPRDJ doesn’t modulate as well or sound as clean as those other circuits.

Pat Ryan was reportedly able to get the modulation up to 100% and improve the fidelity by making the following set of modifications:

(1) Unground the emitter of Q1.

(2) Install a 33 to 47 ohm resistor from the emitter of Q1 to ground.

(3) Install a 220 microfarad 16 volt [electrolytic] capacitor across the 33 to 47 ohm resistor (make sure that the negative lead of the capacitor is soldered to ground).

(4) Remove C25 .1uF capacitor.

(5) Re-align the rf output trimmer cap for maximum output with an insulated alignment tool.

(In a later message, Pat also said “I installed a .001uF cap from the collector to the emitter of Q1.”)

Update: It is possible to do these modifications without removing the circuit board by cutting the traces at the correct locations.

In the newsgroup alt.binaries.pictures.radio, Robert Casey reported: “Looking at the picture at the bottom …and looking at the back of the board after taking the bottom off, I determined where I could cut traces to achieve the mods. I also removed the speaker, and to conserve battery life, disconnected the “ON AIR” LEDs.”

Power supply

When the cassette motor is not running, the WPRDJ draws about 50 milliamps at 6 volts. There is no voltage regulator to protect the CMOS oscillator chips, so one thing you don’t want to do is supply more voltage. The abosulte maximum rating on those chips is 7 volts; with a fresh set of batteries the chips are nearly maxxed out.

Some people will be tempted to replace the batteries with a plug-in transformer (sometimes called a “wall wart”). Keep this in mind: AM transmitters that are powered by wall warts often interact with the AC wiring in unpredictable ways and this can cause a very annoying hum to appear in the signal. If you try a wall wart and the hum from heck appears in your signal, go back to battery power.

Above is a photo of the board. If you would like to see a parts placement diagram of the RF section with the PCB traces visible underneath, read Jonathan Smick’s analysis of the circuit with suggested modifications and a nicely done schematic of the RF section, go to home.att.net/~weatheradio/wprdj.htm

Please note

This product reveiw includes comments from SCWIS, as a user of the unit in support of a part 15 broadcast operation, and includes a significant technical discussing contributed by the MWA website.

Part15.us is grateful to MWA, the Medium Wave Alliance, for permission to include the technical discussion in this review.

Please note

This product reveiw includes comments from SCWIS, as a user of the unit in support of a part 15 broadcast operation, and includes a significant technical discussing contributed by the MWA website.

Part15.us is grateful to MWA, the Medium Wave Alliance, for permission to include the technical discussion in this review.

User Report

This is a top quality product, and an amazing value. The PLL circuit is well designed and runs accurately across the dial. This is a “set it and forget it” system with an external power supply for improved reliability. Frequency is rock solid and modulation is controlled at the source.

Gizmo AM Broadcast Transmitter
• Payment via Paypal
• Order placed 6/20
• Item received 7/3
• Well packed, bubble wrap & box

Includes
• Transmitter in plastic case
• Three meter wire antenna
• Mono patch cord
• Twelve page manual

Nice, clean layout inside. PLL circuit selector easy to use, signal rock solid. Note the threaded brass inserts in the case closure corners. Very nice touch. Includes the ubiquitous adhesive rubber feet. Appears to be a hand soldered, single sided PC board, clean and straight. Antenna terminal is a rugged post, dip switches are easy to reach and set.

The Gizmo is a great place for beginners to start, and it’s a simple way to add a new channel to an existing operation.

When you add the Wizard antenna, also available from Vintage Components, broadcasting is simplicity itself. All you do is plug in your power supply, plug in your source, set up the antenna, select your frequency, turn on the power, and fine tune the antenna match. That’s all there is to it, and you’re on the air!

Here’s an example of a simple set up used to broadcast Old Time Radio MP3s to the author’s apartment complex. This set up is running in the corner of a closet, with only the base of the antenna and two ground radials used for transmitting. Range covered 30 units.

This product is entirely too much fun. Many hobbyists might say “Gee, I could probably build that myself” and many of us could – the point is, why bother? For a very reasonable price, and little more than 10 minutes set-up time, you can be on the air RIGHT NOW!!!

Technical notes on the Gizmo low power AM transmitter

What do you get when you marry a classic PLL oscillator circuit to a slightly modified version of the Wenzel transmitter circuit, and build it on a compact and well-laid-out circuit board the size of a baseball card? You get the Gizmo, an affordable, pocket-size AM transmitter.

In early 2003, the Gizmo was the only assembled, PLL-controlled transmitter available for under $90. It has good audio quality and its range with the supplied wire antenna is sufficient to put a strong signal throughout your house, and possibly your immediate neighbor’s house, with weaker reception extending out a block or two.

The Gizmo does have room for improvement in a few areas. First off, it is extremely vulnerable to having a buzz or whine in the signal unless you power it from a super highly filtered, ultra-perfect power supply. With the specimen Gizmo that I borrowed from a friend to write this review, this problem was somewhat relieved when I put my finger on the middle chip in the oscillator section, which suggests that a little extra bypass capacitance somewhere in that neighborhood might help.

I’d like to thank Mr. Phil Bolyn for proofreading the diagrams below, and for offering the following comments about the design of the Gizmo:

“The RF output consists of a differential amplifier (Q3 and Q4). When Q3 is on, Q4 is off, and vice versa. The transistors are driven to saturation during their respective half cycles. The differential pair method produces highly linear modulation and can be driven to 100% modulation (not true for other, simpler schemes). ”

“Q5 modulates the RF by varying the current to the diff pair at audio rate. The resulting current at Q4 collector is a square wave at RF frequency, amplitude modulated by audio from Q5. The output tank circuit filters out the harmonics. C14 and C15 help to reduce ringing at the current peaks in Q2 and Q4. This should improve the output signal, but I am not convinced they are needed based on spice simulations I have done on a similar circuit. ”

“One problem I see with the Gizmo is the method of tuning the output using a fixed inductor and 4 switch-selectable capacitors. The 4-pole dip switch allows selecting 16 different cap values, but the Q of the output tank is high enough to make this a real compromise. To get max output you would need to carefully select a frequency that matches one of the 16 cap values. Since quiet channels are few in populated areas, it would be pure luck if one of the cap values exactly matched a vacant channel frequency. I would suggest an improvement to the circuit.”

“Mouser Electronics sells an inexpensive 100pf variable trimmer cap. I would add one in parallel to the dip switch to fine tune after selecting the nearest value with the switches.”

You can’t help admiring the Gizmo’s elegance and compactness, and it has helped dozens of people to “get their feet wet” in the LPAM waters without having to build something from scratch or purchase a much more expensive assembled transmitter.

New review here: www.radiointel.com/review-sstran.htm

And Paul Stenning’s review here: www.vintage-radio.com/reviews/amt3000.html

Posted here with permission from Popular Communications Magazine,

New review here: www.radiointel.com/review-sstran.htm

And Paul Stenning’s review here: www.vintage-radio.com/reviews/amt3000.html

Posted here with permission from Popular Communications Magazine,
www.popular-communications.com

Join The Part 15 Revolution With The SSTRAN AMT-3000 AM Broadcast Transmitter!

Perhaps you didn’t know, but you can legally own and operate your own AM radio station. FCC Part 15 Rules permit unlicensed transmitter input powers of 100 mW between 510 and 1705 kHz—enough power to span a small portion of many urban neighborhoods!

Micro-broadcasting has many useful applications, including broadcasting your favorite programming over your favorite old radio. Why be at the mercy of ever dwindling AM program variety? Suitable audio sources for your micro-broadcast AM station include cassette, CD or MP3 players, satellite audio channels, or even Internet radio stations. AM radio is going digital, and when the sad day comes when analog AM radio goes dark, you’ll be ready with your own radio station to fill the gap! Here’s how!

The cornerstone for effective microbroadcasting begins with a good quality transmitter, and the SSTRAN AMT-3000 kit is the best we’ve seen to date. The feature- packed transmitter is synthesizer controlled in 10-kHz channel steps (9-kHz for European export models) from 510 kHz to 1710 kHz, easily set via dip-switch entry.

Audio Features

Although this is a monophonic transmitter, the audio input will accommodate two RCA plugs from a stereo source. The stereo signal is summed in the transmitter to prevent loss of right or left channel information. The two inputs can also allow the transmitter to mix multiple mono audio sources for broadcast.

Photo A shows my AMT-3000 with my portable Sony CD player. When the picture was taken the combo was broadcasting ’60s era oldies to a nearby vintage radio. It sounded great! An on-board jumper permits an 8-dB treble boost at 2 kHz; normally the transmitter audio is flat within 1 dB from 20 Hz to 20 kHz. The transmitter is capable of full 100 percent negative modulation (tested and proven in our lab) and also features a frontpanel adjustable 1:1 to 5:1 dB compression ratio. Audio levels exceeding the adjustable limiter level are compressed at a 15:1 ratio. The transmitter audio response is flat from 20 Hz to 20 kHz with low distortion—this is unit is capable of outstanding AM audio quality!

Three front-panel controls for Gain, Modulation, and Compression give the user full control of how the transmitter sounds. The elaborate audio processing (compression and limiting) is handled by an Analog Devices SSM2166 integrated circuit, a powerful level of audio processing that I haven’t seen offered in competitive units. The manual shows how to quickly set these three controls for the best sounding signal and it also gives a more detailed explanation of the Analog Devices’ features than space allows here.

The Kit

Photo B shows what you’ll see when the AMT-3000 is unboxed. Most of the smaller electrical parts are carefully presorted in plastic bags at the factory. The enclosure, knobs, and wall-wart power supply are included in the kit (too often these by Peter J. Bertini, [email protected] technology technology showcase new product performance analysis are expensive “options” in competitor kits; everything needed is here, and for one price). The enclosure is robust and professional looking. Considering the quality, complexity, and parts count, this kit is an exceptional bargain.

Starting Assembly

Before picking up a tool or soldering iron, you need to inventory the parts and read the manual first! The kit is moderately difficult to assemble, so the builder should have basic soldering skills and experience with simple kit construction before attempting to assemble the AMT-3000. This is not the kit for inexperienced first-time kit builders. If you’ve never held a soldering iron, you might have a friend do the assembly for you, or first hone your skills on simpler kits.

On the plus side, the manual is clear, concise, and well written. It shows you how to solder and is amply illustrated to aid with construction steps or in parts identification.

The Hard Part

The most difficult task is soldering the SO-14 SMT (surface mount) Analog Devices audio IC with its .05-inch lead spacing! (Unfortunately, the through-hole version of this IC is discontinued.) The IC is shown in Photo C. If you don’t feel up to the task, the IC can be factory installed for a modest $3 fee. One thing I’d suggest is to have a good hand magnifier nearby when building the kit! My near vision isn’t what it used to be, and a reading glass proved to be most useful during assembly when my eyes wouldn’t focus!

Another tricky construction step involves mounting two monolithic capacitors between IC socket pins on the bottom of the pc board. This was a design revision to clean up some artifacts from the synthesizer that were audible on the transmitter signal. The pc board is silkscreened—all of the component legends and body layouts are clearly printed on it! Also, the solder side of the board is masked, which helps prevent unwanted solder bridges across adjacent runs or between component pins. Photo D shows the PC board nearly 50 percent assembled. Note that the larger IC packages mount in IC sockets for easy replacement.

Plan to spend at least a weekend, interspersed with ample rest periods, to build this kit. It’s all too easy to become complacent and rush assembly, and rushing assembly is what leads to errors. My Waterloo was confusing a TO-92 packaged IC with a similarly packaged transistor. Had I read the instructions, I wouldn’t have erred. The assembled pc board, installed in the enclosure, is shown in Photo E.

Hum Reduction

Many in-home AM transmitters are plagued by “tunable hum.” The kit designer felt most of these problems relate to poor power supply design and inadequate RF bypassing. Tunable hum is a loud hum on the receiver audio that’s only present when tuned into a station.

This kit includes an AC wall wart supply, and the diode rectifiers are RF bypassed to prevent incidental 120-Hz modulation of RF currents flowing through the power supply cables. The transmitter also features (jumper removable) RF chokes in the power and audio lines to eliminate stray RF antenna currents from flowing on these cables (if needed) to control tunable hum problems. I’m pleased to report that I never experienced this problem.

Technical Tidbits

The synthesizer uses four 74HC series ICs to produce a crystal- referenced signal between 20 kHz to 2560 kHz (FCC Part 15 rules permit operation between 510 and 1705 kHz, and the actual transmitter operating range is limited to 530 to 1710 kHz by the RF stage tuning. The synthesizer uses a 4-MHz crystal oscillator for the reference.)

The mumbo-jumbo means that, unlike inexpensive competitor models using free-running oscillators, this transmitter’s frequency is locked to a very stable and accurate crystal oscillator; it won’t drift with temperature, changes in humidity, or over time. My frequency counter showed the transmitter was within a few cycles of the frequency it was set to. This avoids audible heterodynes from weak distant AM stations on the same frequency, especially at night. The S4 dip-switch settings set the binary divider count in the synthesizer to determine the transmitter frequency, based on the following formula:

Frequency in kHz = 10 * (S4_value + 1)

Besides setting the transmitter frequency via S4, the transmitter RF stage output is set to the corresponding frequency range using S5, a four-position dip switch. SSTRAN breaks the broadcast band into several different band ranges, and this switch selects the optimal component values for each of those frequency ranges. The transmitter output tuning is peaked (via a ceramic trimmer) for the highest DC voltage at a meter test point on the PC board.

A considerable amount of engineering time went to making the modulation quality relatively immune to improper transmitter tuning. This all sounds more complicated than it really is, since the manual clearly describes the steps for setting the audio controls, transmitter frequency, and tuning.

Antenna Choices

The RF stage uses a tunable Pi-Net output for maximum power transfer to the integral Part 15 antenna system. The Pi- Net also limits harmonic radiation to better than -20 dBc to meet FCC regulations. The supplied antenna is a 118-inch length of white antenna wire and a 72-inch black ground wire counterpoise per Part 15 Regulations.

The manual instructs the builder to solder the supplied antenna and ground wires directly to the pc board to meet FCC Part 15 antenna requirements. However, the RF output is also available through an optional (but supplied) RCA jack should the builder elect to use another antenna system. This runs the risk of not meeting FCC Part 15 compliance if a different antenna system is used. If you live outside the United States, you will need to determine the local laws applying to unlicensed broadcasting.

Getting The Most Range

The SSTRAN antenna yields best efficiency at the high end of the broadcast band; that is, you’ll transmit farther at 1700 kHz than at 530 kHz. Look for a relatively quiet frequency at the high end of the band to get the best range. In many areas of the country this means going into the relatively unoccupied expanded AM band above 1600 kHz, especially at night when distant stations are barreling in across the dial. Many vintage radios won’t tune above 1500 or 1600 kHz; while others will tune to 1700 kHz, which was the old police band used prior to WWII.

How far you can transmit depends on a lot of variables— every location is different. Some Part 15 broadcasters claim they can cover several blocks; others have trouble reaching across a ranch house! Regardless, with everything being done legally, a properly engineered and efficient Part 15 transmitter with good audio and decent modulation depth will have the edge at any location.

Wrapping It Up

Here’s the bottom line: This is a well-engineered product that delivers what it promises. It’s an exceptional value. The transmitter audio is great. You won’t be disappointed with this transmitter. Since a small family-run company produces the kit, I suspect the product is more a labor of love than a profit-motivated venture. The e-mail customer support was good.

My transmitter is in daily use broadcasting old time radios across neighborhood, a topic we’ll be discussing in a future “Wireless Connection” column! Stay tuned for more details and join our micro-broadcast revolution.

Ordering Information

Full kit, AMT-3000, AMT-3000-9K (9-kHz European)

SMT Chip pre-soldered, AMT-3000-SM, AMT-30000-9KSM (9 kHz European)

Contact:

SSTRAN, 3053 Griffith Rd., Norristown, PA 19403;

Web: www.SSTRAN.com

E-mail: info (at) SSTRAN.com