WEAK-AM

Well, I suppose it had to happen… with all of the talk about reasonably priced Talking House transmitters on eBay, I could not resist… a Talking House is now on the way! Incidentally, I received a “push” when I discovered a new Talking House station operated by one of the local real estate agents on my lake. I checked the range, and it doesn’t do too badly, although the audio is atrocious and is accompanied by copious amounts of hum and noise… that should be very fixable, I think.

The thing I find most intriguing about the TH is the automatic antenna tuning feature. That’s got to be one of the coolest things around! Needless to say, in order for a real estate agent to set it up, they needed a feature like that. I plan on using mine indoors with a 3M wire antenna.

Of course, I’ll take care to keep it away from glass. Radio waves are scared of glass, you know! :o)

Update: drove by the home again tonight, and considering that it is an indoor installation, I am very impressed with the range. I struggle to do that well with my SSTRAN on an indoor antenna, at a much higher frequency on the dial. Also; this one does not seem to have the frequency warble I’ve noticed on others, and it seems to be close to frequency. So they must have improved them significantly. I am not sure why there is so much hum… maybe it was recorded that way, or maybe they are lacking a good ground. But I would tend to think it was recorded that way, since the range is pretty decent.

It took me a week, but I finally found the time to fire up Chris Cuff’s creation on the test bench. My unit came set for operation on 1500 kHz (at my request). You can change the crystal to operate on other frequencies. Lacking a power supply with the correct connector, I fired it up on batteries (an internal battery holder is included). Eight AA’s will apparently power the unit for 8-10 hours continuously.

I connected a short wire antenna to the Fahnestock clip on the back and grounded it to the wall outlet. Next, I hooked up a stereo audio source to the RCA input connectors and fired it up! Immediately, I heard the program material in stereo on my tuner. I have only one AM stereo radio left; it is a modified Radio Shack tuner. The sound quality was not perfect, but it was in stereo!

I reduced the level a bit until the audio sounded cleaner. I seem to recall that you have to limit the modulation to 100% with AM stereo. In any event, I am not certain that the transmitter is capable of more than 100% peak positive modulation.

Although the transmitter comes with a few explanatory notes on how to set it up, there is no schematic. The output circuit is interesting; the coil appears to be wound on a glass test tube (with great care, apparently), along with a nice vernier tuned air variable capacitor. You adjust the capacitor for the strongest signal, as observed on a field strength meter or nearby radio. I jumped in the car and drove down the street. My preliminary observation is that the range is comparable to my SSTRAN when operated under similar conditions. The output appears to be taken directly from the “hot” side of the parallel tuned tank circuit.

This is an interesting unit, and probably destined to become a collector’s item. Chris told me that he has built 350 of them by hand and that he does not have parts to build any more. Classical music sounds great in AM stereo, as those of you who live in the Kansas City area can probably attest, due to the presence of KXTR.

Update: I have discovered that the carrier frequency is off by quite a bit; probably in excess of several hundred Hz. I’m not sure if I’ll bother fixing it as I intend to purchase the programmable oscillator from Digi-Key (if I can remember the part number)! But for anyone who wants to use it crystal controlled, it should be an easy matter to add a trimmer on the board. The microprocessor crystal Chris sent with the unit is probably not very accurate.

I know I’ve read comments on Part 15 from others looking for the famous Chris Cuff stereo transmitter. Well– a word to the wise; there are just TWO left. I happened to be browsing a very popular trading site (you guess) and idly did a search for “AM transmitter”. What to my wondering eyes should appear but a listing from the famed engineer with just a few (the last few) AM stereo units up for sale.

Now to keep this non-commercial; first of all, I am not selling anything. I am not going to tell you how much they are going for, or even where to find them. If you are smart enough to frequent this board, you will figure it out. But if you want one, better HURRY (I am not kidding)!

Update: They are all gone now! If you wanted one, I hope you got it!

WEAK-AM got one and will be soon be broadcasting classical music in AM stereo!

I know someone out there has the answer to this!

I know someone out there has the answer to this! I am contemplating using some programming from the Internet. That’s fine if I’m around to monitor the station and see that everything goes OK, but what about those times when I’m not? I’ve heard the local ethnic AM station broadcast nothing but hum and noise for hours on end when they lost their Internet program feed and nobody was around to restore it.

So, what techniques do you ladies and gentlemen use to set up your PC so it will “self-restore” when the audio feed is lost for any of a variety of reasons, such as loss of power (resulting in a PC reboot), or network problems that just cause the feed to become disconnected?

Any ideas gratefully received!

All–

I’d like to recommend the following article by Ben Tongue, of Blonder-Tongue fame, on maximizing the Q of ferrite rod inductors:

http://www.bentongue.com/xtalset/29MxQFL/29MxQFL.html

All–

I’d like to recommend the following article by Ben Tongue, of Blonder-Tongue fame, on maximizing the Q of ferrite rod inductors:

http://www.bentongue.com/xtalset/29MxQFL/29MxQFL.html

Also, I commend to your attention the following web site of LA8AK, which has construction articles on LC and Q meters and modifications for same, along with actual measurements of various inductors:

http://www.noding.com/la8ak/m1.htm

Regards,

I promised that I would share the results of my toroid winding experiment with the SSTran AMT3000. This should be of interest to anyone who is using this transmitter with an indoor antenna and looking to gain a little more coverage over what is possible with the stock version. Important disclaimer: this modification does not change the input power of the transmitter, but you must decide if you have the necessary expertise to perform it.

I currently have my SSTran set up in the living room (it is cold here in the north right now). The ground wire is connected to the electrical conduit ground via the screw on the outlet box. The 10′ wire antenna is just draped over the top of the TV set for now. Not a very fancy setup!

Before I made the modification I am about to describe, the transmitter covered my house, but that’s about it. I couldn’t even hear it out to the curb reliably. As soon as I backed the car out of the garage, the signal would fade out.

I’ve been thinking about trying some toroidal inductors in place of the small axial leaded ones in the output of the SSTran. Based on the data in the Amidon product guidebook, I ordered a few different sizes of #2 material powdered iron toroids from them. At Phil’s recommendation, I tried a T157-2 core with #26 wire. Type 2 material has a permeability of 10, and this particular core has an A(L) value of 140. According to the data book, you should be able to wind about 168 turns on it. I used 100 turns to approximate the inductance obtained with L4 and L5 in the circuit. This allows me to peak the output with C5 at the upper end of the AM band.

To make testing simple, I removed inductor L7, which I didn’t need anyway, since I only operate at the high end of the band. I connected the wires from the toroid in its place, which have to be a few inches long because it will not fit on the board. Right now, the toroid is positioned off to the side of the unit. I didn’t want to lay it on top of any components because it develops a pretty high field (even though the field is better controlled than that of a solenoid). With this arrangement, I can put the toroid in the circuit by closing positions 1-3 of S5 and leaving position 4 open.

My new inductor resonates with slightly less capacitance at C5 than the L4+L5 combo, so it must have a little more inductance. The tuning is very, very sharp– much more so than before. The DC voltage at the test points went from about 5-6V before to over 12. I have gotten more than 130V p-p at the antenna jack with a 33 pF capacitive dummy load in place of the antenna, but C5 is way too “touchy” to maintain this.

How does this work? Well, the results are much better than I expected. I put on a Schubert symphony and listened around the house. A few nearby radios were completely overloaded to the point that the signal came in for about 150 kHz at the top of the dial. I got in the car and drove out of the garage, expecting the signal to fade out, but I was pleasantly surprised– it remained strong as I turned onto the street. There was no perceptible change all the way to the first corner, then a slight dip, and after that the signal rebounded. Schubert boomed out of the speaker all the way to the end of the block! As soon as I turned the corner, it faded out in a hurry. Most of the coverage is due to re-radiation from the overhead power lines, as it turns out (even though I’m making no attempt to drive the ac line). Amazingly, I have discovered places more than 6 blocks away where there are “hot spots” and the signal comes in strong! This was never the case before.

I could go into more details, but keep in mind that this is just an initial experiment. Ideally, you would short all of the switches in S5 and connect the toroid in series with the antenna like you would with the coil for the base loaded antenna. Then you would tune the antenna by lengthening or shortening it. This way, none of the signal would be shunted to ground by C5. Even though it is just an experiment, the results to date are so good that I definitely will continue to work on this further. If you want more details of measurements that I’ve taken, please ask and I will post them here.

Again, keep in mind that the high Q of the toroid coil means that the output has to be tuned “dead on”, or the power goes to nothing. It is very tricky to set C5 using the existing circuit topology. But when it is in tune, the signal is much stronger! One other possible side effect is that the high audio frequencies seem to be rolled off a little (I have not yet made any measurements of this). I think this would tend to be less of a problem when using the toroid as an external loading coil with C5 operating as a fine tune in parallel with C23, because the ground losses in a typical indoor installation would de-Q the circuit significantly. But the radiated signal should still be stronger than running it the way I currently have it set up.

I would guess that with a favorable indoor antenna setup, it could be possible to obtain 1/2 mile of range or more on a clear frequency. Another possibility would be to use the toroid in place of the large coil, in an outdoor setup, in which case it could be mounted inside of the transmitter enclosure, protected from the elements. Note that the coil probably still has the edge over the toroid on Q, but maybe not by as much as you might think. I estimate the Q of the toroid to be around 400 or so. Q’s higher than that may not mean much, if you can’t keep the antenna-ground circuit in tune.

Please post any questions you may have and I will attempt to answer them.