I ordered an SStran AMT3000 kit from Phil just before xmas, it arrived in very reasonable time. I put off starting it right when I got it due to some other family get-togethers and working around my disabled son's school schedule. I contented myself with inventorying the parts, reading and re-reading the manual/instructions, stripping a few old boards for my junkbox to get my hands in practice for the kit, and etc. I'd also gotten a new used soldering/rework station as one of my presents and I wanted to put a little time in with it before jumping right into the kit.
Finally I quit putting it off and when the house was quiet for a couple hours, I cleared off some table space and set up my assorted toys..
Full sized photo of SSTran build.
Not the most orderly or the best equipped, but it proved quite up to the task. The aluminum foil is a "poor man's antistat mat".
I broke up the build into 3 sessions on almost successive evenings. Total time for the build was about 3.5 hrs. Figured it was best to do it in sessions to avoid headache, eyestrain and etc, since I don't do this sort of work every day anymore.
The kit went together really easy. All the parts were there, the instructions were clear and easy to follow. Very possibly the best kit of any nature I've ever built so far as being good parts and good instructions.
The unit powered up perfectly, no problem finding it on the reciever. Signal was nice and strong and with just a bit of adjustment on the SStran audio controls the sound was excellent! No noticeable hum, and once the audio controls had been adjusted, it was clear as a bell. I tried the treble boost with jumper 6, didn't like the sound, took it back off. I am picky about audio, so I kept the compression fairly low.
Now the antenna was just the wire one that comes with the kit and all I'd done for testing was to take off the wire tie and toss the coil of wire to the other side of the kitchen table. It wasn't even straight. I didn't hook up any ground or bother with uncoiling the ground wire, since this was just power up testing. It peaked to about 6 V.
So then I tried radios around the house to see how it sounded more than a few feet away. Even to a small GPX clock radio all the way on the other end of the house (about 60 ft) the signal was clear and strong and sounded really good. The same GPX does *not* pick up my FM transmitter near as well.
Over the next few days I'll be hunting for a good operating frequency to settle it on and seeing how it does with the antenna more straight (and actually vertical as opposed to draped across a kitchen table and hanging partway down to the floor).
So far I'd give the SStran top marks as a kit, and it seems to be a very nice little transmitter.
Kudos to Phil.
Daniel
Daniel,
Thanks for the writeup.
May you have megahours of fun and picoseconds of frustration.
Neil
I would not recommend using aluminum foil for an antistatic mat. It's just too conductive, and can produce sparks. It can cause a "poor man" to become even poorer. Simply touching one's hands on some large metal object, like a metal cabinet (before beginning work), should provide enough static electricity protection.
Part 15 AM is no place for audiophiles. If a Part 15 AM station is heard at all, it is likely to sound like a dim voice buried in noise. The treble boost would help improve intelligibility a bit.
I also wonder if the kit was ordered with the surface-mount chip soldered in place.
Although I roll my own, I like the SSTRAN. From what I learned from the posts on part15.us, PhilB seems to be a really good circuit designer. I also recommend a kit over a certified unit for anybody who is at all handy with electronics. If a certified unit has a design problem, there is nothing you can do about it. Any change would void the certification. With a kit, the builder assumes all responsibility for the performance of the transmitter, and any changes that keep the transmitter within legal limits are allowed.
Hi Ermi
I've heard logic both ways on using a metal surface as an antistatic measure. The alternative was working on a table surface with a plastic veneer which I felt was more of a risk. The foil was taped down before I started work and a short wire run from the foil to a cabinet screw of the soldering station. The board (when it was out of it's antistat bag) was on the foil or in the clamps of the metal "helping hand". My hands also were usually in contact with the foil. In any case, in contact with it before touching the board. Not optimal, perhaps, but it was a way that was recommended to me by a bench technician I used to work with years ago and I've used it for some years and had no problems.
Correct, "part 15 is no place for audiophiles" but there is also finding the effects of treble boost or more than a very modest amount of compression unpleasant/annoying. All through my own house it sounded quite good without the treble boost. My location is far from optimal and at present I'm using the wire antenna indoors, so how it sounds in the distance is not a prime concern. I don't expect to be setting any distance records in the near future. More treble can improve the legibility of speech, but for musical content I feel that by the time anyone is hearing it as a dim voice buried in noise, they would already have very sensibly tuned to some other station.
Admittedly, I touch up the treble setting on my mixing board a little when I'm going to be doing a read of any length. Most of my evening programming is reading, playing pre-recorded reads, etc. Stories. Classic literature and folklore. But I don't care for the "high pressure" sound and would rather put out a pleasant sounding voice than compromise it for maximizing range. Trim the bass a teeny bit and touch up the treble, a very slight reverb, just a touch of compression, and a brick wall limiter and that's my sound for voice. I mix in a little pre-recorded environmental background sound or instrumental music. I use a large diaphragm condenser mic because I love the sound and smoothness of it. But I'm getting off the track of replying here. LOL
Yes, the kit was ordered with the surface mount audio chip soldered in place. I've never installed a surface-mount component and considered 3$ a bargain to not be trying to learn how to install one during this build. Ideally that's what "introduction to surface-mount soldering" projects/kits/lessons would be for.
I homebrew some sorts of circuits. But RF is not an area where I feel comfortable relying on my own design ability. If I need a clipper, a ground lift box or device, or if I was setting out to build say a console for my station.. Ok, there I'd prefer to work up my own ideas from scratch. Audio and some high voltage circuits are places where I've done enough tinkering and cobbling over the years to feel comfortable working something up from scratch.
PhilB was great. I had a question, how to measure the power to the final RF amplifier to make sure that my unit was compliant. He explained how to go about it with enough detail and explanation that I was comfortable with buying the kit. Especially after seeing the schematics and doing the build it was not difficult at all. That was the only real question I'd had before buying the kit, if it would be reasonably simple for me to check for the power at the final RF being in compliance using a multimeter gear and ohm's law. If he hadn't had a good answer to that question, I wouldn't have bought it.
Daniel
The idea of adding pre-emphasis to AM was the result of studies by the National Radio Systems Committee (NRSC) some years ago, which came to the conclusion that high frequency boost was necessary to help compensate for high frequency losses inherent in many AM radio designs. At the same time, they added low pass filtering at 10 kHz to attenuate adjacent channel interference (what a concept-- compare that to today's nasty IBOC sidebands)!! I believe the Inovonics 222 incorporates NRSC processing.
If you have a radio that is NRSC-compliant either by accident or by design, pre-emphasis restores the proper tonal balance and generally sounds good. If for some reason your radio has a flat response out to 10 kHz (this is rare), the pre-emphasis would sound a bit excessive. But you can compensate for it by reducing the treble if the radio has a tone control. My Scott Philharmonic is the only radio I own that has a flat frequency response to 10 kHz (and beyond) in the wide IF position.
Just like on FM, pre-emphasis allows high frequency noise to be attenuated at the receiver, which is a good thing. I use pre-emphasis on my SSTran.
WEAK-AM
Classical Music and More!
"Static [i.e. radio receiver noise], like the poor, will always be with us." John Carson
The great mathematician, John Carson, was wrong when he made this statement, because he was arguing against Armstrong's claim that receiver noise is suppressed in an FM system. However, the statement applies perfectly to Part 15 AM.
The good signal obtained inside the house, even with a short piece of wire as an antenna, is not representative of Part 15 AM performance. This signal is produced by the near field, which is strong near the antenna, but it diminishes rapidly as the distance from the antenna increases. The near field does not propagate. For transmitting to other houses, one must depend on the far field, which is very weak.
In a recent thread, it was pointed out that the radiated power of a good Part 15 AM installation may be around 22 uW. This is a system efficiency of only .022%. With such a low radiated power, the signal is likely to have noticeable noise even across the street. The listener must have some special motivation to stay tuned to a Part 15 AM signal. His reason for listening will not be because the station sounds good. Perhaps the station is providing some service to the neigborhood. Or, maybe the pastor from the listener's church is on the air. It would be interesting to learn how various operators keep their audiences.
So, who was John Carson?
He was not a late-night talk show host. Probably, his greatest achievement was introducing the Laplace transform to electrical engineering. The Laplace transform significantly simpifies electrical calculations by allowing differential equations to be solved using only algebra, and not calculus.
The Laplace transform was originally known as "Heaviside's operational calculus." The trouble with Heaviside was that he was not merely a genius, but a savant. A genius thinks a lot like an ordinary person, except he is faster and better. A savant can't explain how he always comes up with the correct answer. He just knows the answer, but doesn't know why. As a result, some prominent mathematicaticians thought that operational calculus had no mathematical basis. Carson showed that operational calculus can be explained by the use of an integral equation studied by the famous mathematician, Laplace. By attributing Heaviside's method to Laplace, Carson managed to get operation calculus generally accepted in electrical engineering.
Unfortunately, almost nobody knows about Carson's role in developing the Laplace transform. He is best known for being wrong while giving Armstrong a hard time. Carson used his mathematical expertise studying FM, and he concluded that it has practically no advantages over AM. It looks like he studied narrowband FM, and he was not able to understand Armstrong's wideband FM. Carson's legacy became defined by his famous mistake, and practically all of his positive achievements have been forgotten.
On January 17th, 2008 Ermi Roos wrote: The good signal obtained inside the house, even with a short piece of wire as an antenna, is not representative of Part 15 AM performance. This signal is produced by the near field, which is strong near the antenna, but it diminishes rapidly as the distance from the antenna increases. The near field does not propagate. For transmitting to other houses, one must depend on the far field, which is very weak.
Antenna engineering texts show that the near field of a transmit antenna extends to about 2*(L^2)/lambda, where L is the greatest dimension of that antenna, and lambda is the wavelength (same units of measure).
The mathematical result of this for legal Part 15 AM systems shows that distances from the transmit antenna to the AM receivers of neighbors, and even including most receivers in the same home where the Part 15 AM system is installed are beyond the near-field radius of that system.
In such circumstances, the received far field basically has an inverse distance relationship to/with the field strength existing at the near-field radius. That is, twice the distance yields half the field strength.
In a recent thread, it was pointed out that the radiated power of a good Part 15 AM installation may be around 22 uW. This is a system efficiency of only .022%. With such a low radiated power, the signal is likely to have noticeable noise even across the street.
The field strength generated over an unobstructed groundwave propagation path 90 meters in length (more than "across the street") using an ERP of 22 µW is about 0.5 mV/m. That is not a trivial signal, even though it may have some audible noise when the AM signal has low, and zero modulation. In some parts of the country, commercial AM broadcast stations have regular listeners using such signal strengths -- and in some cases those signal strengths even are protected by the FCC.
Unfortunately, almost nobody knows about Carson's role in developing the Laplace transform. He is best known for being wrong while giving Armstrong a hard time. Carson used his mathematical expertise studying FM, and he concluded that it has practically no advantages over AM. It looks like he studied narrowband FM, and he was not able to understand Armstrong's wideband FM. Carson's legacy became defined by his famous mistake, and practically all of his positive achievements have been forgotten.
But for most r-f voltages across the receiver input terminals, even narrowband FM has a noise advantage over DSB, full-carrier AM as long as the FM receiver r-f bandwidth is appropriate, and the AM limiters in the FM receiver are effective (which limiters remove most of the r-f noise).
RF
As far as I can tell, the whole idea here is to have fun and learn something! So if you just want to play tapes through your old time radios around the house, that's fine. And if you want to spin records for the neighbors, that's fine! And if you want to try to attract a bigger audience somehow, and manage to be successful at it, that's fine too! Each situation might lend itself to different types of program material, audio processing, etc. That's what makes Part 15 fun!
My station is experimental. At this point I don't really care if I have any listeners or not. I must say though, it sounds quite good on my neighbors' Wave Radio in their home three doors down. And it is full bandwidth, high quality AM, with very little processing.
WEAK-AM
Classical Music and More!
The post-detection PSD of FM is parabolic. That's why there's a lot more noise at high audio frequencies, relatively speaking. That's the "hiss" you hear on a weak station. It is for this reason that most FM transmission systems use pre-emphasis, and sophisticated ones add double-ended companding systems. Limiters don't remove this high frequency noise.
WEAK-AM
Classical Music and More!
Electrically short monopoles have a near field that extends to about .1 lambda, regardless of the height of the monopole. [2*(L^2)]/(lambda) does not apply to antennas much shorter than a wavelength. This can be seen from formulas for the electric and magnetic fields from differential current elements. These formulas are in antenna engineering books. I have seen Bode-like graphs of near and far fields for short antennas that show that the radius of the near field is about .1 lambda from the antenna. I could not find a reference that gave the exact factor. I calculated the near field radius as.0983634*(lambda) by finding the radius at which the real and imaginary parts of the vertical electric field in the differential current element formulas were equal. At 1620 kHz, the near field radius is (.0983634)*(185.2) = 18.2 m = 59.7 feet.
A typical urban atmospheric noise level in the AM broadcast band is .15 mV/m. This is a S/N ratio of just over 3 if the signal is at .5 mV/m. This is very noisy. .5 mV/m might be good enough in a rural setting if the receiver is of good quality.
An effective limiter does, indeed, improve noise level in a narrow band FM system. By elimiting amplitude noise, only phase jitter remains. However, the noise suppression of a wideband FM system is much greater than what would be obtained with a limiter alone. By increasing the FM deviation, and therefore the bandwidth of the FM signal, output noise is greatly suppressed. It is this aspect of FM that Armstrong claimed, but Carson did not understand.
Ermi Roos wrote "Electrically short monopoles have a near field that extends to about .1 lambda, regardless of the height of the monopole. [2*(L^2)]/(lambda) does not apply to antennas much shorter than a wavelength."
Your statement above prompted a trip to my copy of ANTENNA THEORY, ANALYSIS AND DESIGN by C. Balanis (2nd edition) to refresh my memory. On page 32 Balanis states this about the reactive, near-field region, "For a very short dipole, or equivalent radiator, the outer boundary is taken to exist at a distance of lambda/2*pi from the antenna surface."
That boundary distance is about 96.7 feet for a very short, linear radiator on 1620 kHz. Beyond that boundary the radiating far field voltage dominates, as the reactive near-field values decline by the inverse square and cube of their distance in wavelengths from the radiator.
So in the case of very short radiators your statement was more correct than mine. Thanks for pointing this out.
Rich
At first, I suspected that the boundary of the near field may be lambda/(3*pi) (not lambda/2*pi) because of the following reasons:
1. It is close to .1*lambda, which is the number I got from field graphs for short antennas. 1/3*pi = .106103295. 1/(2*pi) = .159154943.
2. Factors of the form 1/(n*pi), where n is the integer 1,2,3,4, or 6, occur frequently in antenna theory. Strictly speaking, this is not a logical reason for selecting a number from this set, but mathematics is more of an experimental science than one would expect. One starts with a conjecture, and then tests it with calculations. Then, if the conjecture appears like it may be correct, a mathematician attempts a formal proof.
When I used the factor lambda/(3*pi) for the radial distance from the short antenna in the formula for the vertical electric field of a differential current element (a differential current element is the limiting case of a short antenna), the real part of the field had a magnitude that was 8/3 of the magnitude of the imaginary part. I did not see any particular significance of this result for defining the boundary between the near field and the far field. Using the way the break points in a Bode plot are defined, I found the radial distance at which the real part of the electric field has the same magnitude as the imaginary part, which is (.0983634)*lambda. I reported this value in my previous post.
After reading about the Balanis reference, I checked the the vertical electric field if the distance from the antenna is lambda/(2*pi). The real part of the electic field is zero, and the imaginary part has a finite value.
The graphs I used are much closer to my value for the radius of the near field boundary than that of Balanis. I think that the reason for this is because the graphs are drawn in the manner of Bode plots, with straight lines drawn on a logarithmic scale. My method for determining the boundary calculates the Bode break point. The Balanis value is the radianlength, which is often used in antenna theory.
I think that both the Bode break point and the radianlength are equally valid values for the near field boundary for short antennas. The near field and the far field do not have a sharp boundary. The transition from the near field to the far field is gradual. The location of the boundary is rather arbitrary.
Probably you and I are the only ones left reading this thread, Ermi, but I thought you'd be interested in what John Kraus says on this topic.
The link below leads to a scan from Kraus' Antennas, 3rd edition. His point that two of the fields cancel at the radian distance was interesting.
Agreed that the region boundary can be considered arbitrary, although Kraus and Balanis both use the same 1/2*pi value for it.
Rgds,
Rich
I'm at least still reading it, Rich. I'll be the first to admit that some of it is going a bit over my head, but I think I get at least some of the basic ideas implied. Could you clarify what Kraus means by the energy being mostly stored at distances less than 1/(2 pi) and being mostly radiated at distances larger than that? I don't think I'm getting the sense of how he's referring to it as being stored.
Daniel
Daniel asks: Could you clarify what Kraus means by the energy being mostly stored at distances less than 1/(2 pi) and being mostly radiated at distances larger than that? I don't think I'm getting the sense of how he's referring to it as being stored.
The mathematical development showing this is too much to post here, but Kraus writes, "Many antennas behave like the dipole, with large energy storage close to the antenna. The region near the dipole is one of stored energy (reactive power) while regions remote from the dipole are ones of radiation. The situation is like that inside a resonator with high-density pulsating energy accompanied by leakage which is radiated."
Kraus defines the radius of this imaginary, spherical resonator as 1/2*pi wavelengths from the radiator, as at that distance the magnitudes of the reactive and radiating fields have equal values.
Balanis wrote, "For an antenna, the radian sphere represents the volume occupied mainly by the stored energy of the antenna's electric and magnetic fields."
Rich