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.
Toroid Test
WEAK,
Thank you very much for your detailed report on your experiment. I think the improvement in range you reported is due to Schubert’s music and not your coil. Hayden and Beethoven would probably provide equivalent range.
Anyway, it is great that you are trying to improve things. You wrote:
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)
Be careful with this. I made the mistake once while experimenting with base loaded antennas where I noticed the diminished high frequency response which I though was due to antenna Q. A quick calculation alerted me to the fact that the Q required to cause this was horribly higher than any Q I could get from my coil and antenna. What I found was that the load to the SSTRAN from my antenna system was so far off the target of about 800 ohms transformed to the output stage collector that the RF waveform was trash. If you haven’t done so yet, check the RF waveform on a scope. Things are not always as they seem but your results seem encouraging.
It is hard to argue with success but it is surprising to me that given the estimated ground losses for a typical part 15 AM system that reducing the losses in the tuning inductor would have much benefit. But, I am only going on best guesstimates and you have some real data…I could be really wrong about this.
Neil
Impedance control
Hi Neil,
Thanks for your comments! In the lab, I made a number of tests using various loads on the output. I found that the final can become unstable under certain conditions, as you mentioned. If you install the 820 ohm resistor included in the kit in the output stage collector circuit, it is 100% stable all of the time according to my experiments, but this results in wasting almost half of the available power. I experimented with various resistor values and found that you can actually go up to around 5 kilohms and maintain stability under reasonable loading conditions. That is the value I have in the circuit now. It might not provide enough of a load in all cases.
I observe the transmitter output using a Tektronix 475 analog oscilloscope and a 10x passive probe. It is sensitive enough to sample the field without a direct connection. If the probe is attached directly to the antenna, it adds an additional 11 pF, which can be compensated for by retuning C5. The 475 has a bandwidth of 200 MHz. I also have a FET probe that I can use. This further reduces the loading to a few pF when used with a 10:1 attenuator.
Currently, C5 is tuned such that it is responsible for about half of the antenna loading (35 pF). Thus, half of the output current returns through it to ground, rather than through the antenna. This results in low efficiency, but it does mean that the output circuit Q is probably higher than it would be if the antenna provided the only load. You can disconnect the antenna completely and tune C5 to resonance, and then you will see a very high Q since there is little loss in the circuit (only that due to the coil and C5).
I believe this higher loaded Q is the primary reason for the slight loss of higher frequencies. It’s not excessive, but it does seem to be noticeable. Again, I think this problem will go away once the antenna represents the full load on the output, with the attendant ground losses included.
WEAK-AM
Classical Music and More!
Toroid
WEAK,
You are on the right track here. I solved my RF distortion problem by reinstalling the 820 ohm resistor. Your are correct that it robs power from the antenna but for what I do (yardcasting) it is a simple solution. Your checks with higher resistances is interestng and I may try this.
BTW, I used a Tektronix 465 years back and it was a great scope. The trace just wouldn’t focus like the old 546 it replaced but it was about on third the length. Shorter beam, faster sweep but harder to focus. I monitor things here as you do. The only add on is I place an inductor between the tip and ground to kill the 60 Hz hum and use about 2 feet of wire on the tip to get more signal. Usually I get tens of millivolts with this about 6 feet from the tx. antenna.
Neil
TOROIDS,Yikes!
I have a huge junkbox which includes thousands of toroid cores. I would love to put them to use,but most dont have any identifying numbers on them. Any way to tell or test these? Will post some pics shortly.
Regards,Lee
http://www.freewebs.com/wilcomlabs/index.htm
Measure or Buy New
Hi Lee,
If you have the proper equipment to measure inductance accurately, you could wind up some of the toroids with test coils and measure the inductance. This will tell you the permeability. You can physically measure the cores to learn what size they are. Then you can compare them to published data from Amidon and others.
But if you don’t have the proper measuring equipment, unless you just want to tinker, it’s probably better to order some new ones so you know what you’ve got. Otherwise, it could be a lesson in frustration, since you probably don’t even know if they’re powdered iron or ferrite. The powdered iron cores are typically painted, so you might get a handle on it that way. But if they’re not painted, it’s pretty hard to know what you have unless you test them.
Incidentally, I’m thinking of trying Litz wire on my next toroid coils. Litz wire is expensive, but can yield coils with very high Q. Since we are talking about miniscule amounts of power here, there should be no problem handling the current.
WEAK-AM
Classical Music and More!