Wind a Toroid!
Posted on February 1, 2008
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.