Test Results with an AMT-5000: Stronger Signal with External Loading Coil

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Test Results with an AMT-5000: Stronger Signal with External Loading Coil

[The following is excerpted from an email I sent to Phil Bolyn (founder of SSTran) last month after finally being able to conduct careful tests with the SSTran AMT-5000 comparing the internal loading coil/wire antenna combination with the external loading coil/pipe antenna combination. If you own an AMT-5000 and you want the best range and audio power, build the external loading coil/pipe antenna combination described on the SSTran website!]

Well, it has taken me three years to get around to this test. But today I had an opportunity that was an efficient use of my time. I had gotten my hands on one of Sean Cuthbert's joke of a Cquam Part 15 transmitter. I could go on and on about his unit, but I'll cut to the chase to say that it has an internal coil that you can't defeat. Knowing the trouble I have had in the past trying to peak the output of transmitters with internal coils using the pipe antenna, I opted for outdoor testing with a temporary 108" 22 guage wire antenna attached to 1/2" schedule 40 PVC pipe mounted in place of my external coil/pipe antenna setup. I used my signal at 1710 kHz to perform the test.

After having a good laugh over the results with Sean's excuse of a transmitter (... it failed to stay in stereo on my TX-11a. Even the ASMAX transmitter could do that when mounted outside...), I decided to test the AMT-5000 with the internal coil and vertical wire antenna since I already had the temporary wire antenna in place. The difference between the AMT-5000 using the internal coil and vertical wire antenna versus the external coil and pipe antenna is simply amazing.

A couple years ago, I build a poor man's home brewed field strength meter using a germanium diode and a few other components in a project box that connects to my volt meter. Before disassembling the 1710 kHz pipe antenna, I measured the RF output in volts on the FSM. I moved it all over the lower antenna and coil area coming within 2 inches of the pipe. With the way my FSM was adjusted, I was able to get highest readings above 7 volts (7.1 at least). I went over to my 1640 kHz pipe antenna, did the same thing and got similar results.

After I finished with the Cuthbert transmitter, I hooked up the 1710 kHz AMT-5000 to the temporary wire antenna and followed all of your tuning instructions. When I was done and got everything peaked and adjusted for the high efficiency, I took more readings. This time, I couldn't get any reading above 3.5 volts! Yes, the performance was acceptable (and considerably better than the Cuthbert transmitter), but it was no match for the external coil/pipe antenna. I took a ride around the neighborhood just to confirm, comparing the wire antenna setup at 1710 with the pipe antenna setup at 1640. The pipe antenna/external coil not only had noticeably more range. It also had louder, more robust audio without distortion. (The audio and modulation on both transmitters were adjusted to the same perimeters before leaving the house.)

So, there was no contest. You can get almost twice the RF output with the external coil and pipe antenna than you can with a vertical wire antenna and internal coil. I know I'm just one tester and according to the scientific method, other testers have to have the same results under the same conditions. But the differences I saw were so remarkable, you couldn't miss them. I think it is safe to say that you should recommend the construction of the external coil (in-line is best)/pipe antenna combination for the best range and audio with the AMT-5000 just as you do for the AMT-3000. (The performance of the AMT-5000 with the wire antenna is somewhat similar to the performance of the AMT-3000 on the external coil/pipe antenna.)

Sure, constructing that coil and antenna is considerable extra work and expense. It's a little bit more difficult to adjust. A number of connections stay exposed to the weather and you may have to check on things every few months. But the improved range and more robust audio is worth it.


First Test of Its Kind

In the several years since the AMT5000 has been around fjockey is the first person on any forum to report trying external and internal loading coils in carefully observed tests.

Therefore this post means a lot to those of us who have always wondered "what if".

I built an external coil for my AMT3000 which really kicked it to life, but have never tried the external coil on the AMT5000, which as is (with internal coil) out-does the AMT3000 with its external coil.

Therefore we can expect that an AMT5000 with a well-built external coil will be in the top class of part 15 transmitters.

What we will wait for now is a well done comparison between the AMT5000/External Coil and the Procaster, which member Jim Henry has found to be exceptional for range.

Carl Blare

Loading Coils

The test results provided tend to reinforce my suspicion that internal toroid loading coils are not as efficient as air core loading coils at the base of an antenna.

For instance, my 3 inch diameter air core coil has a measured RF resistance at 1680 kHz of 18 ohms which represents about half the loss of my antenna system. 

I have never seen data on toroid loading coils for this application and it has always seemed unfair to use a "calculated efficiency" for a final amplifier as has been done when this neglects the loss in the toriod.  Providing the toroid resistance would help a user assess the overall system performance since a change in this value from say 18 to 30 ohms would have a big impact on the power delivered to the radiator.



Sean told me the following:

Sean told me the following:


Hi Rick.The c-quam requires more bandwidth so the 100mw is more spread out resulting in a slightly lower main carrier.


 So really you're comparing apples and oranges, because of the lower main carrier and that's due to the qualities of cquam.


calling his transmitter a joke was pretty rude.

Question for Neil

What is inductance of your 3 inch coil?

Druid Hills Radio AM-1710- Dade City, FL. Unlicensed operation authorized by the Part 15 Department of the FCC and our Resident Hobby Agent.  

Answer to Post #5


The "3 inch" coil I use for my outdoor antenna is comprised of 68 turns of number 18 enameled wire with a 5 turn trim coil used for fine tuning.  The inductance is nominally 254 uH.  The measured resistance at 1680 kHz is 18 ohms.

The coil was designed by trial and error to achieve resonance with my antenna system.



To Neil

Thank you. Would you benefit from a larger coil with say #10 wire?

Druid Hills Radio AM-1710- Dade City, FL. Unlicensed operation authorized by the Part 15 Department of the FCC and our Resident Hobby Agent.  

Larger Coil or Wire

"Would you benefit from a larger coil with say #10 wire?"

I don't know.  The larger diameter wire would increase the cross section of the skin effect conductive area lowering the resistance but the larger wire would increase the length of the coil requiring more turns and hence a longer wire in the coil which would work to increase the resistance.  The interwinding capacitace would also increase.

It would be interesting to build one and make some measurements but at present I am not so inclined.



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