[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.