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Ramsey AM25
Fellows, I have a SSTRAN AMT3000 and have built the prescribed antenna and have had some moderate success with it, and considering that my location is very poor and there are still some installation improvements to be made, consider it an excellent system. I also have a Ramsey AM25 from an earlier time. An expert in the Part15 field has given me a full explanation as to why the AM25 won’t tune well to a 3 meter base loaded antenna, some of that being that the AM25 has a fixed 50 ohm output that is untuned and uses a 7 element low pass harmonic filter. I’ve experimented with it quite a bit over the summer using various combinations of loading coil taps, variable capacitors, etc. and can’t get the thing to a good resonance peak. Also eliminated the low pass filter thinking that the Q of the antenna might antenuate hormonics only to wipe out my neighbor’s TV. Have any of you had good success with the AM25 loading it to a base loaded 3 meter antenna? JimB
Total posts : 45366Hello JimB,
I am a AM25 user. In my side, i’ve tried to connect my 3-meter antenna with my loading coil made myself ([i:8e2328010b]the inductance is [b:8e2328010b]345[/b:8e2328010b] uH for the frequency of [b:8e2328010b]1470[/b:8e2328010b] kHZ[/i:8e2328010b]). The signal from AM25 propagates no more than 150 meters. 🙁 I still don’t know why i have such problem with my unit.
However, if i put my horizontal antenna of a quarter wavelength, it works very great ([i:8e2328010b]but it is largely illegal — Antenna length must not exceed than 3 meters[/i:8e2328010b])
My friend who is a CB hobbyist and professional electrician is supposed to contact me within a few days to look at the problem of my AM25.
I will let you know as soon as i (or my friend) resolve my problem.
Regards.
Yves Roy
Total posts : 45366Hi,
Here is what I did. This was not engineered and is not based on science, but it worked OK for me.
I am not interested in outdoor long range antennas with range that gives me millions of listeners, so I built this indoors in my basement and it covers my property just fine (one half acre).
I wound a coil using #12 THHN insulated wire on a 1.5 inch PVC pipe using 45 turns. One end goes to a copper water pipe ground, the other to the 10 foot aerial. In parallel, I connected a 30 to 400 pF variable cap. Then I wound a coil on the cold end of this coil consisting of 7 Turns of the same wire. Each end of this is connected to the AM 25 hot and gnd. Tuned the cap for maximum fieldstrength and it works OK. I get a pretty sharp resonance with it.
This is in no way an optimized antenna, but maybe it will give you something to build on.
Neil
Total posts : 45366Yves. The expert that explained the AM25 loading problem to me said that account it has a fixed 50 ohm untuned output that it would work well with a quarter wave antenna ie your long wire, yet wouldn’t tune to the lower impedence of a 3 meter base coil loaded antenna, so it looks like you and I are looking for the same solution. One of our knowledgeable people recommended that I connect the antenna before the 7 element harmonic filter at the output, that being the 3 donut shaped coils and capacitors connected to ground between them. I had already tried this and wiped out the next door neighbor’s TV reception. He also suggested a buffer of some kind, yet I haven’t gotten there yet. Another knowledgeable person said that the high Q of a base loaded 3 meter antenna would tend to suppress harmonics yet that didn’t work with the AM25 with its harmonic filter disconnected.
Neil, Thanks for the suggestion. That second winding and grounding one end of the coil may be something to work with. I’ve tried the variable cap from top to bottom of the SSTRAN tapped coil while connecting the xmtr ant to various taps without getting any good resonance point. JimB
Total posts : 45366Hi Jim,
I suspect that you did not get resonance with the SSTRAN coil because the typical 365 pF air variable cap has a minimum capacitance around 30 pF and this probably sets the resonant frequency too low. The coil I described has much less inductance that the SSTRAN coil and therefore resonates at a higher frequency with more capacitance. I definitely get a sharp peak with this. I operate at 1500 KHz.
Again, this was just guessineered and thrown together, but the idea was to get the LC resonant. A parallel LC at resonance is a high inpedance, so the 7 turn coupling acts as the primary of a transformer and transforms the high secondary impedance down so it matches the transmitter. The seven turns was my first try, and it worked well enough for covering my property that I quit experimenting. Maybe more or fewer turns would be optimum.
I can hear my signal from my basement antenna about .2 miles away with a car radio.
Neil
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