Good points, Bruce, and after the schooling I got from RFB (a member who once posted here) I actually have some knowledge on the subject. All the subjects.
The standard impedance for professional broadcast transmitters is 50-ohms. It's the industry standard.
LPB transmitters are made to the industry standard so the input to the coupler is a standard impedance of 50-ohms. By the way, LPB transmitters have also been used for licensed stations with low power nighttime authority.
What does the coupler do? It matches the transmitter to the impedance of the power line.
What is the impedance of the power line? That's where things go into chaos, because everybodies power panel will have a different impedance based on all the appliances in the house that are plugged into the line as well as all the other houses being fed by the same power transformer on the pole.
That's why the output selector of the LPB TCU 30 Coupler offers 21 different impedances! No joke. They range from 1.1 ohms up to 72 ohms.
Yes, there is a 50-ohm output choice, which allows the very same coupler to drive a TIS (Traffic Information System) antenna! True story.
I'd have to go down in the basement to see what impedance my power line best matched with, but I'm thinking it was 25 or 32 ohms.
The AMT3000 and 5000 are not built to the industry standard of 50 ohms, and PhilB explains why in his manuals. The 50 ohm standard requires a fully matched antenna, which would be at least 1/4 wave, but only 3-meters antenna length is allowed with Part 15, A FACT WHICH CHANGES THE TECHNICAL RULES! By providing the SPECIAL IMPEDANCES at the output of AMT3000 and AMT5000 the best possible match to a short antenna is made possible! It's a thing called good engineering.
To adapt the AMT3000 or AMT5000 transmitters for use with carrier current it would be necessary to add a linear amplifier that provided a 50 ohm output.
That's what I want to try with my
SS-Tran to feed a CC coupler. I have already fed other
tuning coils, variometers, etc., that
way - and I have bypassed the last
tuning caps/coils completely inside my
AMT-3000. The old Panaxis AM-100
final stage was just a transistor into
a blocking cap. Whatever was needed
after that you had to make. I have seen
other Part 15 transmitters that have been
that way - but I don't remember which ones.
(The above objective was to make a 3 meter
stick work with a different kind of matching
arrangement. it is still very different from trying
to match to a 50 ohm CC coupling circuit.)
But still - I think it is easier to make the
output of the AMT-3000 50 ohms, by bypassing
the output network - making another one (which
for me would feed an attenuation network for the
sake of experimentation) which would then go to
the CC coupler with all the necessary precautions -
the isolation cap - and then onto the AC line.
However, this is just an idea. Don't try it just
because I mentioned it as a possibliity. I can't
be responsible for any problems that might occur.
Besides - my wife wouldn't be happy either.
(The above might not be explained very well,
but I have to go and can't proofread it.
My ride home is here.
Best wishes to everyone.
Bruce, Monitoring station, CT
"The 50 ohm standard requires a fully matched antenna, which would be at least 1/4 wave,..."
Quarter wave antennas are in the vicinity of 35 ohms.
Add some ground losses and it's pretty close.
The standard impedance for professional broadcast transmitters is 50-ohms. It's the industry standard.
AM broadcast transmitters typically are designed for a load impedance of 50 ohms.
However the feedpoint impedance of a monopole antenna system typically used by AM broadcast stations varies with its height in electrical wavelength, and with the r-f resistance of the antenna system connection to the earth (just as with Part 15 AM).
Even when driven against a very loss-loss r-f ground (2 ohms or less), the "real" term of the feedpoint impedance* of a single, base-fed broadcast station monopole can vary from maybe 25 ohms to 700 ohms or more, due to the variation in the radiation resistance of the monopole as a function of its wavelength.
Radiation resistance does not constitute an undesirable loss -- it is the equivalent resistance presented by the monopole to the antenna system feedpoint as a result of the r-f energy radiated into space by the radiating conductors of that antenna.
The feedpoint impedance of an antenna system is what it is. If it varies from the impedance that the transmitter is designed to drive (as they almost all do), then a matching network is used at the tower base to transform that feedpoint impedance to the impedance the transmitter is designed to drive.
Maximum energy is transferred to the antenna when system impedances are matched. If the load impedance is not matched to what is expected by the transmitter, then part of the power available at the antenna feedpoint is reflected back to the source (transmitter), where most of it is converted to heat.
*the only resistance that dissipates power
A Carrier Current Coupler is, from Rich's notes, a variable and adjustable impedance matching network.
Whereas a licensed station will build a fixed "permanent" matching network.
The LPB TCU-30 is rated for a maximum of 30 Watts, and could be used for stations with nighttime authority 30 Watts or less.
There is a 60 Watt LPB Coupler in their literature, but we've never seen one on eBay.
The Radio Systems CP-15 Coupler is rated to a maximum of 20 Watts.
The matching networks for high power are made out of much bigger parts.