wrote in message
ps.com...

i guess what i meant was more along these lines: you need to make sure
the peak-to-peak input voltage and current capabilities of the input is
matched with the amp, right? or does the amp have a lot of leeway in
those regards?

Actually, no. Typically the amp doesn't have a lot of flexibility there,
but in general the exciter does.

In amateur practice, the impedance of almost anything that connects together
at RF is 50 ohms or converted to 50 ohms. Given that, the power level then
converts directly to voltage and current.

Typically, HF exciters have a lot of range in output power. The FCC
prohibits the sale of HF power amplifiers having a required drive of less
than 80 watts. Since most HF exciters have 100 watts out, the required
input power of HF amps is rarely specified, since it will almost always
match almost all exciters. I don't think other countries have the same
rules, but the FCC also requires that the amps cannot easily be modified to
accept lower drive, so it is expensive for manufacturers to make amps
differently for different markets, since that typically would require a
complete redesign.

On VHF, it's a little different. Most VHF exciters only have a few output
power settings. VHF amps generally specify the input power, and typically,
it will be a fairly wide range. But still, if you want, say, 200 watts at 2
meters, you would buy a different amp for a 5 watt HT than you would for a
50 watt mobile.

just to make sure i have it straight, if i were to transmit a sine wave
at 146mhz, anyone listening in on 146 mhz wouldn't hear anything
(except maybe less noise than usual). if i were to vary the frequency
between 146.0001 and 145.9999 at a rate of 100hz, then anyone recieving
would hear a quiet 100hz tone. now if i were to vary the frequency
between 146.001 and 145.999 (holding all previous listeners constant),
then anyone listening would hear a much louder tone...correct? or am i
not understanding it yet? ;-)

That is exactly right.

There is a weird behavior of FM receivers called the "capture effect". A
signal of sufficient amplitude at the input of an FM receiver cose to where
the receiver is tuned will totally quiet the receiver. In an AM, CW or SSB
signal, the amplitude is converted to audio almost directly. As a result,
noise on the input appears as noise on the output.

FM is different, though. In FM, we want to hear the frequency modulation.
When there is no signal, we hear the detector randomly wandering around
trying to interpret the noise as signal, but the amplitude of the noise
really has no effect. Once a signal is detected that is strong enough for
the detector to follow, the detector follows the signal and there is no more
noise. To exaggerate this effect, most FM receivers amplify the signal so
much that the amplifiers become saturated and amplitude variation in the
input signal is clipped (well, softly clipped), so that the later stages of
the receiver see the same amplitude. This is different than AM/SSB/CW where
it is the amplitude changes you are looking for.

This is one of the reasons that FM signals have so much higher quality ...
any noise is actually a result of noise in the receiver (or transmitter)
rather than the atmosphere. With other modes, not only can you hear the
atmospheric noise, but at lower frequencies, the atmospheric noise is many
times higher than the noise inherent in the receiver.

The other reason is bandwidth. In amateur practice, FM signals are 5 kHz
wide, compared to about 2.5 for SSB. In commercial practice, FM signals are
wider still.

There is another "gotcha" in your description above. If you vary the
carrier at, say, 1000 Hz, from say, 146.999 to 147.001, the actual bandwidth
will be somewhat wider than you expect, and it will be dependent on the
frequency of the modulation. I know this doesn't make sense, it has to do
with some weird math. If you studied Fourier series back in school it was
some abstract mathematical thing that had nothing to do with the real world.
Well, guess what. Fourier has everything to do with radio! The result is
that to stay within the 5 kHz bandwidth, the highest modulating frequency
has to be somewhat lower than 5 kHz. This is one reason why the FCC
prohibits amateurs from broadcasting music; reasonable fidelity of music
requires higher bandwidth than voice.

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