"Twistedhed" wrote in message
...
An otherwise clean transmitter coupled to an antenna with the above
characteristics can cross the line between no RFI and significant RFI.
In some cases, this can be mitigated somewhat by moving the antenna
(usually raising it) to another area, where it's radiation will not
couple as much R.F. into neighboring premises. A better solution would
be to run an antenna with ground plane radials, such as a Sigma 5/8 wave
or similar.
Dave
"Sandbagger"
http://home.ptd.net/~n3cvj
_
And the running of certain amplifiers will NOT compound or add
additional RFI in such cases. A perfect linear for cbers is a palomar,
as it is not class C, which certain long-time hammie posters in
rec.radio.cb consider to be some kind of devil-spawned conspiratorial
gadget to reek havoc and cause misery in their radio lives.

Twist the amp doesn't have to be class "C" to cause problems. Even a class
"B" linear amp is not completely clean. In particular solid state amps are a
problem because many are push-pull designs with no filtering on the output.
A push-pull design has low levels of even harmonic output since the even
order signals cancel out in the output transformer. And that is only for
"well matched" power transistors. The problem with the push-pull design is
the odd order harmonics are not attenuated at all! If you care to look at
Ham rigs that use wide band solid state push-pull amplifier output for HF
operation you will see band pass filters on the output. In fact these
filters are switched in and out depending on the band in use. That's how the
manufactures keep the output clean enough to meet FCC specifications.

Tube amplifiers are another story. These amplifiers can be relativity clean
IF the output is designed correctly. The critical section is the output
matching circuit. This circuit performs two important functions; load
matching and harmonic filtering. Some of the posters on this board in the
past don't seem to grasp these two ideas well. They seem to think that if
the amp loads up OK then all is well - not true. If you go through the math
you will discover that there is no unique solution for the value of the
matching section component values. So how do the designers pick the values
they do? They pick an operation "Q" for the matching section, that forces a
particular value for the components. The significance of the "Q" value is it
is a measure of how frequency selective the matching section becomes. The
higher the "Q" the sharper the response becomes, thus the less harmonic
energy is coupled to the output. Pick a "Q" too low and you have significant
harmonic output from the amplifier. Pick a value too high makes the
amplifier a pain to use, in other words the amplifier has to be retuned for
small shifts in operating frequency. From either experience, or through
detailed and complex calculations, acceptable values for "Q" have been
determined that would yield a tube amplifier matching circuit that is likely
to have acceptably low levels of harmonic output content.

If somebody says their amp is "clean" well the only sure way to know is to
test it with a spectrum analyzer. If you look at reviews for Ham rigs, and
power amplifiers in particular, you will see spectrum analyzer screen shots
so the buyers can see for themselves just how clean the output happens to
be. And I haven't seen a spectrum analyzer screen shot for a 11m solid state
amplifier yet. If you know of any it would be interesting to see them.


--
Leland C. Scott
KC8LDO

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