|[AMRadio] antenna tuners transmision lines and more|
k4kyv at hotmail.com
Mon Apr 24 00:16:49 EDT 2006
>From: "Brian Carling" <bcarling at cfl.rr.com>
>The power may not be wasted very much in the tuner, BUT
>REFLECTED power goes back into the RF final and is disippated
>in the famil amplifier device(s) - at least many people have
>written articles for decades describing that marticular "myth"
>or so-called "FALSE STATEMENT." I am not so sure it is false
That is a myth. The power that is not radiated by the antenna or burnt up
as resistive loss in the wire, goes back to tank circuit of the transmitter,
contributes to the circulating rf current in the tank circuit, and is
re-reflected back to the antenna. It may take several oscillations back and
forth before all the energy is dissipated, but it is eventually dissipated
in the antenna as radiated power and in the wire as resistive loss, not in
the final amplifier tubes. The standing waves can be thought of as
circulating current on the feedline.
If the open wire line is left open with nothing connected, or if it is
shorted, no rf escapes the feedline to excite an antenna. It is nearly all
reflected back to the transmitter, and then back to the opposite end, until
it is all dissipated as heat due to resistive losses. The current on the
feeders, as measured with an rf ammeter, might be very high, but there is no
radiation resistance or radiation. The rf ammeter may read seveal amperes
while the final amp is dipped at resonance to near zero place current.
There is very high circulating current in the tank circuit and the feeder,
and at some points the voltage is very high - basically a Tesla coil, but
The myth I have often heard is that the rf is delivered back to the final
and is dissipated in the plates of the output tubes. That is not true. If
the plates of the tubes glow, it is due to plate dissipation (DC input to
the final minus the power delivered to the tank circuit). This dissipation
is due to operating conditions of the tube, not rf power being reflected
back into the tube.
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