[AMRadio] antenna tuners


Gary Schafer garyschafer at comcast.net
Sun Apr 23 12:48:55 EDT 2006



> 
> The point is, adjusting any one or combination of the following: the PA
> tank
> settings, the antenna tuner settings, the antenna turner coupling coil (if
> link coupling is used), the length of the open wire feeder, the length of
> the antenna, will affect the resonant frequency and thus the reactance vs
> resistance of the network that couples the amplifying  device at the final
> amplifier to the aether.
> 
> Don k4kyv
> 

The question was meant to invoke some thought about what parts of the system
come into play when we say things are "resonant".

As we see from Don's explanation we can go right down to the final tank
circuit when talking about making the antenna system resonant.

The following will be true when the tuner is adjusted to provide a non
reactive 50 ohm output to the transmitter:

Assuming a 50 ohm link, if the link in the final is resonant (reactance is
zero) then once the plate is dipped, moving the link to change coupling
should not change plate resonance. If the link is not resonant then it will
affect the plate circuit resonance when its coupling is changed.

If the antenna tuner does not match the link on the transmitter the link
will have a reactive component that will effect transmitter plate tuning.
Now the transmitter plate tuning will be part of the whole "antenna system
resonance".

This is why I don't like to refer to the "antenna system" being resonant.
Too many things can be involved and we loose sight of what we really mean.

The same is true if you feed a coax fed dipole directly from the pi network
output of your transmitter. Or use a tuner that is not tuned for a flat
match to the transmitter. The plate and load tuning becomes part of the
"antenna system tuning" if you are to use that terminology. 

If we think of feed lines as transformers (whether they are open wire or
coax) and antenna tuners as variable transformers it makes things easier to
see. 

When using an antenna tuner and we tune the reactance out of the circuit so
we only see a resistive component we can say it is resonant. But what is
really resonant? The antenna is not, the feed line is not, the tuner is not.


The only thing resonance means in this case is that the capacitive and
inductive reactances at the tuner are equal. It confuses many people.

73
Gary K4FMX










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