From: "Gary Schafer" <[EMAIL PROTECTED]>
Let's say you changed the output impedance of your transmitter from 50 ohms
to say 200 ohms. (changing nothing on the tuner) Would the "antenna system"
(antenna, feed line and tuner) still be "resonant" as you had them tuned
when you had the transmitter set for 50 ohms output?
The antenna system might still be in resonance, depending on what the
reflected power on the open wire line "sees" when reflected back to the
transmitter output. The transmitter puts a load, with a specific impedance,
on the transmit end of the feedline just as the antenna puts a load on the
antenna end. It's possible that the impedance that the transmitter presents
to the feedline could have a combination of reactance and resistance that
would alter the resonant frequency of the system. Another possibility is
that it would stay in resonance, with no reactive component, but due to the
load mismatch, there would be a reduction in coupling so that the
transmitter would not load up to full power.
A comparable example would be, if the above system used a balanced tuner
with swinging link: You could tune the tuner to resonance, and then vary the
link to achieve just the right amount of coupling to load the final to the
desired load. Moving the link in or out would vary the load on the final,
and thus final amp plate courrent, but if everything is tuned up to cancel
out the reactance, after you vary the coupling with the link, you would
re-check the dip at the final amplifier, and it would still be dipped at
resonance, ever though the final might be loaded more lightly or heavily.
I can do the same thing with my link coupled tranmitters. Adjust the tuner
to resonance, which usually gives maximum coupling to the final. Dip the
final. Now re-tune the antenna tuner. Adjusting the antenna tunerl should
give a peak plate current, while adjusting the final amp plate tank cap
gives a dip to the plate current. If everything is tuned to resonance, tune
the ant tuner to peak plate current. Now re-check dip. It should still be
dipped to resonance. Now adjust the link coupling. The plate current will
increase or decrease, depending on whether the link is moved in or out of
the coil. But once the coupling is changed, the PA plate current should
still be very close to the minimum point (dip).
If it is not at resonance, changing the loading with the link will require
retuning the PA plate tank cap to resonance to maintain the PA plate current
dip. If that is the case, no problem. Re-dipping the final brings the
system back into resonance. With my link coupled transmitters, I usually
have to touch up the dip after I change coupling with the link.
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
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