[AMRadio] antenna tuners


Donald Chester k4kyv at hotmail.com
Sat Apr 22 22:46:03 EDT 2006


>From: "Gary Schafer" <garyschafer at comcast.net>
>
>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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