[Amps] T networks and harmonics

[Bill L. Fuqua]

     If the impedances are quite different there will be attenuation. You have the same input and output impedances.  I was referring to a T network as an output impedance network which has to match very different impedances. 
     I thought we were talking about pi-l, Pi networks as   output coupling networks.  
     In fact any time you try to design a matching network  with out transformers and use a pi or t or what ever you  can't avoid resonance and thus attenuation of both the high  and low frequencies.       
73
Bill wa4lav



-----Original Message-----
From: "Gary Schafer" <garyschafer at comcast.net>
To: <amps at contesting.com>
Date: Sun, 5 Feb 2006 22:48:48 -0500
Subject: [Amps] T networks and harmonics

There was some discussion a few days ago about pi/L networks and harmonics.
Someone suggested that a T network tuner would also help reduce harmonic
energy out of an amp as "it did not only act as a high pass filter, but
being a resonant circuit it would provide some harmonic attenuation".

I had always thought of a T network with series capacitors looking like a
high pass filter. I drug out the spectrum analyzer and a dentron KW T
network antenna tuner. Hooked it up to the dummy load and the transmitter.
Tuned things up and checked 2nd and 3rd harmonic levels on the spectrum
analyzer. Then disconnected the tuner and again checked the harmonic levels.


They were exactly the same with and without the tuner in line!

I could see other spurious trash from the transmitter that was below the
transmit frequency being attenuated some but nothing above the transmit
frequency was attenuated.

I then hooked up a comb generator with 1 MHZ harmonics to the input of the
tuner, still tuned as before, and observed the various harmonics of the comb
generator. 

No attenuation above the frequency that the tuner was tuned to was observed!
It passed harmonics all the way up to around the 2 meter frequency range
just as well as it passed 14 MHZ where the tuner was tuned. You could see
the comb generator was attenuated below 14 MHZ, just as you would expect
from a high pass filter!

Removed the tuner and observed the comb generator directly to verify levels
out of the comb generator and they were the same as with the tuner at 14 MHZ
and above.

So from what I saw, a T network antenna tuner does nothing at all to
attenuate harmonics.

Perhaps if it was feeding a reactive load rather than a fairly flat load it
would make some difference?

73
Gary K4FMX


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