I know most tuner designs of this style use series capacitance with a
shunt inductance. But that doesn't mean it could not be reversed, ie.
series inductance with a shunt capacitance.
Shawn
N7LQ
On Sun, 5 Feb 2006 22:48:48 -0500 "Gary Schafer" <garyschafer@comcast.net>
exclaimed:
> 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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