>
>Has anyone tried lining the input or output cavities of PAs with the
>"lossy" material used in anechioc chambers to suppress RF reflections?
>The effect is to reduce the "Q" of the cavity, so that instead of
>reducing the series "Q", as applied in nichrome wire transmission leads,
>etc.(and which might reduce power slightly by introducing series loss in
>the tuned circuit), the overall cavity "Q" is reduced, thus greatly
>damping any oscillatory tendencies. If this works, the reduction in "Q"
>is free/lossless.
>
>This technique is highly effective is stabilising pre-amps with a
>tendency to oscillate.
Hi Ian,
Good idea. The problem with it though is that most absorber type
material is designed for high UHF or microwave use. I suppose that
absorber at HF could be manufactured but it might be kind of thick.
The second thing is that most of the parasitic and instability problems
are not caused by resonances in the PA cavities themselves. On occasion
they are but those are immediately noticeable. While building my amp, I
had a cavity resonance for a bit. I ended up with a design that
eliminated it but until that point, I would get oscillations every time I
put the cover on the amp. I was actually going to experiment with
various microwave absorbers to see if they did anything (at an RF trade
show I attended in February this year I scammed numerous absorber sample
kits from the various absorber manufacturers). Fortunately, I solved the
problem without the need for the material!
Your point is well thought out though and in VHF/UHF and microwave amps,
absorber can make all the difference in the world. In my commercial
amplifier building days at 6 GHz and also at 800 MHz, we used absorber
material all the time in our devices to eliminate effects of the
amplifier housing. We were dealing with situations though were the
housing itself could easily have been on the order of a quarter of a
wavelength and thus signal re-generation could easily take place. On HF
though to get a 1/4 wavelength cavity, you'd need a rather large PA!
Your idea has merit and thanks for the suggestion!
73,
Jon
KE9NA
--------------------------------------------------------------------------
The Second Amendment is NOT about duck hunting!
Jon Ogden
jono@enteract.com
www.qsl.net/ke9na
"A life lived in fear is a life half lived."
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