>>The average HF tank is a low-pass filter, so how does VHF-energy get
>>past
>>C-Tune? .
>>- thanks
>
>Very simple if you bothered to analyze a real circuit....not just theory.
>The SB-220 is a prime example of what you claim does not exist.
Ok, yes, I was talking a theoretical example. The "filter" could have a
flyback where at higher frequencies the attenuation will actually start
to degrade. Also, since no filter provides INFINITE attenuation, yes
some energy will get through. However, it may be about 20 to 30 dB down.
So taking all those factors into affect, yes, I'll agree with you. Some
energy will get through. A significant enough amount of energy?
Possibly not.
Any VHF/UHF energy that does manage to get through the PI-net, will
travel to the load. At the load, some of the energy will be delivered to
it, but depending on the impedance of the load at the given frequency,
some will be reflected. As in the case of an HF antenna, probably a
majority will be reflected. It will travel down the transmission line
back to the amp. Then it will hit the filter. Some of that energy will
get through but it will be further attenuated by the rejection action of
the filter.
If the VHF attenuation of the output PI-network is only 15 dB (not very
good, IMHO), then even with a completely open circuit at the output of
the PI-network, the whole thing will look like a 30 dB load at VHF. This
is because the energy gets attenuated by 15 dB going through the
Pi-network, gets 100% reflected by the open and then gets attenuated by
ANOTHER 15 dB going back through the Pi-net a second time. And actually
due to the fact that the impedance of the filter may not match the
impedance of the xmission line at VHF, you would probably get an
effective attenuation of greater than 30 dB total. A 30 dB return loss
is a pretty damn good load if you ask me!
So IMHO, unless you have some sort of incredible flyback (just a couple
dB attenuation) in the pi-net at the particular frequency of VHF
oscillation, any VHF/UHF energy escaping from the SB220 won't really
affect the internal workings of the amp.
So Carl, please explain given all this how you feel that I am wrong.
73,
Jon
KE9NA
-------------------------------------
Jon Ogden
KE9NA
http://www.qsl.net/ke9na
"A life lived in fear is a life half lived."
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