Thanks for doing the sums, Jim, I was interested in the theory. I thought
energy would be reflected in both cases of open and closed circuit stubs.
Something new to learn every day.
David
G3UNA
----- Original Message -----
From: James Colville
To: David Cutter
Cc: Jim Brown ; amps@contesting.com
Sent: Saturday, April 03, 2010 5:48 PM
Subject: Re: [Amps] coaxial stub on amplifier output
Get your calculator out. 30dB down from 1500 watts is how much power?
1500 watts=+61.8 dBm minus 30 db (which is what the harmonic would be down
normally) = +30dBm. +30dBm = 1 watt. Evenif the harmonic is only down 25 db,
that is only 3 watts.
Not much power do dissipate is it?
73
Jim W7RY
On Sat, Apr 3, 2010 at 9:32 AM, David Cutter <d.cutter@ntlworld.com> wrote:
Jim
Thanks for the tutorial.
I've often wondered about the movement of energy in and around a stub.
Harmonic energy is delivered from the PA to the output feeder and onwards to
the stub. Would I be right in saying that the harmonic energy is reflected
from the stub back to the PA where it is absorbed in the output filter? If
so, this energy is absorbed as heat, it has to go somewhere. Does any of it
get back to the output device? A pi output filter is a low pass filter in
either direction, so, I'm guessing that harmonics are mostly absorbed there,
but is there much attenuation in that direction?
David
G3UNA
> On Tue, 30 Mar 2010 09:19:15 -0400, Carl wrote:
>
>>Those are fairly common for multi TX stations to knock down harmonics and
>>phase noise.
>
> Yes. I'm currently working on a Q&A style tutorial on coax and stubs. The
> first draft is at http://audiosystemsgroup.com/Coax-Stubs.pdf
>
> Comments are appreciated. I would especially appreciate additions to the
> table of amps and output circuits.
>
> W2VJN's book is excellent, and worth the money if you're doing any sort of
> multi-transmitter station. Available from Inrad (the crystal filter
> people)
> for $20. There are also four excellent pieces in QST written by Ward
> Silver, N0AX. They are cited in my tutorial. And George has contributed
> some of his work on stubs to the 2010 ARRL Handbook.
>
> As to the effect of a stub on the current waveform -- a proper stub (or
> combination of stubs) should be invisible at the TX frequency, but a short
> circuit at even order harmonics. If the amp has reasonable output
> filtering
> without the stub, the stub should have no significant effect on that
> current waveform.
>
> 73,
>
> Jim Brown K9YC
>
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