[Amps] coaxial stub on amplifier output

[James Colville]

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 at 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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