When I first started using HF stubs in the early 80's I used a noise bridge
and RG-58 figuring it would be fine and cheap.
The null was rather broad and the stub heated up considerably as the
frequency increased. I suspect that either the bridge was inaccurate or
there is considerable fundamental energy involved.
I then went with very low loss quad shield CATV RG-11A and took them to work
and trimmed on a VNA that had the 75 Ohm option, not that it really
mattered. No more heating.
Carl
KM1H
----- Original Message -----
From: "James Colville" <jimw7ry@gmail.com>
To: "David Cutter" <d.cutter@ntlworld.com>
Cc: <amps@contesting.com>
Sent: Saturday, April 03, 2010 12:48 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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