It is for a cavity amplifier, with capacitive-coupled output paddle. Coax is 9
3/16 inch EIA, so I despise having to modify it. The stub is designed on paper
and in machine shop, having an acme threaded screw inside that will change the
penetration of the paddle, all while running at ~500 kW average power.
I was just curious for the collective experiences and wisdom here. The Amps
forum contains a world resource of people who have done many things with many
RF amplifier variations.
Yes, you can expect some plate current (or drain, or collector) variation if
you put a LPF on the output, and an out of band harmonic is not terminated but
is reflected back. I had a lot of trouble with a particular FM broadcast
amplifier cavity years ago, when it played wonderfully into a flat Bird
resistor load, but when
LPF was added (to meed FCC requirements) the thing when ape and created
spurious oscillations. You can bet that the plate current waveform was
modified. Now the second harmonic might be 20-30 dB down naturally in a cavity
circuit due to Q. Still there is a slight effect, although small. The harmonic
termination could change things a percent or two. But it can affect the
propensity for a power tube to behave as expected, also.
The plate current waveform may be distorted, which really isn't a fundamental
frequency problem in the end result due to the Q of the cavity. However, in a
more conventional Pi network or link coupled output with moderate Q and some
bandpass or lowpass response, one might see something. It could mainly effect
the DC current in a small way, which might bump or drop efficiency a small
degree. I haven't tried it. Gerald, it can work as a way to feed B+ but only
over a narrow band, like 10% or less BW. Some industrial RF heaters shunt feed
their B+ this way, through inside of a tubing that is the output inductor.
While it isn't a 1/4 wave stub, it is essentially the same thing, feeding at a
high impedance for RF. Just what you'd like a choke to be.
In the 1980s I built quarter wave stubs for putting on the front of a 2m
repeater site near Philadelphia to protect the receiver from both nearby FM
stations (overload) and from harmonics of transmitters. I used semi-rigid
0.25inch coax.
I put a 1/4 wave DC short on a GMRS base station radio last summer, 162 MHz,
that prevented static build up from the monopole on the roof top. Before that,
they were seeing static buildup problems. These little stubs are wonderful
devices if you have a network analyzer to make sure they are on the correct
side of the Smith Chart, accounting for the connector insertion phase and stray
L at the soldered shorted end.
I am concerned with this big one as I don't want fireworks in my amplifier.
73
John
K5PRO
>
> ## I bought george's [ w2VJN] book from inrad a few yrs back
> It's superb. There is also excellent material in the latest ON4UN
> low band dx book. George's book show's other designs too.. all using
> coax.
>
> ## apparently, whether you put the say shorted 1/4 wave stub right at the
> output of the amp.. vs a 1/4 wave away from the amp depends on whether you
> have a simple PI net... or a PI-L. They work great on the output of the 2
> x GS35B
> 6m amp. Attenutaion improves by a whole bunch, when bigger coax is used.
> Huge
> diff between 213 and heliax.... for the same 1/4 wave stub. Somebody
> pointed out
> to W8JI that they could hear his 2nd harmonic... when JI was TX on 160m...
> and folks
> could hear him easily.. when operating on 80m. JI installed the stub
> right at the 160m ant
> feed point... end of problem.
>
> ## I didn't think of the stub interacting with plate current waveform
> modification. Will you
> get the same waveform modification, when using a LP Filter... say with a cut
> off of 33 mhz?
> What if the LP filter has a cut off just a bit higher than the band in use...
> like monoband LP
> filters ??
>
> Jim VE7RF
> ------------------------------
> Message: 5
> Date: Tue, 30 Mar 2010 08:26:21 EDT
> From: TexasRF@aol.com
> Subject: Re: [Amps] coaxial stub on amplifier output
> To: jtml@vla.com, amps@contesting.com
> I will display some ignorance here:
>
> Does the plate current waveform distortion cause any problem at the
> fundamental frequency?
>
> Your idea for using a quarter wave stub like that is interesting. For a
> single band amplifier, it could replace the rf choke for B+ and avoid all
> those pesky resonance problems. Of coarse the plate blocking C would need to
> be
> removed and used to connect the plate load C (in a pinet) to the output to
> keep the B+ off the load C.
>
> 73,
> Gerald K5GW
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