[Amps] Transmission line stubs

[John Lyles]

Thanks Jim for the informative summary presentation and paper on stub 
placement and design. I built a small 1/4wave shorted stub to protect 
GMRS base station receiver long ago in a remote location 
(lightning/static build up on antenna).

In my job, I designed 1/4 wave stub for the output of a 3 MW peak power 
200 MHz amplifier, that was commercially produced (well, 8 of them). It 
is in 9-3/16 inch coax, so you can imagine the trick in deciding where 
the Tee junction ends for the stub. When line is that large in diameter, 
there are some fudge factors related to the location of the junction, so 
that you get the exact quarter wave. Robert Pound at the MIT Rad Lab in 
WWII wrote about this. He was an interesting chap. Back then coaxial 
cables didn't exist so they build air insulated transmission lines at 
first, for high powered radar. The center conductor supports were 1/4 
wave stubs until Teflon and Rexolite became usable insulators.

https://ethw.org/Oral-History:Robert_Pound

We designed empirically at first, cut and try, then modeled it using CST 
Microwave Studio and succeeded. The purpose of this stub was not to 
provide 2nd harmonic attenuation but to be able to short the output 
enough to get a drive shaft in there to be able adjust the output 
coupling in the big cavity amplifier while it is running. It works 
beautifully, having a bellows on the center conductor of the big coax, 
and a capacitive paddle in the PA. However, the benefit of the second 
harmonic trap effect was welcome of course. The f2 component of the PA 
is suppressed -50 dBc after the stub. At least 25-30 dB added notch there.

There is another benefit to using a second or third harmonic stub after 
a tube or solid state power amplifier, but it is a second order effect 
that may conflict with your other design goals. You can modify the PA 
anode current waveform in a tube amplifier and likewise affect the 
waveforms in transistor drains by judicious spacing of the stub from the 
PA. Its not effective for multiple path binary combiners such as used to 
combine a lot of solid state pallets (in broadcasting for FM or TV for 
example) unless every cable length is identical to each module from the 
combiner. In tube PA's, this sort of 'waveform engineering' in PA design 
is essentially how class F works. By suppressing the first even or odd 
harmonic with a stub placed a specific distance away from the active 
amplifying device, you can square up the waveform a smidgen which 
increases the PA efficiency. Its based on the Fourier reconstruction of 
the modified sinewave. There are numerous papers on this, I can supply 
if interested.

This isn't of much use on multiband amplifiers or systems that must 
change frequency significantly.

73

John Lyles

K5PRO

> Date: Thu, 24 Nov 2022 18:03:39 -0800
> From: Jim Brown <jim at audiosystemsgroup.com>
> To: amps at contesting.com
> Subject: Re: [Amps] 10 KW CCS ON 6M...USING THE 3CX-6000A7... PART 12
>
>
> On 11/24/2022 2:35 PM, Steve Thompson wrote:
>> For sure the stub (looking like a short to even harmonics) works best
>> when combined with some series inductance to work against. Hopefully the
>> load cap is already a low impedance to harmonics so the short circuit of
>> stub won't do so much when added across it - but it shouldn't do any
>> harm either, and appeals to my laziness compared with having to
>> manufacture a nice choke like you did.
> Take a look at the fundamental concepts in these two links, which
> address the placement of stubs for harmonic suppression at HF. The first
> doc ran in National Contest Journal about five years ago, correcting
> some less than ideal work published a year earlier, the second is the
> slide deck for a talk I did at Visalia around the same time. My work got
> pretty thoroughly peer reviewed.
>
> http://k9yc.com/LocatingStubs.pdf
> http://k9yc.com/StubPlacement.pdf
>
> Some of the same techniques are used with cavity filter networks.
>
> 73, Jim K9YC
>