[CQ-Contest] Coax Stubs for SO2R

[Jim Brown]

Hi Rudy,

All those "amp people" are WRONG.  I'm not "an amp person," but I am an 
EE. Another EE, N6BV, retired editor of the ARRL Antenna Book and other 
ARRL tech publications says that the second harmonic content of a 
typical ham output stage (the tubes or transistors themselves) is only 
about 6 dB down from the fundamental. All power amplifiers must include 
an output network that filters out the second harmonic and other higher 
harmonics that are produced by the tubes/transistors. The second 
harmonic content at the output terminals should be at least 40 dB down 
from the fundamental WORKING INTO A 50 OHM LOAD. Caps added for emphasis.

If you're running SO2R with closely spaced antennas, a second harmonic 
that is only 40 dB below the fundamental can make it difficult to hear 
anything but your own harmonic. THAT'S why we care. This mostly matters 
on 80, 40, and 20 CW, where the harmonic of the lower band ends up right 
where we would like to work on the harmonic band.

As a simple example, I have a 40M wire Yagi with the driven element a 
few feet below my 3-el SteppIR. I often work 40 and 20 at the same time 
-- that is, as 20 is shutting down and there's activity on 40). Without 
stubs, my own 40M signal wipes out about 80 kHz of 20 CW. A double stub 
on the 40M antenna knocks that harmonic down enough that I only lose 
about 10 kHz of 20M.

Filters are passive networks, and ALL passive networks, including those 
at the output of a power amp, depend on the impedance at their input and 
output to work properly. K6XX, an EE who is an "amp person,"worked on 
the Elecraft KPA500, and part of what he did for that project was study 
a dozen or more popular ham power amps. A few years ago, Bob gave an 
excellent talk to NCCC about how to make your power amp be clean. One of 
the most important thing was to carefully TUNE it. With a "no-tune" 
solid state amp, that means using a good antenna tuner at the output so 
that it's always working into 50 ohms.  This makes the output filters 
work better, and it minimizes IMD (splatter, clicks) that are generated 
by the amp.

Bob's my neighbor and a good friend, but also a serious competitor. When 
I moved in down the road from him 10 years ago, he made sure that I knew 
how to tune my power amps!

I strongly urge you to study the link to my applications note on stub 
placement. It covers these issues as background, but more important, it 
shows how we can put the stubs at the optimum place on the line (that 
is, length of line to the antenna, length of line to the power amp) so 
that the stub is most effective, AND so that the stub takes full 
advantage of the filtering provided by the power amp's output stage. A 
stub that is poorly placed with respect to the power amp is FAR less 
effective, for reasons discussed in the applications note.

http://k9yc.com/LocatingStubs.pdf

73, Jim K9YC

Mon,7/25/2016 3:48 AM, Rudy Bakalov via CQ-Contest wrote:
> All amp people I know, including those that manufacture commercial amps, categorically state that amps do not generate harmonics on their own. That is, amps merely amplify them.
>
> To me this seemingly minor difference is huge. If amps only amplify but do not generate harmonics then there is no scientific reason for filtering after the amp assuming the same amount of filtering is applied before the amp. In practical terms, this would mean that low power BPFs before the amp are as effective as high power BPFs after the amp.