[CQ-Contest] Coax Stubs for SO2R

[Jim Brown]

Your logic is exactly backwards, Jukka, and it applies ONLY to 
optimizing the relationship between the stub and the power amp. When the 
output circuit element is a capacitor, the stub should be 1/4 wavelength 
from the amplifier at the harmonic it is intended to suppress. When the 
output circuit element is an inductor, the stub should be directly at 
the amplifier output, OR a halfwave up the line at the harmonic.

But the relationship between the antenna and the stub also matters a 
LOT. A stub works by shorting the line at the frequency of the harmonic, 
and will be most effective when applied at a HIGH Z point on the line AT 
THE HARMONIC FREQUENCY. Most single-band antennas present a HIGH Z at 
the second harmonic, so the stub should be some multiple of half waves 
from the antenna.

Note also the advantage of double stubs in portable setups where length 
of the line between antenna and stub cannot easily be controlled. Even 
if the first stub (nearest the antenna) is poorly placed, the second 
stub, a quarter-wave nearer the transmitter, provides its full attenuation.

All of this is covered in the applications note for which I posted a 
link last night.

http://k9yc.com/LocatingStubs.pdf

The next link shows how we implement stubs in the trailer that W6GJB put 
together for or CQP and 7QP expeditions.  Note how close the 80 and 40 
antennas were to each other in the last setup, and the 40M dipole and 
C3SS were to each other in the daytime setups. At 600W from an Elecraft 
KPA500, we had zero problems with harmonics with CW stations on both bands.

http://k9yc.com/7QP.pdf

73, Jim K9YC

On Sun,7/17/2016 10:37 AM, Jukka Klemola wrote:
> Placing a stub at output connector helps a tad in case your output circuit
> that shows a low impedance to ground at the harmonic. That is typical for
> nearly all our equipment that have a low pass circuit with a capacitor
> going to ground as the last component of the output circuitry; a Pi circuit
> output.