[CQ-Contest] Coax Stubs for SO2R

[Jim Brown]

On Mon,7/18/2016 12:58 PM, Jukka Klemola wrote:
> Thanks Rudy ..
>
>
> In case you really split the hairs, there is a connection between the two;
> placement and length; but it is weak.

This is false.
>
> In case you really want to experiment, make the stub a little long.
>    -if you want to be precise, I would suggest to tune the stub to harmonic
> band lower edge
> Second, find a place where it attenuates the harmonic more than elsewhere.
>    -you will likely notice the maximum attenuation placement is not critical
>    -you will likely notice finding the spot where the attenuation is not too
> great is narrow

There is no need for "experimentation -- this is all well-settled 
science that I learned in my university EE classes more than 50 years 
ago. The resonant frequency of a quarter-wave  stub can easily be 
measured with even the simplest of antenna analyzers. Connect the 
open-circuit stub to the analyzer, then tune the analyzer frequency to 
find the lowest impedance (it should be very near zero ohms).

> Disclaimer: this is depending on the output stage and antenna; but
> typically it is like that with tube amps and monobanders.

The output stage and the antenna ONLY affect PLACEMENT of the stub(s) 
along the line. They do NOT affect stub tuning.

> After finding the adequate placement, then prune the stub length to get the
> compromise for the attenuation over the harmonic band.
>
> Please note it is possible in a case of a 40m stub that the stub placement
> is not at all the same for 20m attenuating or 15m attenuating stub.
>
>
> ** Warning, scientific content:
> Then, if you really measure the stub carefully, you will notice the 20m
> attenuation maximum is not the half of the attenuation maximum on 10m.
> It is because the speed of radio wave changes in the coax dielectric.

> You will definitely notice this if you build a 160 or 80m stub and tune it
> to attenuate 80 or 40.
> Then measure it on 20 or 10 .. it is not centered on 20 and the maximum
> attenuation may be just outside the 10 band.

RIGHT! And too few people realize this. The good news is that the 
difference is pretty small between 20 and 10, but can be significant on 
160, 80, and 40M.

> About all generalizations we see written and drawn:
> All rule of thumb gut feelings are approximations because the equipment
> output stages vary.
> Some amplifiers have Pi outputs on some bands and Pi-L on others.
> That means the absolute maximum result stub placement requirements differ
> for that amplifier when changing bands.

73, Jim K9YC