[CQ-Contest] Coax Stubs for SO2R

[Jukka Klemola]

>
>> 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).
>

In case a person wants to experiment, he can do some experimentation.
My email was written so there is no complicated measuring equipment.

That is, there is no need for antenna analyzers.
You can use the station you have as the signal source and receiver on the
harmonic band to evaluate the outcome.
You can park the stub close enough to the band you want to attenuate using
just two radios you have.

You need your radio to analyze your system anyway to experiment the stub
placement.

You will notice the level of harmonic signal at the lower edge, in the
center and at the upper edge of the band.
Then decide what to do about the stub length. Shortening the stub will rise
the attenuation frequency.
I would suggest to leave it just a bit long .. I like CW more anyway.


To do all that properly by scientific terms, you might need a bunch of
Benjamins you can let go in return to a few boxes with some electronics
inside.

But as you have your station and want to experiment a little - it does not
have to be any more complicated.


Ward has the writing series about all technical matters .. this might be
one tale to tell.


73,
Jukka OH6LI


PS.
**Warning, scientific content:
The stub length requirement along the feedline may get a small twist
because of your system properties on the unwanted band .. in case there
just happens to be s steep curve affecting the bands interaction, the
antenna, the transmitter output or any other part of the system may
attenuate the upper or lower part of the harmonic band by itself and you
would need the stub more for the other part of the band.

This is the weak connection between stub length and positioning ..the
connection is not direct and is definitely not in any university books. You
have to run into this phenomena and ponder a while what it is before you
get the idea how it works.

You do not have to notice this at all if you make and tune your stub when
it is a part of your system with two or more radios and accessories you
have.

And, furthermore, if you make all analysis and pruning using lab
instruments, you can easily forget the instruments output circuitry for
sure have different output criteria than your transmitters' outputs.
Same applies for the receivers' inputs. How they interact with the world
that is outside the band they are on.

This is precisely what harmonics are. They are a set of phenomena outside
the intended use of the equipment and these phenomena do vary.

Using lab equipment beyond a point will mislead you, will bring you
extended work because you need to evaluate the outcome anyway with the real
station, in addition to the lab equipment.
For nearly any setup, the lab equipment will bring you to the ballpark ..
but to really see how your station works, you must use the radios and other
equipment at your station.
The equipment have their characteristics and that is the real world where
you need to be able to make the QSOs.

The simplest example of this is connecting the stub to the RF testing
equipment may differ a little from the way you connect it to your
transmission system .. I have done that error and I hope also many others.
I did that before I got a two port analyzer. Tuning a stub with one port
tool, like the Autek RF-1 small black box, can bring you into a journey you
will actually see real life phenomena through experimentation.