[CQ-Contest] Coax Stubs for SO2R

[Jukka Klemola]

Umm .. I did not exactly finalize the email before sending.
The missing content is here:


There are two things to recognize when making RF filtering; in this case
with stubs:
Stub tuning
Placement of stub


Stub tuning:
Stub may be a little off the exact frequency and it still gives you
benefits.
The attenuation it will give, is more than 20dB at the worse band edge even
if the stub is not exactly centered on the band.
The condition to provide more than 20dB is, if the stub is placed at the
correct place on the feedline.


Stub placement:
The stub placement should be on the feedline at a point where the impedance
is high on the frequency we want to attenuate.
How to determine where that impedance is high:
As the amplifiers differ and transceiver outputs differ, it is better to
experiment the placing.

Decide a place where you want to place the stub.
Rudy wants to place the stub after the remote antenna switch.
Likely, the stub will not fit exactly at the switch output.

Make a coax, maybe 2 or 3 ft long and try. If it helps so much you feel you
have what you needed, let it be.
If you want to test and try different lengths, go ahead. It will take time
and effort to improve to perfection.


If you have a possibility, to avoid the need for optimizing, you might want
to build a double stub.


I hope this email is a better answer to Rudy.


73,
Jukka OH6LI


2016-07-18 21:24 GMT+03:00 Jukka Klemola <jpklemola at gmail.com>:

> I will try and answer each question below:
>
> 2016-07-18 20:37 GMT+03:00 Rudy Bakalov via CQ-Contest <
> cq-contest at contesting.com>:
>
>> If we are learning that stubs cannot be placed randomly,
>
>
> They can be placed randomly.
> Even if the stub is not exactly on the hertz .. if it is within, say 50kHz
> of the frequency you want to damp / attenuate, it will provide to your
> system.
>
> Being like less than 50 ft of coax, the cost is very reasonable, even if
> the contribution is 10dB.
>
> The cost (USD or EUR) per dB is very good if not excellent.
>
>
>
>> is there a method of tuning a stub that is placed randomly along the feed
>> line?
>
>
> Yes.
>
> Build a station with more than one radio.
> Build a stub to one of the radios. Then try some lengths of coax between
> the amp output, coax switch, some panel or where ever you have been
> thinking to place the stub.
> Then change the coax length a little.
>
> I have made male-female coaxes.
> One is about 2 or 3ft, other is about 5ft or so and third is maybe 10ft.
> I can test any length from about 2 to about 17ft in a matter of minutes.
>
> It is a very quick and effective method to produce actual measurable
> results.
>
>
>
>> For example, for an existing installation, there is the existing coax run
>> from the shack to the remote antenna switch. It is not practical to change
>> the length of the coax run. What can be done in this situation, which I'd
>> assume is very common, to squeeze the best performance out of a stub?
>>
>
> After the remote antenna switch, you have coaxes to monoband antennas, I
> presume?
>
> Change the length from coax switch to antenna i.e. place some cable
> between the switch and the stub.
> No need to touch the coax from station to switch.
>
> Basically .. try.and.measure before applying full power.
> This approach usually avoids the .err -phase that is typical to us hams
> when we go to deep experimenting mode.
>
>
>>
>> Rudy N2WQ
>>
>>
> 73,
> Jukka OH6LI
>
>