[CQ-Contest] Coax Stubs for SO2R

[Rudy Bakalov]

Hi Jukka,

Thanks for the detailed response. Your original email was clear...at least to me :-)

I assumed from the very beginning that stub tuning and placement are two different steps. What I was wondering is if, in the case of random or unknown stub placement, the stub can/should be optimized after it has been placed. Sounds like the answer is NO

Rudy N2WQ

Sent using a tiny keyboard.  Please excuse brevity, typos, or inappropriate autocorrect.


> On Jul 18, 2016, at 2:57 PM, Jukka Klemola <jpklemola at gmail.com> wrote:
> 
> 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
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