[CQ-Contest] Coax Stubs for SO2R

[Kenneth Wolff]

"*Yes, a very good move. But when we're talking stubs for harmonic
suppression, we must measure that coupling at the frequency of the
harmonic.*"

Jim is correct. By measuring the coupling of 10 into 20, not just the
coupling of 20 into 10 we can get, by reciprocity, the data we need to
predict interference on 10 from the harmonic on 20. Obviously it means we
should measure the harmonic levels produced by the amplifier which depends
in turn on the load presented by the antenna!

A network analyzer is a very, very nice tool for this kind of work,

- Ken K1EA

On Mon, Jul 18, 2016 at 6:10 PM, Jim Brown <k9yc at audiosystemsgroup.com>
wrote:

> On Mon,7/18/2016 12:08 PM, Kenneth Wolff wrote:
>
>> Jim right of course -- the exception being the old Alphas with Pi-L
>> outputs.
>>
>
> There are many amps with Pi-L outputs.  There's a table of those I've been
> able to learn about (with help from friends) at this link
> http://k9yc.com/Coax-Stubs.pdf
>
>
> At the the old K1AR M2 we ran from my house we used a lot of stubs because
>> we only had two towers.
>>
>> Stubs made with hard line have less loss and are deeper,  but narrower
>> nulls.
>>
>
> Yes, and they can be TOO narrow, locking you into a narrow range of
> operating frequencies.
>
>   Fixing 40 into 15 is a little trickier because it as a third
>> harmonic. I used an open 1/4 stub on 15m with a compensation stub to tune
>> out the imaginary part of the leftover conductance on 40.
>>
>> Double and even triple stubs slightly offset across the band can be used
>> to
>> make a broader null.
>>
>
> Yes.
>
> The most important thing to do is to measure the coupling from each
>> transmit antenna into each other antenna using 10w transmit and either a
>> 50
>> ohm scope or a calibrated receiver.
>>
>
> Yes, a very good move. But when we're talking stubs for harmonic
> suppression, we must measure that coupling at the frequency of the harmonic.
>
> I made a spreadsheet of all the coupling (transcontuce) between antennas to
>> plan my defense between bands. Some bands needed only a receive filer.
>> 1500W on 160m used to couple over 50W into the 80m vertical array. You can
>> smoke receivers if you don't watch out!
>>
>
> Great advice.  A vector network analyzer or a calibrated spectrum analyzer
> and signal generator can also do this well.
>
> 73, Jim K9YC
>
>
>> - Ken K1EA
>>
>> On Mon, Jul 18, 2016 at 12:04 PM, Jim Brown <k9yc at audiosystemsgroup.com>
>> wrote:
>>
>> 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.
>>>>
>>>>
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