[CQ-Contest] Coax Stubs for SO2R

Sun Jul 17 12:11:52 EDT 2016

What happens if you have a single stub, and it is placed right at the 
output connector?

Joe WB9SBD
Sig
The Original Rolling Ball Clock
Idle Tyme
Idle-Tyme.com
http://www.idle-tyme.com
On 10/5/2015 12:07 PM, Jim Brown wrote:
> On Mon,10/5/2015 7:20 AM, Jukka Klemola wrote:
>> Jim,
>> I thought placing double stubs on the TX line is not so critical.
>> The two band stopping stubs are supposed to be about quarter wave 
>> apart; on the harmonic band; and that should effectively remove the 
>> need for extended optimization.
>>
>> Also, in my experience, if you get approx 30 or 35dB attenuation 
>> using one stub, you will get more than 45 dB attenuation over the 
>> whole band when measured in a 50 ohm system.
>
> Sure -- using my DG8SAQ VNWA, I measured peak attenuation of these 
> double stubs of 55 dB for the 80M stub pair and 59 dB for the 40M 
> pair. BUT  a monoband antenna is NOT a 50 ohm system at the harmonic 
> frequency, and the transmitter is NOT a 50 ohm source at the harmonic 
> frequency.
>
> Most monoband antennas present a very high Z to the line at their 2nd 
> harmonic, which establishes a very high SWR for the harmonic, so the Z 
> varies over a wide range along the line. A stub works by placing a 
> short on the line at the harmonic frequency, and is most effective if 
> placed on the line where the Z is high. It is MUCH less effective when 
> at a place in the line where the impedance is much less than 50 ohms.
>
> SO -- with double stubs separated by 90 degrees at the harmonic, the 
> second stub will always be at a high Z spot on the line, but the first 
> will depend on where it is along the line.
>
> There is a second issue.  By their nature, the output stages of modern 
> power amps, both tubes and solid state, produce 2nd harmonic that is 
> only about 6 dB below the fundamental, and must be filtered by the 
> amplifier's output network. Most of these networks are designed for 50 
> ohms, and that is how they are tested.  If the last element of that 
> network is a capacitor, it will be most effective if it sees 50 ohms 
> or more as a load AT THE HARMONIC, and least effective if it sees a 
> short. Likewise, if the last element of the filter is an inductor 
> (Pi-L), it will be most effective if it sees Z of 50 ohms or less.
>
> I rigged two power amps, a Ten Tec Titan and an Elecraft KPA500 with a 
> voltage tap at their output terminal into a dummy load and into a 
> double stub network in line with my 40M dipole. With the stub feeding 
> either the antenna or the dummy load,  the second harmonic at the 
> output of both power amps varied by +/- 10 dB as I added short 
> sections of coax to vary the length of the line between the amp and 
> the stub over more than 180 degrees at the harmonic frequency.
>
> SO -- if we put the stub in the "right" place for the power amp, we 
> get full performance of it's harmonic suppression network plus the 
> suppression of our stub(s), but if we put it in the "wrong" place we 
> can lose as much as 20 dB less of the 2nd harmonic suppression of the 
> output network.
>
>> At least my measurements show these results:
>> -placement not critical for double stub
>
> As you can see from above, that's only true if the antenna is near 50 
> ohms at the harmonic.
>
>> -attenuation almost doubles in dB compared to one stub
>
> Yes.
>
>>
>> --> I have been building only double stubs nowadays.
>
> Me too.
>
> Another point. When you have done this amount of suppression, you will 
> likely begin to find other sources of 2nd harmonic, as W3LPL recently 
> observed (perhaps on another reflector), AND you may also find leakage 
> paths in your antenna switching system.  I recently replaced a vintage 
> Six Pack with a 6x2 sold by 4O3A. I measured crosstalk in the 4O3A 
> unit before installing it and the Six Pack after removing it and found 
> the 4O3A unit had more than 20dB better isolation on 20M.
>
> In hopes of picking up another 6-10 dB of isolation, I am also 
> planning to replace some of the random vintage patch cables in my 
> station with newly made jumpers using high quality RG213 and Amphenol 
> 83-1SP connectors.
>
> 73, Jim K9YC
>
>>
>>
>> 73,
>> Jukka OH6LI
>>
>> 2015-10-05 9:57 GMT+03:00 Jim Brown <k9yc at audiosystemsgroup.com 
>> <mailto:k9yc at audiosystemsgroup.com>>:
>>
>>     On Sun,10/4/2015 2:59 PM, Jeff AC0C wrote:
>>
>>         That's a great way to start.  Especially as the solar cycle
>>         fades and 10/15 are not open.  Then in the day, you run 40/20,
>>         and in the night you run 80/40.
>>
>>
>>     Yes, but there's a LOT more to it if you want to maximize the
>>     effectiveness of the stubs. It can matter a LOT (20-30 dB) where
>>     along the line stubs are placed, both with respect to the antenna
>>     and to the power amp.
>>     See my piece in NCJ one issue back, or download it from my website.
>>
>>     For our CQP expedition, I made up double stub packs for the 40M
>>     and 80M CW stations only. Each pack was two stubs cut to kill the
>>     second harmonic, with a quarter wave (at the harmonic) connecting
>>     them. I didn't have time to optimize their placement, but using
>>     two stubs insures that you'll get at least 25-30 dB, and with
>>     optimized placement, another 25-30 dB.
>>
>>     Stubs are less important on SSB because the likely operating
>>     frequencies are not directly harmonically related, whereas the
>>     harmonic of 3525 hits 7050, and 7025 hits 14050.
>>
>>     73, Jim K9YC
>>
>>
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>>
>
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