[CQ-Contest] Coax Stubs for SO2R
Jukka Klemola
jpklemola at gmail.com
Sun Jul 17 13:37:38 EDT 2016
Hi Joe and others,
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.
The stub at output connector provides a low impedance at the harmonic.
It will be very close to being parallel to the output circuit.
You can expect harmonic attenuation of anything between, say, about 6 to
something above 10dB or so by adding the stub.
Please note this is the case for a properly tuned amplifier.
Even in this case the stub at the output connector, the stub may save your
equipment on the harmonic band if your station design is marginal.
In case the output circuit is not properly tuned; I mean for example a
situation where you have hopped to a new band and you have approximated
settings for that new band, the attenuation the stub at output connector
provides can be more than 10dB.
In case your output circuit is such it does not provide a low impedance at
output on the harmonic frequency, the stub will give you good service by
attenuating the harmonic signal by more than 30dB.
Pi-L output amplifier is one such example.
Making a double stub guarantees you will get excellent service by the stub.
Construction is simple.
1. you have that one stub
2. you add about an eight wave coax that goes from putput connector towards
antenna
3. you make another stub to the end of that eight wave coax
..the system output is at the second stub.
There you go with a stub arrangement that gives a perfect service with any
output construction or any tuning.
73,
Jukka OH6LI
2016-07-17 19:11 GMT+03:00 Joe <nss at mwt.net>:
> 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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