[CQ-Contest] Coax Stubs for SO2R
Joe
nss at mwt.net
Mon Jul 18 09:04:19 EDT 2016
Here is a video of the stub, switching the end short.
https://youtu.be/wgn_L_LUXcI
Depending on the band it either passes or blocks.
And quite a bit.
They are identical,
I off the top of my head do not remember, but I know they are electrical
1/4 wave on 40.
So when inserted, I could have this backwards, Honestly I do not
remember. But say it is this way, when open it passes 40 and blocks 20,
when shorted it blocks 40 and passes 20.
It could be the other way around. but you get the idea.
Joe WB9SBD
Sig
The Original Rolling Ball Clock
Idle Tyme
Idle-Tyme.com
http://www.idle-tyme.com
On 7/18/2016 3:32 AM, Jukka Klemola wrote:
> Well .. with this kind of arrangement, as you are not exactly optimizing
> the last deciBel out of your system, experimenting is the key word.
> Dual stubs are out of the question as you want to have that much
> flexibility.
>
> In a Field Day setup you have different equipment on different bands.
>
> That leads to suggesting to experiment a little at each station you have,
> if you get different results with different radio/amp/tuner/stub setups.
>
> It would be interesting to see a youtube video of your stub switch and how
> it performs at your station.
> Very fascinating. I like the approach to take filtering ideas into that
> kind of a station setup with various equipment.
> It is a mixture of fine and complex ideas within a quick and dirty
> implementation.
> Using some patience that leads to fine and quick operating station.
>
> The bottom line is it does not have to be perfect to function better.
>
>
> 73,
> Jukka OH6LI
>
>
>
> 2016-07-17 21:46 GMT+03:00 Joe <nss at mwt.net>:
>
>> This has been really interesting. I hope you all are learning as much as I
>> am.
>>
>> So, I really made some crude stubs from some old RG-8. A "T" on one end
>> and a shorting switch on the other end.
>>
>> Both electrical 1/4 wave on 40.
>>
>> Depending on the switch it either passes on band and blocks the other or
>> visa versa.
>>
>> We have been putting them right at the end of the radios during field day,
>> and seem to have helped a LOT.
>>
>> Now am I to believe by not placing them there we can make them even more
>> effective?
>>
>> Joe WB9SBD
>> Sig
>> The Original Rolling Ball Clock
>> Idle Tyme
>> Idle-Tyme.com
>> http://www.idle-tyme.com
>> On 7/17/2016 12:37 PM, Jukka Klemola wrote:
>>
>>> 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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