[CQ-Contest] SO2R Coax Stub location; outside?

[wc1m73 at gmail.com]

This discussion reminded me of TowerTalk posts made by Bill Fisher W4AN (SK) about 25 years ago concerning the location of stubs. I think he used two stubs 1/4-wave from the transmitter. I found a 1999 post from Bill on CQ-Contest referring to a thread about stubs on TowerTalk, but that thread didn't have the information I was looking for. I tried searching the TowerTalk archives for "stubs W4AN", but it looks like the searchable archives only go back to 2010. So I manually searched the archives from 1999 to 1996, one month at a time, and found only one 1996 post from Bill when he was KM9P saying that two stubs 1/4-wave apart provide significantly more attenuation than a single stub, and that's what he used for all his antennas. I'm almost certain he also recommended putting the first stub 1/4-wave from the transmitter, but can't find any posts about that even though I'm pretty sure that's what he recommended.

Can anyone confirm that stubs work best 1/4-wave from the transmitter?

I also ran across a post that recommended using two stubs tuned to different frequencies in order to cover an entire band.

73, Dick WC1M

-----Original Message-----
From: Jeff Blaine <KeepWalking188 at ac0c.com> 
Sent: Saturday, February 22, 2020 11:44 PM
To: cq-contest at contesting.com
Subject: Re: [CQ-Contest] SO2R Coax Stub location; outside?

Here's the quick & dirty reply to the "get it in the right place along the line" placement worry.

1. Build your stub and get it ready for install

2. Measure the amplitude of the 2nd harmonic on whatever the band you are wanting to stick a stub in.  Say it's 40m transmitting on rig 1 so you would look at the 20m 2nd on rig 2.  This is the baseline.

3. Now go out and stick that stub in whatever the MOST CONVENIENT PLACE IS along the line.  If you have a remote antenna switch, that's likely the point where the feed line comes off the tower and hooks into that switch.  Don't sweat the location.  Just stick it on.

4. Back in the shack, check the level of that 2nd harmonic.  If it's "good enough" - then you are done.  If it's not good enough, then you have more work to do.

I find the generic single stub, pretty much randomly placed, to be good for say 15 dB or better.  Maybe I'm just lucky.  Or maybe I'm just that good.  ha ha.  But if the stub measures 25 dB on the bench and I'm getting 15 dB in actual use, and let's say I need a few more to reach "good enough" - then I can add in some extra feed line and see if the attenuation improves (because that "moves" the stub to a more favorable point).  Or I can whip up a double stub (which will have more attenuation).  Or do consider something else that would reduce the coupling.

My point is that building and using the stub is far more important and worrying to death about line placement.  You can always refine the line placement later, if needed.

Stubs are a marvelous and wonderful thing!

73/jeff/ac0c
alpha-charlie-zero-charlie
www.ac0c.com


On 2/22/20 7:27 PM, Jeff Blaine wrote:
> Mike,
>
> Good you are getting that filtering added.  SO2r without filtering is 
> a recipe for frying front ends.  Guess luck has been in your favor.
>
> On the stub location, outside is fine.  In fact all my stubs have been 
> located outside.  There is some magic as to the optimal place along 
> the feedline to place stubs but I've never fine tuned to that point.
> I load the stub at the base of the tower where the antenna stack 
> matches are and they work fine.
>
> 73/jeff/ac0c
> alpha-charlie-zero-charlie
> www.ac0c.com
>
>
> On 2/22/20 3:52 PM, Mike Smith VE9AA wrote:
>> Is it reasonable to assume I could locate all my coax stubs 165' away 
>> from the shack outside at the antenna switch?
>>
>>
>> Currently I run SO2R with no band pass filters, nor coax stubs, but I 
>> have a
>> 'relatively' large lot (2.2ac) and "radio 2" has a single antenna
>> (horizontal ZS6BKW) located at the opposite end of the lot and the 
>> primary
>> "radio 1" has a variety of verticals or vertical arrays, so it works.
>>
>>
>> Sometimes that's an advantage (speed of QSY's; not ever having to worry
>> about switching in stubs or filters) but sometimes there is a very small
>> amount of crossband QRM (self inflicted) and it always prevents 
>> running on 2
>> bands with my 'best' antennas.  For example, when I am running on 
>> 40m/80m I
>> have to choose one 4-Sq and always the horizontal antenna is parked on
>> "radio 2" (a ZS6BKW w/ 15m add-on element).
>>
>>
>> If I bought 2 sets of bandpass filters and made up 6 coax stubs that 
>> I could
>> locate out at the antenna switch, I think it would be a small step up 
>> for my
>> signal on some band combos.  The relatively low ZS6' ain't exactly a 
>> pileup
>> buster ;-)
>>
>>
>> Reason is, there is NO ROOM in the shack for stubs.  Just none.
>>
>>
>> So, any downside to doing it this way?
>>
>>
>> Mike VE9AA "NB"
>>
>>
>> Mike, Coreen & Corey
>>
>> Keswick Ridge, NB
>>
>>
>> _______________________________________________
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