[TowerTalk] Tuning coax stubs w/ shack test equipment
Mason, Ken R
ken.r.mason@intel.com
Tue, 30 Mar 1999 10:48:03 -0800
Hello Fellow Hams,
I had tried to work with stubs to prevent intermod during a VHF contest.
Station consisted of 1500 watts on 6M and 1500 watts on 2M. Of course the 6M
station was killing the 2M receiver. I made a 19 inch approx. 1/2 inch
hard-line shorted stub and put it on the 2M radio using a tee connector.
The results were poor. The 2M radio worked fine but the 6M signal still got
in. Checking it with the mfj 259 showed it was super wide. using a tee
connector with a dummy load and the shorted stub showed about 40 ohms on 6M
using the mfj 259.
Is this to be expected? I need to remove the intermod signals on my 2M and
432 radios. Any ideas for my single tower contest station? ( no, not another
tower)
73,
Ken
KC6TEU
mailto:ken.r.mason@intel.com
http://www.qsl.net/kc6teu/
http://home.earthlink.net/~kmcm/
-----Original Message-----
From: k6ll@juno.com [mailto:k6ll@juno.com]
Sent: Monday, March 29, 1999 9:13 PM
To: towertalk@contesting.com
Subject: Re: [TowerTalk] Tuning coax stubs w/ shack test equipment
On Mon, 29 Mar 1999 18:36:15 -0800 Steve Gehring / KL7DC
<gehring@alaska.net> writes:
> I grabbed an "odd" piece of coax (128 inches of
>cheap
>RG-11) from the shack closet and connected it to my Autek RF-1. For
>the
>open stub test, the impedance bottomed out at 15.05 MHz with 7 ohms,
>and
>only rose to 13 ohms at 14.05 MHz, or 1 MHz lower.....
> Now, my Autek worked fine for this
>application,
>and made finding the center of that low Q, broad notch and/or
>passband
>rather uneventful. I've never used an MFJ, but would guess the SWR
>would
>peak during the stub's electrical shorts and opens. I feel they are a
>good
>way to check your work, especially with possibility of length
>miscalculations and varying velocity factors, etc.
I didn't see the original posting on this, but something is wrong
here. A 10' stub made out of rg-8 or rg-11 should have an impedance
minimum of about 0.5 ohm. I just tested my 40m shorted stub, made out
of rg-8, and it read 0 ohms from 14.0 to 14.7 Mhz on the Autek
meter. This is a false reading, though, because of the "diode
suck-out" problem discussed in the Autek manual for very low impedance
measurements.
I still think the best way to tune a stub is to
beg, borrow, steal or buy a grid dip meter, temporarily connect a
1 inch piece of wire from the pin to shell of the stub's pl-259,
and couple the gdo into the small square loop formed by the little
wire, the pin, and the shell of the pl-259.
Dave Hachadorian, K6LL
Yuma, AZ
K6LL@juno.com
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