While we are guessing here, I'm going to take a stab at it.
You are using a vertical and feeding it with an unbalanced line. The outer
surface of the coax braid has become a part of the antenna. By changing the
length of the feed line you are changing the length of the braid and thus
the antenna. You need to decouple the outer surface of the braid from the
vertical and you can do this by adding an UNUN between the coax and the
antenna feed point.
UNUNs are available from Amidon. An UNUN is similar to a Balun but is used
in UNbalanced to UNbalanced situations, which a vertical fed with coax
presents. A Balun is used in a BALenced to UNbanced configuration, such as
feeding a balanced antenna (dipole) with coax. Check this Amidon site for
more information, http://www.amidoncorp.com/f_ham.htm
I've seen some advertisements for "decouplers" which I believe are like
UNUNs. Their purpose is to keep RF off of the outer surface of the coax braid.
73..de John/K4WJ
At 09:03 AM 10/19/00 -0500, James Duffer wrote:
>My guess is the velocity constant(Vo) for the replacement (RG-8) was not
>the same as the Vo of the coax replaced (RG-58), if this is indeed the
>fact, then the physical length of the RG-8 would not be equal to the
>physical length of the RG-58 for the "same electrical length".
>
>It would appear that you need to adjust the length of the RG-8 so that it
>is electrically equal to the replaced RG-58. This is assuming that there
>is no problem with the new connectors.
>
>I hope this may lead to a solution to your problem and you will be back on
>12 meters shortly.
>
>73,
>
>Jim WD4AIR
>
>
>>From: Alan Bryant <wd0evx@moonworks.com>
>>To: tentec@contesting.com
>>Subject: [TenTec] Antenna Matching Weirdness
To: <tentec@contesting.com>
>>Date: Wed, 18 Oct 2000 18:17:10 -0600
>>
>>
>>I realize that by posting this, I'm probably going to look like an idiot,
>>but I'm sincere in wanting to know what's going on with something.
>>Hopefully someone smarter than I am can provide an intelligent answer.
>>So, from the "there are no dumb questions" department...
>>
>>I use a multiband MFJ vertical for an antenna (it's all I have space and
>>neighbor tolerance for), and a small antenna tuner with it to broaden its
>>bandwidth. The antenna is designed for 40m through 10m, plus 6m and 2m,
>>EXCLUDING the WARC bands.
>>
>>When I initially set-up my station, the only short coax patch cables I
>>had with PL-259s on them were made of RG-58 coax, but since it was only a
>>couple of 2' cables, I used them anyway. All seemed to work well enough,
>>and the tuner also allowed me to match with low SWR on the WARC bands in
>>addition to the others. While I know the antenna is far less than optimal
>>on the WARC bands, I've managed to work some DX with this set-up on 12m
>>with relative ease.
>>
>>I recently made new patch cables using RG-8 foam core coax, cut to the
>>exact lengths I needed. Not surprisingly, the configuration of the three
>>tuner knobs was now different to achieve low SWR. But suddenly, I can no
>>longer successfully match on 12m at all. The lowest SWR I can get there
>>is about 3:1, when I could get it clear down to 1:1 before.
>>
>>Understanding that trying to use this antenna on the WARC bands is
>>probably dumb to start with, I'm still left wondering why I could match
>>it successfully with crappy, inappropriate coax (RG-58), and now I can't
>>when using quality coax connections of the proper type (RG-8).
>>
>>Any thoughts on this are welcome (except those, perhaps, telling me I
>>shouldn't expect it to work to begin with).
>>
>>Thanks.
>>
>>
>>
>>---
>>Alan Bryant, WDØEVX
>>Denver, Colorado
>>http://www.moonworks.com/wd0evx
>>
>>
>>--
>>FAQ on WWW: http://www.contesting.com/FAQ/tentec
>>Submissions: tentec@contesting.com
>>Administrative requests: tentec-REQUEST@contesting.com
>>Problems: owner-tentec@contesting.com
>
>_________________________________________________________________________
>Get Your Private, Free E-mail from MSN Hotmail at http://www.hotmail.com.
>
>Share information about yourself, create your own public profile at
>http://profiles.msn.com.
>
>
>--
>FAQ on WWW: http://www.contesting.com/FAQ/tentec
>Submissions: tentec@contesting.com
>Administrative requests: tentec-REQUEST@contesting.com
>Problems: owner-tentec@contesting.com
>
--
FAQ on WWW: http://www.contesting.com/FAQ/tentec
Submissions: tentec@contesting.com
Administrative requests: tentec-REQUEST@contesting.com
Problems: owner-tentec@contesting.com
|