[TenTec] Antenna Matching Weirdness
Dan
tacquire@earthlink.net
Thu, 19 Oct 2000 19:02:11 -0700
Perhaps you could also try making coil of about 10 turns of your cable about 12
inches across in a loop at the base of the antenna. I know this is effective in
keeping the rf where it should be. It's a essentially an RF choke. My butternut
beam specifies to do this with the coax and I did. Guess it worked :)
John K4WJ wrote:
> 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
> >>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
> >>
> >>
> >>--
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