[TenTec] Antenna Matching Weirdness

[George, W5YR]

Alan, I think that yo have answered your own question.

The answer lies not with the condition of the patch cables but with
their length and type. You went from two foot RG-58 solid dielectric
coax to five foot RG-8 foam coax. Not only is the length significantly
different, especially on 12 meters, but the different dielectric
constants of the two cables affects their electrical lengths.

I had a similar experience recently. I terminate two ladderlines (one to
a 20-meter EDZ and the other to a 40-meter full-wave loop) with current
1:1 W2DU baluns to RG-213 coax. With this setup the MFJ 989-C tuner
handles all bands nicely.

I moved the tuner to a different position on the operating desk the
other day and had to add three foot extensions to the coax. After that,
not only were all the 989-C adjustments considerably different, but I
almost "lost" a couple of bands like you did.

I think that as long as you are operating your coax feedline at a
relatively high SWR such that you need your tuner to allow you to
operate on those WARC bands, you are going to have to stick with coax
jumper (and other) lengths that work for you. If the old coax was "too
ratty" then replace it, but with the same type of cable and the same
length. RG-58 is every bit as good as RG-8 for patch cables unless you
are running the legal limit or above.

Keep in mind that with your coax NOT terminated in its characteristic
impedance (Zo) it is acting like an impedance transformer. With your
original setup you just "happened" to have found a set of coax lengths
that presented an impedance range to your tuner that it could handle. 

Also, if that coax run is very long you might be experiencing a little
lost power, but unless the swr is well above 3:1 or so, I wouldn't worry
about it. The ARRL Handbook or the Antenna Book has a chart to show you
how much *more* power you are losing due to the SWR not being 1:1.
Usually it isn't enough to worry about unless it is really high.

Happy hunting!

72/73, George   W5YR - the Yellow Rose of Texas      NETXQRP 6	       
Fairview, TX   30 mi NE Dallas in Collin county	     QRP-L 1373
Amateur Radio W5YR, in the 55th year and it just keeps getting better!
Icom IC-756 PRO #02121 (9/00) Kachina #91900556 (12/99) IC-765 (6/90)

Alan Bryant wrote:
> 
> 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.

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