Hi Mitch: This subject probably brings out more misconceptions
and "old-wives-tales" than any.
And, in each case, there is a bit of truth.
All transmission lines have a characteristic or surge impedance
value. But - since the system impedance is set by the termination,
VSWR can appear on any line. When you have VSWR, the rf on
the line will have voltage and current peaks and nulls, Depending
on where you put your meter in the line, you can have high swr
or low swr.
If however, your termination (Antenna) presents a load that equates
the characteristic impedance of the transline, with no or very
little reactance, then the line will be flat and changes in the position
of your meter, or the length of the line will have no effect.
Bottom Line? Your antenna is not presenting a good load to your
transline! Is this a problem? Probably not of any consequence unless it
indicates a problem with your antenna.
Solid state devices are fairly sensitive to load matches, and it could
cause your transceiver to cut back on the power it delivers, but that is
not a great problem as the difference between 100 watts and 80 watts is
about a dB. And, losses in the line go up too, but at hf, most coax
still won't have a great loss.
Now - RG-213- be careful - that cable is not designed to be used with
PL-259 connectors. The outer conductor diameter is too small, and
for a good transition, it takes some serious work. If you don't want to
use RG-8 for some reason, then go to RG-214. You can use 213, but
you need to heavily tin the braid, and make a real good solder joint
with some serious heat, or count on burning the connection soon.
Good luck.
Del, KL7HF
-----Original Message-----
From: MitchA@pssch.ps.ge.com <MitchA@pssch.ps.ge.com>
To: tentec@contesting.com <tentec@contesting.com>
To: <tentec@contesting.com>
Date: Monday, January 25, 1999 8:06 AM
Subject: [TenTec] Cable Lengths
>
>Hi, Guys:
>
>I have been rearranging the station over here to fit in some new
equipment -
>actually the 1208A 6m Transverter - and have noticed that the SWR and
>observed power output changes as I add lengths of coax. Now, it is 100' to
>my HF antenna and I have always had about 100W output from the Omni V -- I
>can't see a resistance problem with 4 feet of extra coax (I tried multiple
>pieces). BUT...I do remember listening to a Collins net one time where
they
>were discussing ideal lengths of coax that the factory literature
>recommended in order to yield the best SWR. I also remember learning about
>wavelength and transmission line lengths, back in the dark ages of
>engineering school. Would any of you be able to refresh us on this, as it
>relates to 50-ohm coax, such as RG-213 ? Thank you.
>
>73 --
>
>Mitch KG9DB
>
>--
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>
>
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