Mitch:
The stuff on the Collins probably had to with the right cable lenght to get a
180 degree total phase shift through the S-line and KWM-2 Pi-networks when
driving a Collins linear. It was a way to minimize distortion products through
the 30L-1 and 30S-1. If you are getting different readings, not to worry, it's
fairly common. All it means is; that your match is not perfect. If you think
back to those school days, there was probably one fact you can recall. The
terminating impedance of a line will repeat itself every half wave. If the
match is perfect, doesn't matter where in the line you look. it will always look
good. If there is some amount of mismatch, you'll start to see different
measurements on an SWR bridge as you look at different parts of the line. Only
way to know for sure is to use a power meter, that measures reverse power and
take the ratio, or get the bridge at the feed point.
Before good power meters were generally available to amateur, a lot of folks cut
their feed line to multiples of half wave lengths. It gave a better chance of
an accurate reading, but with multiband feeds, it's not easy to do. In
microwave work, we can use slotted lines because the wavelength is so small that
we can slide a probe along it to check max & min standing waves. Try that at 20
M.
Bottom line....if the transmitter is happy.. don't worry. You can add a tuner,
or change the line length to make it happy, but the feed point match at the
antenna will still be the same as it always was, unless something was damaged.
Good luck,
George Maier - K1GXT
MitchA@pssch.ps.ge.com wrote:
> 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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