Just a little note on this thread as it regards converting the 75 ohm CATV
cable to 50.
I,m sure all of you are aware than an electrical half wave of transmission
line, or any
multiple thereof, will repeat the impedence it sees on one end at the other
end, regardless of
the impedence of the line. In other words, a .5 electrical length of line of
any impedence
may be connected to a 50 ohm load at one end, and it will measure 50 ohms at
the other.
All that is really necessary is to ensure that the CATV line is a multiple of
1/2 wave
electrically and the cable will repeat the antenna impedence at the shack end.
As a matter of
fact, this is true of any impedence cable, open wire line, ect. While you might
be unfortunate
enough to have to add about 100' at 3.8, ( if you came up inches short ), it's
only about
50+ feet for 40, 25' at 20 and so forth. (using .8 for a velocity factor.))
In essence, 25' +/- at 20 rolled up in a coil outside the shake adds
practically nothing in
terms of db loss, and is one heck of a lot cheaper than transformers, the
necessary connectors
ect.
This principle is also true even if the .5 ( or multiple ) wavelength of line
is made of
coax of different impedences. In other words, you could have 50 feet of 50 ohm
line, joined
to 50 feet of 75 ohm line, and it will also act as is the line were an
identical section of
cable.
Just one word of caution; published velocity factors cannot be trusted. I have
seen VFs
vary by as much as 10% on the same type line. If you've got the MFJ 259, it's a
snap to
get the line right in almost no time....but remember, while you're doing that,
see what the
effect of cutting about .75" of cable off the end does....because the PL-259 or
whatever,
is going to substract that. If you measure 20khz for 1", cut the line for 20khz
above where
you want it.
73
Ed
-------------------------------------
Name: Ed Sleight
E-mail: k4sb@avana.net
To: <towertalk@contesting.com>
Date: 3/18/97
Time: 5:35:15 PM
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