The idea of using RG!! pigtails to fine tune 75 ohm coax length is great!
There may be a few tunees out there puzzled by how all this is done, so a
couple of comments: You old timers can skip over to the next message.
Per previous postings you have seen that line input impedances are repeated
at half wave intervals as the signal travels down the transmission line. If
your antenna "looks like" 50 ohms, that is what you will measure at the half
wave points. At other points it will vary above and below 50 ohms by up to
75/50 (the theoretical vswr) times 50 ohms and 50/75 times 50 ohms.
So, how do you find this magical length? There are several ways but to me one
of the easiest is through the use of an antenna analyzer such as the MFJ 259.
Remembering the repeat at half wave intervals above, if the far end of the
line is left open (disconnected), the near end will also appear open or
disconnected. ie: it acts as if it is not there! To test the line , the
antenna analyzer is terminated with a tee connector and 50 ohm load on one
side of the tee and the line under test on the other leg of the tee. As the
frequency of the antenna analyzer is varied you will observe frequencies that
the vswr will drop to a very low value. These are the frequencies that the
line is a multiple of a half wave. A purist will remark that there is an
error caused by the half of the tee connected to the line and that is correct
but a small error and easily compensated by trimming the line a bit low in
frequency.
This same method also works well using a transceiver, vswr meter and dummy
load but beware that your frequency coverage is restricted and you may be
operating the transmitter into a very high vswr. A noise bridge is a third
method that uses the receiver frequency calibration for determining half wave
points.
The CATV cable can be quite a bargain as often it is available for free. I
have even seen it laying by the roadside apparently left for scrap by the
installers.
Happy trimming de Gerald Williamson, K5GW, Owner & General Manager, Texas
Towers (yes we have 259 analyzers in stock)
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