Guy, Bob and all, the 50 vs 75 ohm question can be solved with a hand
calculator after taking a couple of measurements; the coax center conductor and
shield diameters. It most likely has a foam dielectric and you can assume a
v.f. of 80% without introducing much error.
Get your handbook out and look up the formula for coax characteristic
impedance and get after it.
The methods of terminating the coax with various resistors and making
measurements does also work but you can't expect meaningful data measuring 75
ohm coax with a 50 ohm measuring system. The coax electrical length will
cause many different readings depending on the frequency.
One method that would remove that uncertainty is to connect the coax in
parallel with a known good load and sweep the frequency until a vswr peak
shows up. You can then take that frequency as one of an odd quarter wave
multiple. Then the known good load can be moved to the end of the coax and
vswr
remeasured. If the coax and load are close in impedance, the vswr will be
low. If they are not close in impedance, the vswr will be elevated by the
ratio of the two impedances squared.
A variation of this test is to simply connect a good load to the coax and
measure the vswr while sweeping the frequency. If the load and coax are
close in impedance, the vswr will be low at all frequencies. If not, the vswr
will vary from 1 to 2.25 and back as the frequency changes. This is assuming
75 ohm coax and 50 ohm load.
While set up, you can also estimated the loss of the cable by leaving it
unterminated and measuring the vswr. The lower the loss, the higher the vswr.
Using VHF for this test accentuates the loss, making it easier to
estimate. Your handbook will have info on how to interpret the test results.
The first method is by far the easiest way to determine the coax impedance!
73,
Gerald K5GW
In a message dated 11/2/2010 9:37:44 P.M. Central Daylight Time,
robert.chudek@gmail.com writes:
Guy,
No, I don't have a clue what it might be. It's kind of like asking "I've
got a set of four black tires with white sidewalls on one side of them. Will
they work on my car?"
But you can test the coax and get a rough idea about the impedance by
putting a 50 ~ 52 Ohm resister at the far end and measuring the VSWR. If it's
1:1 that would tell you it's a 50 Ohm impedance. If it's 1.5:1 that would
suggest a 75 Ohm impedance. Or you could put a 100 Ohm potentiometer at the
far end and adjust it for minimum VSWR, remove the pot, and measure its
resistance. That will be the approximate impedance of the cable.
If you have (or can borrow) an antenna analyzer (MFJ259), you can short
the far end and measure the lowest frequency of the VSWR dip. Then you can
calculate the velocity factor by measuring the physical length compared to
the free-space (calculated) length. This will give you the ratio between the
two (i.e., the velocity factor).
With a couple basic tools, you should be able to determine what you need
to know. It's a good problem to have that will teach you about RF and some
basic measurements. You can Google "measuring unknown cable impedance" and
find more specific details.
73 de Bob - KØRC in MN
Message: 4
Date: Tue, 2 Nov 2010 06:15:28 +0000
From: Guy Molinari<guy_molinari@hotmail.com>
Subject: [TowerTalk] Mystery hardline
To:<towertalk@contesting.com>
Message-ID:<BAY129-W16905245A9B5BD0978CBAE8490@phx.gbl>
Content-Type: text/plain; charset="iso-8859-1"
I picked up some surplus hardline. It has an OD of 5/8" including a
1/16" plastic jacket. The jacket is black and has a green stipe running up 2
sides of the cable.
Any idea of what this stuff is? Specs?
73,
Guy, N7ZG
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