[TowerTalk] Mystery hardline

[TexasRF at aol.com]

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 at 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 at hotmail.com>
Subject: [TowerTalk] Mystery  hardline
To:<towertalk at contesting.com>
Message-ID:<BAY129-W16905245A9B5BD0978CBAE8490 at 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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