[TowerTalk] Finding roof beams, determining impedance of coax cable.

Flanders, Jim jaf00@aag.com
Thu, 26 Jun 1997 14:09:22 -0500


All you have to do is change the resistor value to 72 ohms.  Actually, 
a variable 
Resistor would be great if you can find a noninductive variable.
Jim Flanders
 W0ooG


-----Original Message-----
From:	Edward W. Sleight [SMTP:k4sb@worldnet.att.net]
Sent:	Friday, June 27, 1997 3:17 AM
To:	David O Hachadorian
Cc:	towertalk@contesting.com
Subject:	Re: [TowerTalk] Finding roof beams, determining impedance of 
coax cable.

David O Hachadorian wrote:

> >Take a half wave length of cable and terminate it with a 
non-inductive
> >
> >resistor such as 50 ohms.
> >Place a swr bridge at the TX end.  Tx into the cable with just 
enough
> >power to get a reading on the meter.
> >If the swr is 1 to 1 the cable is the impedance of the resistor. 
 Note
> >
> >that the MFJ antenna analyzer will also
> >do this for you, but not all of us have one, and we all should have 
a
> >swr bridge and a 50 ohm dummy load.
> 
> Actually, it should be almost any length of cable EXCEPT a half
> wave. A half wave will present the 50 ohm termination,
> regardless of the cable's characteristic impedance.
> 
> A better way to do the test is to hook up the 50 ohm resistor to
> a random length of cable and then vary the transmitter frequency
> over a wide range of frequencies. If the swr is flat over a wide
> range of frequencies, then it is 50 ohm cable. If it's not 50
> ohm coax, experiment with various 2 watt carbon terminating
> resistors until you find the proper one that makes the swr curve
> flat.
> 
> Dave Hachadorian, K6LL
> k6ll@juno.com
> 
Sorry, but this won't work either on cables other than 50 ohms, UNLESS
the bridge is set up for 72 ohm operation. ( or whatever the subject
coax actually is )

You might flatten the SWR curve in sucha a manner, but in actuality, 
all
you are doing is another version of changing the length of the coax.
And all you are doing here is fooling the bridge ( or transmitter )
which is actually OK, unless you're dealing with very high SWR figures.
( what you are doing in such a case is trying to find a point on the
line where the outgoing wave is 180 degrees out of phase with the
reflected wave. )

Best

Ed

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