On Thu, 26 Jun 1997 10:09:07 -0500 "Flanders, Jim" <email@example.com>
>Determining impedance of unknown coax cable. (also handy to determine
>ratio of unmarked balun):
> It's been a long time since I've done this myself (~50 years),
>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
Dave Hachadorian, K6LL
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