Saturday, January 7, 2012

SWR: Coax Check

Even a new coaxial cable should be checked.  The coax and connectors may look flawless, check fine with an Ohm meter and still have a serious mismatch, i.e. high SWR. 
You will need a 50 Ohm dummy load, a SWR meter, a known good coax jumper, Ohm meter, and transmitter.

Ohm meter tests:
Check the cable from the center pin on one connector to the center pin of the connector on the other end.  These should be a short circuit, less than one ohm[1].  Next check the shield from the outside of one connector to the other.  This should also be a short circuit.  Now check from the center pin to the shield using either connector.  This should be an open circuit, no reading, on the highest Ohms scale[2].  Be sure that you are not measuring you fingers by coming in contact with the meter probe tips or any part of the connectors when you make this measurement.  Stay off the metal.  When these checks have been made and the connection are all correct you may proceed to the SWR check.

SWR meter tests:
Connect the dummy load to one end of the coax you want to test.  Connect the other end of the coax to the Antenna connection on the SWR meter.  Connect the jumper to the Transmitter connection on the SWR meter.  Connect the other end of the jumper to the transmitter’s output connector.
Turn the transmitter power to minimum and set the mode to CW, AM or FM.  SSB will not work.  Now transmit and read the SWR.  It should be no more than 1.1 to 1 at the frequency that you intend to use the coax.   

If the SWR is higher, carefully do a visual check of the connectors and run the Ohm meter test while wiggling the connectors.  This is especially true for a home build coaxial cable.  It is very easy to short coax when installing connectors of any type.  A cable may not be shorted or open but still have SWR problems.

[1] - 75 feet of RB-8x measured 0.6 Ohms on both the shield and the center conductor.  When the leads are shorted, they measure 0.3 Ohms.  That means the coax is 0.3 Ohms.  This is acceptable
[2] - The meter I use has a 20 MOhm scale.  It has no reading on the same piece of coax.  This is fine.  You can check the meter by wetting a finger on each hand and measuring your resistance to assure that the meter is working.  This reading will vary widely and will even change depending on how tightly you hold the meter lead tips.

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