In order to determine how efficient your transmission line is, you need two numbers. The first is the RF power output from your transmitter. The other number is the RF power at the far end of the transmission line.

Let's say you are using 50 ohm coax cable. Connect your radio to the cable and at the other end of the cable connect a 50 ohm dummy load. The dummy load must of course be able to handle whatever power your radio is delivering to the cable. Also verify that you do indeed have a 1 to 1 match. Next insert the wattmeter between the rig and the cable. Take a power reading an write down that number. Now, move the wattmeter to the end of the cable between the cable and the dummy load. Take another reading. These are the two numbers you will require.

Enter the measured RF output of the transmitter.
( input end of the transmission line )
   Watts

Enter the measured RF output at the end of the feed line.
   Watts

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Your feed line has a      dB loss
Your line is      efficient

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