Does anyone know how to calibrate this meter. There are three
adjustments on the back, (10w, 100w, and 1000w)

Rocky-

What is a Palomar 500 Meter? If it is an RF power meter, then you appear to
have answered your own question.

If you are asking for a way to use those adjustments, you obviously need an
accurate power meter or standard, an RF power source and a dummy load good for
the maximum power.

Do you have an oscilloscope with a built-in calibrator? If you know its rated
bandwidth, you can use it to measure the voltage of a somewhat lower frequency
signal, to sufficient accuracy to be within about a dB. If the scope is DC
coupled, you could use a DC voltage to calibrate it against an accurate DC
voltmeter.

Then, the scope would be connected by a short cable to the dummy load using a
co-ax "T". As long as you stay below the scope's upper frequency limit,
measurements should be accurate. Just remember that you are looking at
peak-to-peak voltage, which is 2.83 times the RMS value.

73, Fred, K4DII