Measuring Ham Radio Coax loss.
I had a few requests asking how I measured the Coax loss on my antenna systems here at Radio Station W7DTG, so I thought I would put in a quick blurb about my super duper awesomely fantastic little old Mighty Fine Junk MFJ259B Antenna Analyzer.
According to the manual:
The second main (or opening) mode is “Coax Loss”. Access this mode by turning the MFJ-259B on and stepping to the Coax Loss display. In this mode, the MFJ-259B LCD indicates frequency and coax loss in dB. The impedance meter is disabled in this mode. This mode was designed to measure 50 ohm cables, but measures differential mode loss in many types of 50 ohm transmission line transformers and choke baluns, as well as loss in 50 ohm attenuator pads.
CAUTION: Do not measure conventional transformers, or attenuators and coaxial cables, with impedances other that 50 ohms. When making measurements, the opposite end of the device under test must have an open circuit, a short circuit, or a pure reactance for termination. Any loss resistance will make attenuation appear worse than it actually is.
To measure loss:
- Connect the MFJ-259B to the 50 ohm cable, attenuator or transmission line type balun or transformer you wish to measure. Be sure the distant end of the component you are testing is not terminated in any resistance.
- Turn the MFJ-259B on. After the display reaches the opening measurement function, press the mode switch once.
- The display should momentarily flash “Coax Loss”
- Read the loss in dB at any frequency this unit covers.
As you can see, the MFJ-259B is an important tool to have in any shack or work bench. It can also be used to tell where a fault line is in a run of 50 Ohm Coax AND be used to measure antennas. It can also be used to tell you how much you should add or subtract from the wire antenna. Using a simple formula you can determine how many feet should be trimmed or added to a chunk of wire to make it resonant.
How cool is that?
73, and see you on the air.
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