[TowerTalk] Measuring Coax Loss

[Gary Schafer]

Hi Rick,

I don't think that I have used the method that you describe. What happens
when the cable length approaches a quarter wave length? Or is this only
valid at a small fraction of a wave length?

73
Gary  K4FMX

> 
> If I wanted to accurately measure the loss on a 10 ft piece of cable,
> I would short circuit one end.  I would put a TEE on my signal
> generator front panel connector and connect a power meter sensor
> to the side port of the TEE.  Next, I would note the power meter
> reading of the sig gen output.  Then, I would then drive the unshorted
> end
> of the 10 ft cable with the third port of the TEE.  The cable loss can
> be
> very accurately calculated by the difference between the power meter
> reading with vs without the coax under test attached.  The lower the
> power
> meter reading, the lower the loss of the cable.  For example, if the
> loss
> of the 10 ft cable is 1% (about 0.043 dB), the power meter will read
> 28 dB lower when loaded by the coax than unloaded.
> 
> You don't need any heroically accurate instruments to make this
> measurement.  You could almost do it using the S-meter on a radio if you
> previously checked the S-meter characteristics.  A 3 dB error in the dB
> measurement only represents a 10% error in the cable loss, due
> to the way the math works.  You can also use a variable attenuator
> on the sig gen to make the measurement.  In that case, the
> S-meter is only used at one setting and its calibration is irrelevant
> (this is a well known technique).
> 
> Rick N6RK
> 
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