Topband: RG-179 Coax

[Richard (Rick) Karlquist]

On 2/14/2020 1:04 PM, Jim Brown wrote:
> On 2/14/2020 11:40 AM, Fred Moeves wrote:
>> Can you figure out impedance of coax with a device like the Rig Expert?
> 
> Yes, if it's a model that can be calibrated, and if the calibration is 
> properly done, and if it's a model that can export measurements to a 
> computer. Calibration requires of Standard Short, Open, and Load parts. 
> Once you've calibrated, sweep a length of the coax with the far end open 
> and again with it shorted. Export the data to a computer, then import it 
> into AC6LA's free ZPlots Excel spreadsheet and follow his instructions. 
> For the measurements, the coax must be connected at precisely the same 
> point as where the calibration was done.
> 
> Before attempting this, study parts of the manual for the equipment that 
> talks about the "measurement plane."
> 
> 73, Jim K9YC
> 

This is the "hard" way to measure Zo.  Been there, done that.

The easy way is to attach a small 100 ohm (non-wire-wound) pot to one 
end of the coax and attach the other end of the coax to any old 
impedance meter.  Even an MFJ-259 will work.  Now adjust the pot until 
you see a constant impedance across all frequencies.  Don't worry about 
what the impedance meter says this impedance is.  Instead, measure the 
DC resistance of the pot.  More accurate.  Foolproof.

Rick N6RK