[TowerTalk] measuring cable loss with an MFJ 259B

Jim Lux jimlux at earthlink.net
Wed Jun 14 15:22:50 EDT 2006

At 12:13 PM 6/14/2006, Pete Smith wrote:
>It's the latter, and I bet you're right about the variations in accuracy 
>with impedance.  The manual states "...The impedance meter is disabled in 
>this mode."  Perhaps all they mean is the front panel meter.  I'm not 
>clear on just what is being measured in this mode.  My old Autek RF-1, if 
>I remember correctly, required you to sweep for lowest X, and then 
>converted R into attenuation through a formula.  Alas, the manual went 
>with the meter.

rather than sweeping the cable with it open, you might try sweeping it with 
a known mismatch at the other end (e.g. two 50 ohm loads in parallel with a 
"T"). that way, the match at the meter will always be reasonably close to 
the design impedance of the meter, and you don't get all those weird 
anomalies at 1/4 and 1/2 wave multiples.

25 ohms would be a 2:1 VSWR, which is a return loss of 9.54 dB.  If you 
actually measure a return loss  of, say, 20dB, then you'd know the cable 
has a loss of about 5.3 dB.

Actually, you could probably work out a nifty equation/table that would 
take "measured Z (or VSWR) with a 25 ohm load" and turn it into cable loss.

>73, Pete
>At 02:55 PM 6/14/2006, Jim Lux wrote:
> >Are these swings with the frequency held constant? Or wavyness as the 
> cable is swept in frequency.
> >
> >Part of it might be that the bridge in the 259 has better accuracies at 
> some impedances than others.  As you sweep the frequency, the impedance 
> being measured varies from a short to an open, with varying amounts of 
> reactance.

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