[TowerTalk] measuring cable loss with an MFJ 259B
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.
>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
More information about the TowerTalk