The method for the Autuk VA1 is as follows:
From Instructions for the Vector RX Antenna Analyst Model VA1
IMPEDANCE (Z) METHOD
Either open or short the line and find the minimum Z at the nulls. The
cable loss at that frequency is given by:
Min. Loss (dB) = 8.69 X Minimum Z/ Line Impedance
(Note: You need not change the VA1 line impedance for this measurement,
since Z does not depend on line impedance)
For 50 ohm line the loss is
Loss - 50 ohm line (dB) = 0.17 X Minimum Z
For example, if you measure a 4 ohm minimum Z with 50 ohm line, the loss
is 0.68 dB.
The Z method works well for any line impedance. Overall, we recommend
the Z method for its greater accuracy. The SWR method can be used for a
quick estimate.
Please note that all loss values are for a line terminated in the line
impedance (SWR = 1. ) The loss is higher at higher SWRs.
END QUOTE
There is a statement to the effect that an open line is the more
accurate method.
Bob Newton W5RN
Rolling Hills Estates, CA
----- Original Message -----
From: "Pete Smith" <n4zr@contesting.com>
To: "Jim Lux" <jimlux@earthlink.net>; <towertalk@contesting.com>
Sent: Wednesday, June 14, 2006 12:13 PM
Subject: Re: [TowerTalk] measuring cable loss with an MFJ 259B
> 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.
>
> 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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