[TowerTalk] measuring cable loss with an MFJ 259B

[Bob Newton W5RN]

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 at contesting.com>
To: "Jim Lux" <jimlux at earthlink.net>; <towertalk at 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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