[TowerTalk] Measuring coax loss with MFJ-259B

[Robert Chudek]

Using a Bird meter and comparing power levels at both ends is one method to determine coaxial cable losses. But this is impractical in many circumstances.

Here's a link to an overview of how coaxial loss can be measured from one end of the cable:  http://www.us.anritsu.com/downloads/files/11410-00320.pdf  They even have a sense of humor when suggesting where you can solve the formula they posted.

This article talks about taking readings between the peaks and valleys of the VSWR measured within the frequency range of interest. But in this commercial application, the impact of this is not very visual because of the limited bandwidth they are testing. The main take-always from this article is the measurement concept and the formula.

I purchased an AEA VIA HF Analyst a couple years ago and made measurements on all kinds of things as I rebuilt my antenna systems. One graph is a 500 foot roll of RG-8x coaxial cable. I plotted the VSWR signature when scanned from 1.5 MHz through 50 MHz. The peaks and valleys are very prominent on this graph and you will see that, depending upon what frequency you measure the VSWR, it will vary. The Return Loss chart looks similar.

You can download a copy of the SWR and Return Loss graphs from here:  http://chudek.aberon.net/  Select the "AEA HF Analyst Charts" album and then the "Coaxial Cables" album. The VSWR graph... is a single page MS Word document (70-Kb) with the VSWR and Return Loss charts.

I doubt there is anything "wrong" with the MFJ-258. But determining cable loss is not a simple "one shot" measurement using that device.

73 de Bob - KØRC in MN


---------------Original thread---------------

Message: 8
Date: Fri, 5 Oct 2007 21:31:07 -0400
From: "jeremy-ca" <km1h at jeremy.mv.com>
Subject: Re: [TowerTalk] Measuring coax loss with MFJ-259B
To: "TowerTalk" <towertalk at contesting.com>, "Pete Smith"
<n4zr at contesting.com>
Message-ID: <013601c807b8$935fb8c0$6500a8c0 at KITTYMA123>
Content-Type: text/plain; format=flowed; charset="iso-8859-1";
reply-type=original

I got so disgusted with the uselessness of my 259B that I dumped it on Ebay.
Back to the old faithful Birds with my son in the shack driving a cable with 
an exact 10W on 440 MHz when I signal him. The Birds are as close to a 
perfect match as I could get and I can refer to the recorded as installed 
loss for any cable. With cable runs up to 450' I like to keep an eye on 
losses.

Carl
KM1H


----- Original Message ----- 
From: "Pete Smith" <n4zr at contesting.com>
To: "TowerTalk" <towertalk at contesting.com>
Sent: Friday, October 05, 2007 5:43 PM
Subject: [TowerTalk] Measuring coax loss with MFJ-259B


>I recently replaced my two main 190-foot feedlines to the tower with
> LDF4-50A hardline.  On the shack end, there is about 50 feet of fairly old
> RG-213 (12 years or so).  Today I used my 259B to measure the combined 
> loss
> at 30 MHz, with the far end open-circuited.  I got 1.3 dB on one cable and
> 1.4 on the other, which correlates very nicely with the rated attenuation
> of the cables.  However, there was one strange thing - strange to me,
> anyway.  with the 259B in coax loss mode, I found that relatively small
> frequency excursions would cause a relatively large change in the reported
> loss - going, for example, from 1.3 dB at 30 MHz to 1.6 at 29.6 MHz - and
> of course, if anything, you'd expect the attenuation to go down with
> decreasiong frequency.  The variation in reported loss is also cyclical as
> I tune further down.
>
> The manual for the 259B doesn't say anything about this sort of frequency
> sensitivity, but I remember that with my old Autek it was necessary to 
> tune
> for minimum R and then apply a formula to determine loss.  Is this just an
> omission in the MFJ manual, or something I don't understand, or maybe a
> problem with the unit itself?
>
> 73, Pete N4ZR
> The World HF Contest Station Database
> Full details on 3300 contest stations
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