[TowerTalk] Measuring Coax Loss

[Richard (Rick) Karlquist]

I also have an AIM-4170 network.  It is considerably better than the 
MFJ-259B.  It would have allowed additional resolution and accuracy
in a reflection mode measurement of loss.

If you still want to do a through measurement, it would probably be
better to use a power meter than a spectrum analyzer at the far end,
since a power meter is more accurate than a spectrum analyzer, plus
being smaller and cheaper.

It is also worth noting that when you start talking about 0.1 dB
or even 0.01 dB differences in insertion loss, you have to start
worrying about whether the characteristic impedance of the coax
is exactly 50 ohms or not.  Otherwise, there will be a "mismatch error".

Notice that at 50 MHz the loss was 2.07 dB and at 3 MHz, the loss was
0.62 dB.  If there were no "low frequency effects", the loss at 3 MHz
should have been about .50 dB.

Rick N6RK

Jim Brown wrote:
> N6RK suggested that the MFJ259B would be a good way to measure coax loss with 
> a reflection measurement at one end only. I responded I was looking for more 
> precision than that. Several years ago, I carefully calibrated my 259B using 
> W8JI's procedure. Today, I compared loss measurements of a 208 ft length of 
> Commscope 2427K (the plenum version of 3227) made both with my 259B and with 
> my HP generator/spectrum analyzer setup. 
> 
> The HP data has one digit better precision -- the spectrum analyzer readout 
> is 0.01dB, while the 259B reads 0.1 dB. Within those limits, the MFJ yielded 
> results that were usually within rounding error of the HP setup between 10MHz 
> and 150 MHz, which is where I stopped measuring (because that's the highest 
> frequency I cared about for my applications). For example, at 50, 100, and 
> 150 MHz the HP numbers were 2.07 dB, 2.84 dB, and 3.37 dB; the MFJ yielded 
> 2.1 dB, 2.8 dB, and 3.3 dB. At 3 MHz (the lowest frequency I could measure 
> with the MFJ), the numbers were 0.62 dB and 0.6 dB. 
> 
> For obvious reasons, you really need the greater precision of the HP setup to 
> get good data for frequencies and lengths where the loss is relatively small.
> 
> 73,
> 
> Jim K9YC
> 
> 
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