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Measuring Coax Loss

 

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I've just assembled my FA-VA5 and it is an excellent device. However, I am a bit confused about using the FA-VA5 to measure coax loss. In the "S" mode I am seeing "RL" reflection loss and also "ML," which the manual describes as "Matched Line."

If "RL" indicates the coax loss (with one end of the cable open), I am seeing more than double the amount of loss that I would expect. For example, with a calibrated wattmeter in my station I set the transmitter output to 15 W on 20 meters. With the same wattmeter at the antenna, I measure 11 W, or about 1.3 dB of loss, which is what I would expect. When measuring the same cable, with one end open, at the same frequency with my FA-VA5, however, the "RL" is indicating a little more than 3 dB and the "ML" indicates 6 dB.

Perhaps I am not using the analyzer correctly, or I may be misunderstanding RL and ML.

Steve
 

Hi Steve
Welcome and nice to hear from you.

That looks like a normal measurement to me. We have to keep in mind, that both the forward wave and the reflected wave are attenuated by the cable loss. Therefore, the transmission loss is half of the return loss. If you take a 6-dB pad for example, it will show 12 dB return loss.
The difference between your power measurement and the FA-VA5 could be the normal effect of measurement accuracy.

73
Dieter, VK3FFB
 

Had another look. "RL" indicates return loss or 20 x log /S11/. With reference to my earlier response, the cable loss is the return loss divided by two. Or: cable loss = half of peak return loss.
ML is the loss caused by mismatch and ML then does not apply when determining cable loss via a return loss measurement. ML is called "matching loss" in the handbook but people & books also use the term "mismatch loss" or transmission loss caused by mismatch.
For illustration purposes, I am including a graph obtained from circuit simulation. The graph confirms the above said and refines it insofar as a usually small return loss ripple is concerned. The latter is caused by the frequency dependent loss distribution on the mismatched (open ended) cable.
73
Dieter, VK3FFB
 

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That is very helpful, Dieter. When I divide the RL indications by two, the results are almost exactly what I am measuring with my wattmeter.

?

Steve, WB8IMY

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From: [email protected] <[email protected]> On Behalf Of Dieter VK3FFB (group owner)
Sent: Tuesday, March 24, 2020 8:14 PM
To: [email protected]
Subject: Re: [fa-va5-users] Measuring Coax Loss

?

Had another look. "RL" indicates return loss or 20 x log /S11/. With reference to my earlier response, the cable loss is the return loss divided by two. Or: cable loss = half of peak return loss.
ML is the loss caused by mismatch and ML then does not apply when determining cable loss via a return loss measurement. ML is called "matching loss" in the handbook but people & books also use the term "mismatch loss" or transmission loss caused by mismatch.
For illustration purposes, I am including a graph obtained from circuit simulation. The graph confirms the above said and refines it insofar as a usually small return loss ripple is concerned. The latter is caused by the frequency dependent loss distribution on the mismatched (open ended) cable.
73
Dieter, VK3FFB
 

Great. Glad to be able to help.
73
Dieter, VK3FFB
 

Dieter,,

Hi - my FA-VA5 kit working well after 3 years.?

In the past I have only used it for measuring SWR and it has been fine.

I think that I now have problems with my coax feeds to my 4 * 9 2m H/V array so want to check the losses in the coax. And if necessary replace them.

Which value do I take for the coax loss? RL or ML? I am in S mode (144.130 mhz).

Sorry if this seems like a stupid question but I am not an electronics engineer!!!

Thanks, Peter G4URT
 

Hi Peter,

sorry for the delay.

For measuring cable loss, you can use RL (return loss). You disconnect the cable at the antenna side and?leave the cable unterminated. The RL you measure is then twice the cable loss or

Cable Loss = 0.5 x RL

This is so because the forward wave and the reflected wave are attenuated. In a way it is like having twice the actual cable length :)

I hope this helps.

73
Dieter, VK3FFB
 

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Dieter,


Thank you for the reply and information.


I have finished measuring the coaxial cable RL for receive side of my H/V 2m array.


RL * 0.5 makes the results much better and all looks good now!


Again, many thanks,


Regards, peter G4URT




------ Original Message ------
From: "Dieter VK3FFB (group owner)" <dpelz@...>
To: [email protected]
Sent: Wednesday, 1 Mar, 23 At 05:43
Subject: Re: [fa-va5-users] Measuring Coax Loss

Hi Peter,

sorry for the delay.

For measuring cable loss, you can use RL (return loss). You disconnect the cable at the antenna side and leave the cable unterminated. The RL you measure is then twice the cable loss or

Cable Loss = 0.5 x RL

This is so because the forward wave and the reflected wave are attenuated. In a way it is like having twice the actual cable length :)

I hope this helps.

73
Dieter, VK3FFB
 

Most welcome.
Glad it worked out well.

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