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How to Measure return loss ?

 

Hello anyone have any suggestions on how to measure retun loss on 1 5/8 fxl-1873 50 ohm hardline when only 1 end accessible, velocity factor is around 88% using a nano vna
Thanks Terry N8JX
 

Are you asking about adapting the nanoVNA to the hardline connector?
Otherwise, since return loss is a one-port measurement, and you have one port accessible, I do not understand your question.
Since you mention velocity factor, are you trying to locate a discontinuity?
I am puzzled.
73, Don N2VGU
 

Thanks for the reply Don,
I am trying to determine if the buried hard line is bad, and what the rtl is at 50 mhz which I believe should give me a good indication if the hard line is in spec. do I divide the rtl in 1/2 since I only need to measure in one direction. Or is there a better way to check if it is bad?
Thanks Terry
 

Return loss is loss going both ways. Don't divide by two.

On 2024-07-22 05:47, Terry N8JX wrote:

Thanks for the reply Don,
I am trying to determine if the buried hard line is bad, and what the rtl is at 50 mhz which I believe should give me a good indication if the hard line is in spec. do I divide the rtl in 1/2 since I only need to measure in one direction. Or is there a better way to check if it is bad?
Thanks Terry
 

Hi Terry,
Some of the responses you are getting folks don't understand what you are trying to do.
It is correct Return Loss for looking at SWR or Impedance is two way loss, but you are trying to measure the insertion loss of the coax using the return loss function. For this measurement you DO divide the return loss measurement in half.
This technique only works if you have an open or a short at the other end of the cable. The open or short gives you total reflection which is required to measure the cable loss. So you need to be able to disconnect the other end of the hardline and leave it open.
Then measure the return loss. divide by 2 and compare that loss with what the spec sheet says the loss should be.

Good Luck 73 Greg WA1JXR
 

A return loss bridge. I built my own, decades ago. It was for 75 Ohms and
fixed frequency. I used it to troubleshoot some horrendous MATV systems in
local school systems. They had let their electricians install the cheapest
crap and treat it as electrical work, rather than as RF. A lot of coax was
damaged during installation, and some was run in rain gutters on the roof.
The taps weren't sealed, and the ice that formed crushed them. All you need
is a signal source, the bridge and an RF voltmeter for basic tests,
Do a search for "return loss bridge schematic" and you'll find plenty of
information. Mine was built from a '60s ARRL handbook design.

On Sun, Jul 21, 2024 at 11:17 AM Terry N8JX via groups.io <n8jx100=
gmail.com@groups.io> wrote:

Hello anyone have any suggestions on how to measure retun loss on 1 5/8
fxl-1873 50 ohm hardline when only 1 end accessible, velocity factor is
around 88% using a nano vna
Thanks Terry N8JX
 

Short the cable and measure the SWR. Then convert to return loss and divide by two. https://www.everythingrf.com/rf-calculators/vswr-to-return-loss-calculator
 

Set the frequency WELL below the electrical 1/4-wavelength (in the cable).
No need to short the far end of the cable. You can make the measurement
with the far end either open or shorted. However, be sure the far end of
the cable is not connected to anything. Then do a reflection measurement
with return loss selected on the NanoVNA. At the measurement end, connect
the center conductor to the center pin of the source port and the shield to
the backshell of the same port on the VNA. Divide the resulting number by
two. That will reflect loss for one pass of the RF energy down the cable.
Ideally and with no losses, it should measure infinite as the SWR (ideally)
should be infinite. Any deviation from infinite reflects cable losses.

Dave - WØLEV

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On Mon, Jul 22, 2024 at 4:40 PM Terry W7AMI via groups.io <terry.w7ami=
gmail.com@groups.io> wrote:

Short the cable and measure the SWR. Then convert to return loss and
divide by two.
https://www.everythingrf.com/rf-calculators/vswr-to-return-loss-calculator





--

*Dave - WØLEV*


--
Dave - WØLEV
 

Hi Terry,

I have measured the performance hundreds of older coaxial cables.
I have found that return loss with the end of the coax is mismatched
into an open or shorted load detects deterioration only when it
has already reached an advanced state of deterioration. Apparently
the onset of coaxial calbe deterioriation is masked by the much
greater coaxial cable loss when using an extremely mispatched termination.

in my experience return loss is much more able to detect the onset of
coaxial cable deterioriation when the end of the cable terminated in
its charactersitic impedance.

Like new hardlines such as LDF4-50A or AVA4-50 have greater than 24 dB
return loss at frequencies up to well above 1 GHz. As the cable
deteriorates its 24 dB return loss drops to lower and lower frequencies
and the return loss measurements begin to show ripple vs. frequency.

Deterioration of solid dielectric flexible coaxial cables such as RG-213
is almost always caused degradation of the cable shield. The onset of
deterioriation can be reliably detected by simple shield resistance
measurements. Like new RG-213 has shield resistance of about 0.1
ohms per 100 feet. In my experience, return loss measurements
don't reliably detect RG-213 shield deterioriation until shield
resistance is about 0.5 ohms per 100 feet or more.

73
Frank
W3LPL

----- Original Message -----
From: "Terry N8JX" <n8jx100@...>
To: nanovna-users@groups.io
Sent: Monday, July 22, 2024 3:47:57 AM
Subject: Re: [nanovna-users] How to Measure return loss ?

Thanks for the reply Don,
I am trying to determine if the buried hard line is bad, and what the rtl is at 50 mhz which I believe should give me a good indication if the hard line is in spec. do I divide the rtl in 1/2 since I only need to measure in one direction. Or is there a better way to check if it is bad?
Thanks Terry
 

A couple tricks here:
1) if you've got an antenna hooked up that's moderately narrow band (match wise), do your return loss measurement well away from the design frequency.
2) TDR (computed from frequency swept data) is your friend, because it lets you see the loss up to the antenna (which you will almost certainly see)

-----Original Message-----
From: <nanovna-users@groups.io>
Sent: Jul 22, 2024 10:16 AM
To: <nanovna-users@groups.io>
Subject: Re: [nanovna-users] How to Measure return loss ?

Hi Terry,

I have measured the performance hundreds of older coaxial cables.
I have found that return loss with the end of the coax is mismatched
into an open or shorted load detects deterioration only when it
has already reached an advanced state of deterioration. Apparently
the onset of coaxial calbe deterioriation is masked by the much
greater coaxial cable loss when using an extremely mispatched termination.

in my experience return loss is much more able to detect the onset of
coaxial cable deterioriation when the end of the cable terminated in
its charactersitic impedance.

Like new hardlines such as LDF4-50A or AVA4-50 have greater than 24 dB
return loss at frequencies up to well above 1 GHz. As the cable
deteriorates its 24 dB return loss drops to lower and lower frequencies
and the return loss measurements begin to show ripple vs. frequency.

Deterioration of solid dielectric flexible coaxial cables such as RG-213
is almost always caused degradation of the cable shield. The onset of
deterioriation can be reliably detected by simple shield resistance
measurements. Like new RG-213 has shield resistance of about 0.1
ohms per 100 feet. In my experience, return loss measurements
don't reliably detect RG-213 shield deterioriation until shield
resistance is about 0.5 ohms per 100 feet or more.

73
Frank
W3LPL


----- Original Message -----
From: "Terry N8JX"
To: nanovna-users@groups.io
Sent: Monday, July 22, 2024 3:47:57 AM
Subject: Re: [nanovna-users] How to Measure return loss ?

Thanks for the reply Don,
I am trying to determine if the buried hard line is bad, and what the rtl is at 50 mhz which I believe should give me a good indication if the hard line is in spec. do I divide the rtl in 1/2 since I only need to measure in one direction. Or is there a better way to check if it is bad?
Thanks Terry

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