Background
From time to time, ham radio operators may question whether a section of installed and used coax is still good or significantly below spec and needs replacement.
A very common defect in coax installed outside is ingress of water. The earliest symptoms of water ingress are the result of corrosion of braid and possibly centre conductor, increasing conductor loss and therefore matched line loss (MLL). Any test for this must expose increased MLL to be effective.
Introduction
This article describes a simple but effective test of MLL for coax of known Z0 and length using a suitable one port antenna analyser (or VNA such as the NanoVNA), the nominal Z0 is sufficient to demonstrate cable is good.
The test involves measuring the resistance looking into a resonant length of coax with either an open circuit or short circuit termination.
The concept is to measure MLL and compare it to specification. Defects that increase MLL are not usually narrowband, but will be evidenced over a very wide range of frequencies so measurement at the exact operating frequency is not necessary.
Analyser requirements
Frequency
The analyser will need to cover a suitable frequency range for measurement. For cables for use on HF, I would advise measurement above 10MHz as actual Z0 is closer to nominal Z0. For higher frequencies, choose a range near to the operating frequency.
The analyser needs to be able to measure R and X reasonably accurately at a low impedance or high impedance resonance of the line section with either SC or OC termination.
Access
Access is needed to one end for the analyser, and at the other end for a SC or OC termination (the cable has to be disconnected from the antenna). It is not necessary to connect the ‘far end’ back to the analyser as you would for a two port transmission test.
Connectors
If you use connectors with a loose coupling sleeve (UHF, SMA etc), do not use a loose male connector as OC, connect it to a F-F adapter so nothing is loose.
To get accurate results, all connectors must be secure, clean and properly tightened.
Got all that under control? Let’s measure…
A practical example
The DUT is 10m of quite old budget RG58A/U fitted with crimp BNC connectors. A sample of this cable has previously been measured for braid coverage, it is just 78% so we might expect it to be a little poorer than Belden 8259.
Above, the top is a sample of the cable under test, and lower is Belden 8259. One can see the poorer braid coverage of the test cable… but does that alone condemn it?
The analyser is an AA-600 which uses an N(F) connector so a N(M)-BNC(F) adapter is used. The AA-600 uses a 16bit ADC, so it gives very good accuracy of extreme impedances (which is the case for this test).
Taking my own advice to measure above 10MHz, the third low impedance resonance of the cable section with OC termination is about 15MHz… let’s measure that.
Using the AA-600’s measure All facility and scrolling frequency up and down with the arrow keys until X passes through zero, we get the above measurements. I have taken a screen shot for the article, but no USB connection is needed for a practical measurement, just write down the frequency and R when X=0.
Now using Calculate transmission line Matched Line Loss from Rin of o/c or s/c resonant section we will calculate MLL (see Measuring matched line loss).
So, now we know the frequency of measurement and MLL, we need to find the specification MLL at that frequency using a GOOD line loss calculator.
Specification MLL is 0.06dB/m, we measured 0.07dB/m, it is a little higher than spec, probably a result of the budget construction, and no reason to condemn it, it is probably as good as the day it was made.
Can you use a NanoVNA?
Yes, you can any instrument that can measure R and X at resonance. I have demonstrated the technique using a noise bridge, an antenna impedance bridge (GR1606B), and a NanoVNA.
Conclusions
The technique and formulas used gives a practical simple but effective method of measuring matched line loss using a one port analyser (or any instrument that can measure R and X at resonance).