How do I check integrity of my vhf cable?

[demonboy]

Before I thread it up the mast I want to check my RG213 vhf cable is intact. It has a couple of minor kinks in it and I want to ensure the wires aren't broken. I know 213 is rated at 50ohms but I think I read this was its operating impedance, not standing. Any pointers?

Thanks in advance.

 

[john_morris_uk]

Although finding someone with a suitably rated VSWR bridge (a simple device that is inserted into the transmission line between the VHF set and the start (or finish) of the coax) it will only tell you part of the story. The acid test is to actually show that the coax is functioning properly by checking with a distant station that they can hear you!

So firstly find someone with a VSWR meter and check that it says all is ok. Then (because for various technical reasons a 'good SWR reading' doesn't totally gaurentee that everything is ok) make sure that stations at a reasonably long way away can hear you ok.

I'm assuming that the cable is capable of having the antenna fitted and has the plug on the end for the VHF. You will need some sort of termination on each end to test it.

 

[demonboy]

Although finding someone with a suitably rated VSWR bridge (a simple device that is inserted into the transmission line between the VHF set and the start (or finish) of the coax) it will only tell you part of the story. The acid test is to actually show that the coax is functioning properly by checking with a distant station that they can hear you!

So firstly find someone with a VSWR meter and check that it says all is ok. Then (because for various technical reasons a 'good SWR reading' doesn't totally gaurentee that everything is ok) make sure that stations at a reasonably long way away can hear you ok.

I'm assuming that the cable is capable of having the antenna fitted and has the plug on the end for the VHF. You will need some sort of termination on each end to test it.

Thanks for the reply, John. I should have explained that the mast is lying horizontal in the yard and the aerial and unit in storage. I was just wanting to check that the inner and outer conductors are intact. Interested to know how much battering the cable can take (it's been rammed up the mast conduit a couple of times) before it loses its integrity.

 

[Salty John]

With both ends disconnected you should see no circuit between outer braid (connector body) and the inner conductor (centre pin). Also, if the cable is coiled so that you get the two ends close to each other, check that there is continuity end to end of the centre conductor and of the outer braid. That's about the best you can do.
RG213 is pretty tough stuff.

 

[rob2]

Absolutely, any cheap multimeter will allow you to check continuity, or even just a battery and bulb. Don't forget to flex the suspect areas a bit during the test in case a break is making intermittent contact. There is a minor concern about the kinks that it may be less than optimal as the 50 ohm matching is (as I understand it) a function of the spacing between inner and out conductors, but if it kinked while threading it down the mast you wouldn't even know about it, so probably as good as it is likely to get.

Rob.

 

[Como]

With both ends disconnected you should see no circuit between outer braid (connector body) and the inner conductor (centre pin). Also, if the cable is coiled so that you get the two ends close to each other, check that there is continuity end to end of the centre conductor and of the outer braid. That's about the best you can do.
RG213 is pretty tough stuff.

If the coax is in the mast so you cannot get to both ends then, having checked that there is no circuit between the body and the pin, deliberately short one end of the cable and you should see the short (or near short) at the other end.

 

[savageseadog]

I would pull the cable out and have a proper look, pull a mousing line in.

 

[mlines]

A kink means it has effectively exceeded its bending radius at that point. As rob2 says this could affect the internal structure of the cable at this point which may not show up as a break or a short but may affect its RF performance. The only way to test for this properly would be to use an analyser. Having said that, most cable issues/losses are in the connectors so its probably fine for comparatively low powered RF use.

 

[emnick]

Is it not the insulation between core and braid that you want to check? if so perhaps a mega meter will do it. in excess of 999mega ohms on my standard meter, if the cable is damp it will find a lower resistance etc...not saying this is the correct way though. But would give an indication of any breakdown or dampness. Don't hold the cable ends though or you will be checking your own body resistance.