Ham Radio Deals
https://www.hamradiodeals.co.uk/forums/

How to test a roll of coax?
https://www.hamradiodeals.co.uk/forums/viewtopic.php?f=43&t=64515
Page 1 of 2

Author:  G7FQW [ Sun Mar 01, 2015 11:34 am ]
Post subject:  How to test a roll of coax?

Hi all, I am expecting a roll of very cheap RG213U from somewhere in Slovakia to arrive in a week or so courtesy of the bay.

It's cheap, it's white (possibly for marine use?) but the specs given do appear to be far better than Rg8 mini, and I only want it for HF.

So apart from simple DC testing with a multimeter, how can I go about testing this to see if it matches the spec given?
auction & Specs here

What equipment do I need?

Or is it a case of TX into it with the power meter at one end & dummy load the other then swap the meter to the dummy load end & see what the results are?

Author:  Sangoma [ Sun Mar 01, 2015 11:37 am ]
Post subject:  Re: How to test a roll of coax?

If it is white, it may well not be UV resistant, and start to crack/rot quickly outside.

Best way to test it is with an SA, but otherwise put power in one end and measure how much comes out the other. (with a dummy load etc)

Author:  G7FQW [ Sun Mar 01, 2015 11:42 am ]
Post subject:  Re: How to test a roll of coax?

SA?

Do you mean spectrum analyser? if so how? (I have one - just need to replace a PSU cap).

Author:  GW6RRL [ Sun Mar 01, 2015 11:44 am ]
Post subject:  Re: How to test a roll of coax?

If you have a decent scope and a signal generator, you can check the length and impedence as per this youtube clip
https://www.youtube.com/watch?v=Il_eju4D_TM

Author:  Sangoma [ Sun Mar 01, 2015 12:06 pm ]
Post subject:  Re: How to test a roll of coax?

G7FQW wrote:
SA?

Do you mean spectrum analyser? if so how? (I have one - just need to replace a PSU cap).



Yes, the spectrum analyser will most likely be 50 ohms, set the tracking generator to sweep the frequency range you are interested in and you will see the looses over that range. Calibrate first.

There may be some strange results due to the coiled up coax, or resonance, but I didn't find any when I tried it.

Author:  G3TXQ [ Sun Mar 01, 2015 12:24 pm ]
Post subject:  Re: How to test a roll of coax?

Connect your radio to one end and leave the other end open-circuit. Apply a small amount of power at 28MHz - just enough to get an SWR reading.

Anything lower than 2.5:1 and the losses are more than you would expect for 100m of RG213.

Steve G3TXQ

Author:  G7FQW [ Sun Mar 01, 2015 4:44 pm ]
Post subject:  Re: How to test a roll of coax?

Cheers all, I'll have to see what I can do when it arrives.

Author:  M0WYZ [ Sun Mar 01, 2015 4:56 pm ]
Post subject:  Re: How to test a roll of coax?

can do it easy with a antenna analyzer ,don't forget to unroll it though!!!

Author:  G7FQW [ Sun Mar 01, 2015 5:31 pm ]
Post subject:  Re: How to test a roll of coax?

I wish I had one.
Maybe when I get it I'll ask the local club members if anyone has one, might make it quicker.

Author:  G3TXQ [ Sun Mar 01, 2015 5:36 pm ]
Post subject:  Re: How to test a roll of coax?

Try my method - you don't need anything other than the radio and an SWR meter. The cable is likely open-circuit at the end so you don't need to unroll it; even if it's shorted the method works!

Steve G3TXQ

Author:  G7FQW [ Sun Mar 01, 2015 5:58 pm ]
Post subject:  Re: How to test a roll of coax?

Will do.
But how do I compare any result with whats advertised?

Author:  g7pan [ Sun Mar 01, 2015 6:12 pm ]
Post subject:  Re: How to test a roll of coax?

G7FQW wrote:
Will do.
But how do I compare any result with whats advertised?

Connect your radio to one end and leave the other end open-circuit. Apply a small amount of power at 28MHz - just enough to get an SWR reading.

Anything lower than 2.5:1 and the losses are more than you would expect for 100m of RG213.

Steve G3TXQ

Author:  G3TXQ [ Sun Mar 01, 2015 6:17 pm ]
Post subject:  Re: How to test a roll of coax?

Loss = 10 log [ ( SWR + 1) / (SWR - 1) ]

Here are a few to save you the calculation:
1.5:1 = 7dB
1.6:1 = 6.4dB
1.7:1 = 5.9dB
1.8:1 = 5.4dB
1.9:1 = 5.1dB
2.0:1 = 4.8dB
2.1:1 = 4.5dB
2.2:1 = 4.2dB
2.3:1 = 4.0dB
2.4:1 = 3.8dB
2.5:1 = 3.7dB

Your cable is specified as 3.65dB/100m at 28MHz, so you should see an SWR of 2.5:1 or higher at 28MHz if the cable meets specification.

Steve G3TXQ

Author:  G7FQW [ Sun Mar 01, 2015 8:28 pm ]
Post subject:  Re: How to test a roll of coax?

Thank you Steve, you have saved me asking later on :)


I'm wondering if I should bother trying to UV protect it more, after all I'm in the NW of the UK and we get lots of sun (not) lol.
Long lengths of black sleeving, wrapping in tape, or perhaps just leave it & replace in a few years if needs be.

Author:  Sangoma [ Mon Mar 02, 2015 4:29 am ]
Post subject:  Re: How to test a roll of coax?

Might be worth trying too check the specs or intended usage first, it may be OK.

The only white 213 I've seen before was being used inside a building, and the outside cable was all black (different circuits)

May be worth sticking a short piece within easy reach somewhere unshaded so that it can be checked often.

Even with the little sun you may get, there will still be plenty of UV to affect it if it is not stabilised. It may just take a year instead of 6 months.

Page 1 of 2 All times are UTC
Powered by phpBB © 2000, 2002, 2005, 2007 phpBB Group
http://www.phpbb.com/