It’s been a while since I was regularly operating my station and much of the coax that is in use has been exposed to the outside environment for over 10 years. I know the best way of testing coax is by using specialised sweep tools, but having only basic testing equipment available I though I should ask the Ham Radio community over on Google+. I think Google+ is an entirely underrated social platform, and I find myself leaning towards it’s use more and more.
Some of the advice coming back I expected, such as investing in an MFJ-269 or FoxDelta Analyser (which is good advice by the way, I see an antenna analyser being next on my purchasing list). However, I think Tim (VK2XAX) provided me with some excellent advice to get started:
Yes there is a couple of things you can do with basic ham shack items:
- stick your power/SWR meter on a small patch cable with a dummy load directly on the output of a 2m radio. Set you power out to precisely 10 watts aka 40dBm call this p1. Remove meter from rig. attach suspect cable to rig. attach meter and dummy load to end of cable and measure power again without touching rig output power. call this p2.
p1-p2 = loss for your cable.
look up spec sheet for your cable e.g. RG213 has a loss of ~15db@100MHz for 100m so 15/100 = ~0.15dB loss per metre. e.g If your cable is 34m in length then it should have a loss of ~5.1db aka about 3.5w.
If your 34m cable shows a different value to 3.5w then you should be able to calculate that loss and work out how much worse than the specs it is. - If you have a big dummy load, stick that on the end of the cable with the SWR meter at the radio and pump up the power to max and see if there is any SWR – there should be none. If there is SWR then the cable is most likely damaged in some way that is causing reflection These are good “rule of thumb” tests to check your cables.
Tim then went on to detail in a separate Google+ post a demonstration of his testing.
I was quick to check out the second of the two items, as high SWR was my immediate concern (The SWR without ATU seemed a little higher than I remembered, ranging between 2-3:1). I was pleased to find that it is not the coax contributing to the SWR (which means I need to look at the balun next…). The loss measurement will also come later.
Other more obvious inspections I have done are:
- Continuity – just use a multimedia so find any obvious issues like open circuit, or short circuit between the conductor and ground.
- Visual – Inspect Coax and connectors for any physical signs for deterioration such as oxidisation around the connectors and ground braid and any indications that the outer jacket of the coax has been damaged or has become brittle, allowing water to enter the coax.
I have unfinished business with the Coax and dipole, so I am sure there will be a post in the future providing a continuation of the testing.