3
$\begingroup$

I have a situation similar to this: Testing for 'Bad' Coax . In addition to checking for DC continuity or shorts, here was my test method:

My 50Ω dummy load (Elecraft DL1) has a diode and capacitor test point so that you can use a volt meter to measure power at the dummy load.

I connected the dummy load to the rig (transmitting a carrier at 144 MHz) and measured the voltage (using a Fluke 75III DVM), then connected the cable under test between the rig and the dummy load and measured the voltage again. With just the dummy load, I measured 6.2V at the test point (which calculates to the expected 1.6W).

The first cable was short (<5m) and it measured 5.4V (i.e., a significant loss); other cables (all of them RG-8 type, and all <20m long) measured under 1V.

  • Was that a valid test?
  • Am I correct that these cables were junk?
Improve this question
$\endgroup$
3
  • $\begingroup$ reflex, default question: The dummy load is properly impedance-matched to the cabling, right? $\endgroup$
    – Marcus Müller
    Commented Aug 20, 2022 at 19:21
  • $\begingroup$ and, which frequency was this measurement done at? A loss of, say, 9 dB over 20m, would be much, but possibly in spec, for the original cable datasheet, in the 9cm band. For 30 MHz, it would be undisputable inadequate. $\endgroup$
    – Marcus Müller
    Commented Aug 20, 2022 at 19:30
  • $\begingroup$ Test was done on 144MHz. Dummy load is 50 Ohm. $\endgroup$
    – Duston
    Commented Aug 20, 2022 at 19:32

1 Answer 1

Reset to default
2
$\begingroup$

Belden, which typically produce the upper middle-class non-measurement-grade cables, would spec something like 5 dB/100m, or 1 dB for 20m at 150 MHz, for their RG-8/U. Whatever you're seeing significantly exceeds that.

Note that the diode used in your dummy load will not work well for low voltages (as you can see in the figure on the second page of the instructions to your dummy load); as an engineer, I'll allow myself that this is not a great way to measure powers over a large range. However, for your kind of tests it suffices; you don't care how much lower exactly the power is, just that it's more than, say, 2 dB.

So, if you're confident in your connectors, I'd say damaged cabling is the best explanation. Hard to tell with your setup whether it's leaky, i.e., has some kink/cut somewhere that makes it emit more than it carries, or whether it's lossy (i.e., converts RF power to ohmic heat in oxidized conductor or dielectric heat in the insulation), but a) doesn't really matter much b) would notice if turning on a receiver tuned to the same frequency in the vicinity.

So:

  • Yes, I'd call this a valid test, and
  • I'd agree with that assessment.
Improve this answer
$\endgroup$

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .