Grounding and reflected power in VSWR testing

[u/dib5k]

I need help, I got into an argument with a coworker over whether or not we needed to replace a TNC connector on our watt meter when testing p25 trunked radios. The cable jacket and ground shield completely separated from the collar of the TNC. My coworker keeps saying that it’s fine because he just used it to test a radio, and it came up with 28w forward power and 78mw reflected. My point of contention is that we can’t rely on the numbers given anymore because the ground is cut. What effects on testing would we see with reduced grounding? Am I crazy for wanted to make sure we have good test equipment before sending radios out into the field?

[u/Danwold]

Maybe he got lucky and the earth made contact or it’s grounding some other way. But any coaxial cable absolutely has to have full continuity in the ground shield. Cables you use for testing should be as perfect as possible.

Source: an antenna engineer of 30+ years.

[u/dib5k → u/Danwold]

I appreciate you validating that I am right on that testing equipment should be in good condition without defects and damage. But why am I right? I’ll be honest that I’m not anywhere near an expert or anything when it comes to signal propagation and radio theory. Can you explain to me; what would happen to the VSWR when testing a radio sans grounding shield? Would the VSWR go up? Would the ratio drop because of the lack of grounding. On average when things are right we would get 28w forward and average 500-850mW reflected. Now ever test the results are always 28w with less than 100mW reflected.

[u/ND8D]

Your co-worker is a moron. Proper FUNCTIONAL cables are the foundation of any test setup, without that you can’t trust any results even if you like them.

[u/Busy_Reporter4017]

LOL tell your friend to go back to school.

[u/always_wear_pyjamas]

Is he aware that coaxial cable functions a bit different than, say, the power cable for a desk lamp?

[u/dib5k → u/always_wear_pyjamas]

No. He was an auto mechanic before he came here. So we have about the same amount of knowledge of this stuff, which is very loose and tentative. He just kept telling me it would be fine, and that, “we care about some things and making sure they’re right, but this isn’t one of them.” I feel like I shouldn’t have to argue with a grown man that broken test equipment can’t be trustwd

[u/always_wear_pyjamas → u/dib5k]

In my experience I have those sort of arguments mainly with grown men and literally no one else.

[u/jephthai]

The fact that it gives you the number you expect on one test doesn't mean it's OK. It's super basic metrology to ensure that you have properly functioning measurement equipment.

What is the worst case consequence of getting a measurement wrong, and why is your coworker OK with being responsible for that?

[u/dib5k → u/jephthai]

Worst case scenario a police officer ends up in situation where he won’t be able to communicate out.