Lossy RG58? An experiment

[fremont]

I was troubleshooting a 25' length of RG58 today that had continuity where it should (hot to hot; ground to ground; no shorts) but wasn't receiving RF.  I put on a 50 ohm dummy load and connect to my analyzer and SWR was off-the-charts so into the garbage it went.

As a control, I hooked up a 2' RG58 jumper (with dummy load) and saw the expected 1:1 at the lower bands; however, when I tried at GMRS frequencies, it was 1.9:1 with 25 ohms.  Helped visualize the coax loss we often attribute to antennas in the UHF band.

[Lscott]

1 hour ago, fremont said:

I was troubleshooting a 25' length of RG58 today that had continuity where it should (hot to hot; ground to ground; no shorts) but wasn't receiving RF.  I put on a 50 ohm dummy load and connect to my analyzer and SWR was off-the-charts so into the garbage it went.

As a control, I hooked up a 2' RG58 jumper (with dummy load) and saw the expected 1:1 at the lower bands; however, when I tried at GMRS frequencies, it was 1.9:1 with 25 ohms.  Helped visualize the coax loss we often attribute to antennas in the UHF band.

Are you sure your dummy load is spec’ed for GMRS frequencies? Some aren’t and give really crappy readings.

[WSAM454]

Loss does not necessarily equate with SWR.   You can have a perfect 1:1 match, but at the end of 25' of RG58 you would expect to see about 2dB of loss, (approx. 40%) so 5 W in will give you about 3 out.   More likely a dummy load problem at UHF frequencies.

[SteveShannon]

Lossy cable will make an antenna’s SWR appear better than it actually is. 
An SWR meter in the radio or connected next to the radio measures the forward power before it has been attenuated by the cable and measures the reflected power after it has been attenuated twice, making the ratio of reflected power to forward power much lower than it actually is. 
 

[fremont]

5 hours ago, Lscott said:

Are you sure your dummy load is spec’ed for GMRS frequencies? Some aren’t and give really crappy readings.

I'd have to double check but I'm pretty sure it's up to 500 MHz