Mobile install - common mode problem?

[rack201]

Installing a 477mhz collinear antenna on the bull bar of my 4wd - opposite side to my 2m/70cm dual band. Testing off the car in the open it gives close to 1.1:1 Swr, but mounted on the car I can’t get better than ~2:1.

My SWR readings differ based on the length of the patch cable I’m using from the meter (AV400) to the radio, so I’m assuming I have common mode current on the coax and RF is coupling to parts of the vehicle and re-radiating to the coax.

I tried some mix31 ferrites which made no difference - believe I need mix61 for UHF. If it’s caused by re-radiated RF (and the feed point is 50ohms) would they be better placed closer to the radio?

Does it sound like I’m on the right track? Also tested with the 2/70 antenna removed - no change - it’s over a wavelength away.

 

[qc]

Can you send photos of your antenna setup? What antenna manufacturers and model is 477mhz collinear antenna? have you tried to swap the 2m/70cm dual-band antenna to the NMO on your collinear antenna? And recheck your your SWR to see if there is any changes?

 

[mmckenna]

Does it sound like I’m on the right track? Also tested with the 2/70 antenna removed - no change - it’s over a wavelength away.

Got a link to the 477MHz antenna specs? Seems like it is looking for a ground plane or counterpoise, especially if changing the cable length is changing SWR.

 

[tlemke940]

agreed it sounds like a ground plane problem to me too. there may be a bad ground at the antenna, the bull bar mounts, or any ware in between try a ground strap to a good clean ground. also are you testing with the hood up or down and is the antenna in line at least with the hood as in the hood is not too close to the radiating element aka the whip