Antenna Spacing on Car roof

[cpg178]

I currently am running one VHF and one UHF radio in my car running off of a single VHF 1/4 whip in the middle of my roof using a ham diplexer. I am thinking of adding a second VHF radio which would require me to add another antenna. Since the radio shop charges hourly it is little added cost to just add 2 more holes so I can ditch the ham diplexer. I am unsure what the best set up for the antenna placement is. I will have 2 VHF antennas and 1 UHF. Are they better inline with each other or as I did the in quick graphic below? + is my current antenna, O is future ones. (spacing isnt exact hard to draw with laptop trackpad and just a generic SUV cant find my exact car) I understand there should be some sort of minimum spacing

 

[KQA726]

quick question, with the side airbags (curtains) in most cars, where can they run the antenna cables without disturbing this safety feature? I would hate to lose that important safety future just for a trio of antennae. For looks I would put the two VHF in back and UHF front and center.

 

[mmckenna]

There's two things you need to consider:

You need to have a sufficient ground plane under each antenna. That means 6" around the base of the UHF antenna, and 18" around the base of the VHF antenna (more if this is for 2 meter ham stuff). Putting the antennas too far back will result in a lopsided ground plane with a less than ideal radiation pattern.

You need to have enough separation between antennas to avoid overloading the receivers. Too much RF coupled into the other antennas can damage them. The amount of spacing will depend on a number of variables, and there is no one rule that will apply in every situation.

 

[mmckenna]

quick question, with the side airbags (curtains) in most cars, where can they run the antenna cables without disturbing this safety feature? I would hate to lose that important safety future just for a trio of antennae.

Side curtain airbags are usually over the doors. Running coax down the back corner to the RF decks is usually what's done. Some cars have split airbags between the front and rear seats, so the center "B" pillar can be used.

For looks I would put the two VHF in back and UHF front and center.

Looks are a consideration for some, and I'd probably agree that the two VHF in the back might look good.
But spacing requirements and getting enough ground plane on a smaller vehicle might result in that not working. Depends if you prioritize the looks over function. Nothing wrong with that if going on a personal vehicle….

 

[cpg178]

Regarding air bags. All my radios are remote headed. Bricks are in the spare tire well since my car does not have one. So no antenna wires go near airbags. The cables for remote heads run under the bottom of the door sill

 

[cpg178]

Side curtain airbags are usually over the doors. Running coax down the back corner to the RF decks is usually what's done. Some cars have split airbags between the front and rear seats, so the center "B" pillar can be used.



Looks are a consideration for some, and I'd probably agree that the two VHF in the back might look good.
But spacing requirements and getting enough ground plane on a smaller vehicle might result in that not working. Depends if you prioritize the looks over function. Nothing wrong with that if going on a personal vehicle….

Thanks, it is for a personal vehicle and in all honesty I do more monitoring and very seldom TX. One radio is 110w VHF MCS2000 and 2 others are XPR5500s U and V high power. I am gonna go out now and just put antennas on the roof and see what configuration looks best but I am leaning towards the 2 VHF in the back and maybe throw a high gain UHF in the middle front.

 

[mmckenna]

You'll either want to throttle that MCS-2000 back to low power, or make sure you have a good amount of spacing between that antenna and the other VHF radio. The filtering in the UHF radio will keep most of the VHF out.

Ideally, that might be a good situation to have one VHF centered on the roof, the UHF towards the back of the roof, and the other VHF on the trunk.