Performance from different CB antenna locations.

[Kerensky97]

I need some opinion on where to mount a CB antenna. The two options I have are a Mag mount on the center top of the roof of the truck, or on the fender opposite the FM radio antenna. I'm just kind of curious how much performance loss I'm seeing moving a CB antenna from the roof to the fender.

I know the roof is ideal but CB will maybe be used for one group trip a year, casual boredom radio chatter listening on roadtrips, and possible emergencies (although I have other alternatives); so I'm not too worried about optimal performance. I'm also kind of worried about height issues with the roof antenna. It's hard enough fitting a fullsize truck on trails made by Jeep CJs. About what range loss do you see moving from the roof to the fender?

As for alternatives I have an FRS walkie for spotting over rocks or hiking. A Spot3 for "127 hours" type scenarios. And will eventually have a separate HAM Radio GoBox for basecamp setups when I want good performance and distance.

 

[unseenone]

quite a bit difference center of the roof is best. I have one mounted both ways. Even if you get it up on the roof it will be better.

 

[lqdchkn]

http://www.rightchannelradios.com/b...s/18428271-poor-cb-antenna-mounting-locations

http://www.rightchannelradios.com/blogs/selection-guides/18542007-choosing-the-best-cb-antenna

 

[Frdmskr]

Well you are doing car to car and not DX per the rules. So loss should be moot.

Larsen or Motorola had a graphic online showing loss at various points on a vehicle.

Finally, check out K0BG.com for ideas.


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[4x4junkie]

I need some opinion on where to mount a CB antenna. The two options I have are a Mag mount on the center top of the roof of the truck, or on the fender opposite the FM radio antenna. I'm just kind of curious how much performance loss I'm seeing moving a CB antenna from the roof to the fender.

I know the roof is ideal but CB will maybe be used for one group trip a year, casual boredom radio chatter listening on roadtrips, and possible emergencies (although I have other alternatives); so I'm not too worried about optimal performance. I'm also kind of worried about height issues with the roof antenna. It's hard enough fitting a fullsize truck on trails made by Jeep CJs. About what range loss do you see moving from the roof to the fender?

As for alternatives I have an FRS walkie for spotting over rocks or hiking. A Spot3 for "127 hours" type scenarios. And will eventually have a separate HAM Radio GoBox for basecamp setups when I want good performance and distance.

It's about a 40-50% difference with fender or side-mounted CB antennas vs one center of the roof, I've found (the loss also becomes much greater with antennas less than about 5 feet). This range difference is greatest off the side of the vehicle that the antenna is mounted on, being that there is little or no ground plane in that direction.

If a CB is a tool you want to have potentially work in an emergency, then this seems all the more reason to not skimp out on your antenna. Naysayers will say CBs are next to worthless in an emergency, and I cannot argue the possibility of failing to reach someone while in a remote area is fairly high these days (so indeed you do not want it as the only comm tool in your emergency kit), however an inferior antenna and/or mounting location will pretty much guarantee it's ineffectiveness.

A 5-foot (or taller, the taller the better) antenna atop the roof should provide about 5-10 miles average range to another similarly-equipped vehicle, and if you have a SSB CB unit, should also allow you to make effective use of skywave ("skip") signals as well. So even if you can't raise someone locally, if the band is open you may be able to reach out to someone in another state on one of the upper SSB channels who could then place a phone call for you.

You mentioned concerns about antenna height, such concerns are almost entirely mitigated by use of a spring and/or quick-disconnect antenna mount. A spring lets the antenna deflect harmlessly out of the way of low-hanging tree branches / fast-food drive-thru overhangs, etc., and a quick disconnect allows fast removal of the antenna for pulling into a garage or underground parking.

Anyway, just some food for thought.

 

[prerunner1982]

It's about a 40-50% difference with fender or side-mounted CB antennas vs one center of the roof,.

Do your calculations take into account that the roof mounted antenna is a mag mount and the fender mount is not? I don't think the difference is what you state even if both antennas were permanent mount and performance would be much closer considering the roof mounted antenna is using a mag mount.

 

[Kerensky97]

Exactly what I wanted to hear! Thanks all for the input.

I found the Larsen PDF for antenna locations on vehicles, it pretty much confirms everything said here. Permanent is a bit better than mag mount but it makes a big difference moving the antenna below the roofline, and lowers range further as you move away from the center of the ground plane.

Basically fender mounts are really only going to work for "convoy" usage, not for long range comms. But I think that will be ok for now; like I said, I want to have a mobile HAM setup eventually but one expensive hobby at a time. I'm still trying to get this vehicle setup with the basics. Eventually I can use the CB to get the convoy to the mountain top then setup the more legit radio system to see how many states I can communicate across.

 

[4x4junkie]

Do your calculations take into account that the roof mounted antenna is a mag mount and the fender mount is not? I don't think the difference is what you state even if both antennas were permanent mount and performance would be much closer considering the roof mounted antenna is using a mag mount.

Magnetic mounts (good quality ones anyway) have little significance on an antenna's performance (just as the Larsen .PDF Kerensky97 found also illustrates). It also states the minimum amount of effective ground plane needed for several frequencies, and of course CB @ 27MHz will require much more GP than 150MHz will, making it all the more detrimental to the signal pattern to have it on the fender of the vehicle.

I've seen it many times in the real world too where I could clearly hear a buddy of mine who has a magnetic roof mounted antenna, where signals from others who had various non-roof-mounted antennas (most being non-magnetic) were not even detectable.

 

[wirenut]

If you do mount on the fender consider getting an antenna with an elevated loading coil. If part of the antenna must be below any vehicle sheet metal it's best that at least the loading coil be above the vehicle sheet metal. A base loaded antenna would be the worst. One with a rigid mast, then a coil, then a whip would be better. Something like a Wilson 5000.

 

[4x4junkie]

^^
Good advice.

 

[comptiger5000]

Front fender isn't quite as optimal as the roof, but it works pretty well. As long as you put it forward enough, it's not too close to the a-pillars and other metal that sticks up above. And you can see if it'll fit under something before it actually hits.

 

[prerunner1982]

Worked a station almost 1200 miles away this morning while mobile, with a "beautiful signal" report. The antenna is mounted above the driver's rear taillight and the station was behind me and to my driver's side, completely opposite of the "ground plane".... I am ok with the sacrifice of having the antenna mounted on the side vs the top of the vehicle. Of course this was on 20m with 100 watts not 11m (CB) but 20m also requires more "ground plane".


EDIT: Have worked Slovenia, Crete, and Russia... 5000-6300 miles.

 

[Grinder313]

If you go with a mag mount you can run the antenna on the roof most of the timande and when you hit the trail just put it on the hood, trunk, bed...

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