Communications Antennas mounted on front bumper

[craig]

If I understand things correctly, a dual band antenna doesn't require the ground plane. Would mounting a 2m/70cm antenna on a spare tire carrier where it is blocked by the back door of an SUV cause a serious degradation in signal strength in the forward direction?

In the PNW I'm always hitting my roof mounted antenna on trees or it is folded down to protect it. Putting it on the spare tire carrier means one less thing to worry about when on the trail, but if it is going to kill the radios performance I'll consider other options.

Craig

 

[Azrocks]

If I understand things correctly, a dual band antenna doesn't require the ground plane. Would mounting a 2m/70cm antenna on a spare tire carrier where it is blocked by the back door of an SUV cause a serious degradation in signal strength in the forward direction?

In the PNW I'm always hitting my roof mounted antenna on trees or it is folded down to protect it. Putting it on the spare tire carrier means one less thing to worry about when on the trail, but if it is going to kill the radios performance I'll consider other options.

Craig

From my research it sounds like ground plane isn't as much of an issue with a half wave and above antenna. It is more critical with a 1/4 wave. It still is somewhat of a compromise with no ground plane with a half wave. As far as the antenna I would try to get as much of the antenna above the roofline as you can. On my FJ40 I use a lip mount on the back edge of the tub right now with a 1/2 wave and am getting good results.

I think a lot has to do with what you are trying to reach with your set up. If you are reaching the repeaters you need to and are getting the simplex distance you need, then great! I use the short SBB-1 on my cruiser when I don't want the 1/2 wave on there. Even with that short antenna, 1/4 wave, lip mount, bellow roof line, etc. I have no problem hitting the repeaters for the most part that I use. I'm sure simplex would be compromised but haven't tested that yet. I'm moving my 1/2 wave between my truck (mag mount) and FJ40. Eventually I'll get another 1/2 wave for the FJ and keep it in the vehicle along with the SBB1.

If you mount your antenna on your tire rack I would recommend bonding the tire rack/mount to the body of the vehicle with non-insulated braided cable.

 

[craig]

Thanks for the great reply Gary. It sounds like I need to get a meter and take some samples around my vehicle to see how much the location causes the signal to degrade in any direction.

Last weekend I ran with it mounted to the spare tire carrier w/ about 4" sticking above the truck and it worked well. Going towards the rear of the vehicle I was able to verify that I could communicate for at least 50km (I was in Canada), and in the forward direction I was able to test that I could communicate at least 10km. I didn't have a chance to test further, but the signal was crystal clear at those distances (w/ no physical obstructions in the way).

If I have to mount the antenna so that it is above the roofline but attached to the spare tire carrier, I will just move it back to the roof. If it works poorly located on the back of the vehicle but down low I will try the front bullbar next (ugly though). Up high just gets knocked around too much in the NW.

Craig

 

[AndrewP]

If I understand things correctly, a dual band antenna doesn't require the ground plane. Would mounting a 2m/70cm antenna on a spare tire carrier where it is blocked by the back door of an SUV cause a serious degradation in signal strength in the forward direction?

In the PNW I'm always hitting my roof mounted antenna on trees or it is folded down to protect it. Putting it on the spare tire carrier means one less thing to worry about when on the trail, but if it is going to kill the radios performance I'll consider other options.

Craig

Single or dual band makes no difference. For best performance, and low SWR, you need a good ground plane. You can still get OK performance and low SWR with a 1/2 wave antenna. A 5/8 wave antenna with a good ground plane will give optimum performance in a mobile install.

If you have a situation (like a J&&P) where there is not a lot of metal up high where the antenna is, you can get acceptable performance with a 1/2 wave antenna up as high as is convienient.

On my FJ40, I have a 1/2 wave Larsen mounted on the rear roll bar and it works very well, with much better performance than the 1/4 wave SBB1 I had mounted in the same location.

 

[offroad_nomad]

I had a CB and 2m VHF Antennex mounted on my front TJM with zero issues. I've even seen some trucks running non-groundplane antennas on their bumpers. Currently my VHF antenna is on a hood lip mount on the driver's side; again no problems. I may eventually move it back to the front bumper, passenger side.

 

[AndrewP]

I had a CB and 2m VHF Antennex mounted on my front TJM with zero issues. I've even seen some trucks running non-groundplane antennas on their bumpers. Currently my VHF antenna is on a hood lip mount on the driver's side; again no problems. I may eventually move it back to the front bumper, passenger side.

Just because you get acceptable performance(ie your signal gets out a bit, and you can hear a bit), does not mean its good or has "zero issues". Even if you take the time to do a nice RF ground braid to the chassis, the bumper is still a very poor choice. Your VHF on a hood lip mount is a significantly better choice, and should give performance that is quite good. Ignition and alternator noise can be a problem there, but you've probably thought of that. If you want to move it, consider the lip mount at the top corner of your rear hatch. That's about as good as you'll do without drilling a hole in your roof.

With any of the Ham antennas, performance is so good, that even marginal set-ups work great. Still, it's part of the tradition to optimize these tools just like all the other tools in the truck.

 

[Pskhaat]

Those masts are usually very rigid fiberglass with a spring at the bottom. The reason for this is to so the whip itself stays very, very straight. As a whip flops over, the aspect changes (not to mention the impedance) and that changes your radio's ability to receive and transmit an effective wave. There's no reason why we couldn't use the same antennas here, but for some reason the manufacturers don't sell them here very often.

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