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Mobile antenna question

C

cosmos7

Member
Dec 3, 2010
2
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Right now I'm using a 2m/70cm mag-mount in the center of my 4Runner's roof. Obviously the best install would be to drill the roof, but I am loath to do so. The guy at HRO told me that any hard connection to the vehicle would be better than the mag-mount, and I asked him about the tail-light mounting that some guys do and he agreed it would be better.

Guess I'm having trouble understanding... seems to me that a mag-mount with a good counterpoise in the center of the roof would have good omni-directional coverage. I'm not opposed to attaching a bracket in the tail-light area and running in through the tail-light, but it seems counter-intuitive to me since even with a larger antenna much of it will be below the roof-line for all directions except the rear.

Thoughts?
 

In this case, you're right and the salesman at HRO needs to consider the difference between HF and 2m/70cm bands. What you gain with a solid electrical connection to the vehicle body is the removal of the capacitor that the magnet mount forms. Lower frequencies need larger capacitors (magnet mounts) to provide the surface area required to form a low loss RF path to ground.

By the time we get to VHF high band, the impedance path through the magnet mount capacitor is as though there was a direct connection to the RF ground plane. It's no longer a significant factor worth going through the trouble of drilling a hole or giving up the prime mounting location, centered over the ground plane. Keep the antenna exactly where it is and skip drilling anything.

If you have trouble keeping the magnet stuck to the roof, consider spreading a thin layer of clear silicon under the magnet. Striking branches or bridges at speed will not break this bond easily. It will require slow lifting pressure from one edge over several seconds to start breaking the bond. Then any remaining dry silicon can be rubbed off with a rag with no damage.
 
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Something else to consider about a mag-mount is where the feed line will run/rub on the vehicle's surface and how to get it into the vehicle. Along with that magnet being attached to the roof, all of that will cause damage over time.
- 'Doc
 
the rubbing of the coax on the roof of my car has worn down the paint for a couple feet!

so one must weigh what they want to do...drill a hole and cap it later or put up with some worn paint.

best of luck, enjoy the hobby
 
I have found that using what I call a sticky back and have attached them to the vehicle, then tie wrap the coax in place. Here is a photo of one.
enevubyh.jpg
emeqy4e6.jpg

Make sure to clean the area with some alcohol and stick it to the vehicle. These can be purchased at Home Depot and do work. I have had mine on for a while now and no issues with coax rubbing, and it's only double sided tape so I doubt it will damage any paint.
 
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i may give that a try, what area or department of the home depot are these
"sticky backs" found in?
 
Yep in the electrical dept of Home Depot, they sell them in white and black. If you have a grainger or graybar near you they have a better quality kind made by panduit, but the Home Depot ones should work fine as long as the area is prepped properly, I.E., use a bit of running alcohol onto the surface to remove any wax or dirt. Also I try to use the highest percent rubbing alcohol possible as it evaporates much faster, 91%. It costs about a dollar more but is worth it IMO.
 
The black ones I believe have the uv protection. I only used the white as I have a bunch and it's a work truck. Good info though capt. God bless
 
roger all the info, i'll be in the city this weekend and hope to pick some up and give them a try. i already have the high-91% alcohol and the area is well, "rubbed clean" LOL, so I'll swab it and attach the stickies. It will be nice not having that slight thumping sound too while going down the road on those windy days!

thanks for the tip
 
Thanks for the info! I'm not opposed to doing a permanent install, just not drilling the roof where I have the moonroof to deal with. Someone suggested to me that a L-bracket or lipmount in the hood area would be decent, but I'm still thinking that the center of roof is better even with the magmount. I do like the idea of clipping down the coax though so it doesn't flap around.
 

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