Mobile Antennas, Part 2

by Ramon Gandia, AL7X 2/23/2013

OK, you have decided on a mount. Where to put it?

If your car body is metal, you have the fewest problems.

(1) Get a Mag Mount. This ranges from a 2.5" round magnet with a socket to receive an NMO or PL259 antenna. MFJ also makes one with a 3/8-24 female, but also have a corresponding VHF/UHF whip to go into it. The big ones can have 6" or 7" magnets, or even THREE big magnets.

I have found that the little magnets do not like to stay put. Apparently the bouncing over the gravel roads, etc. will dislodge the magnet from your autobody, and pretty soon you are dragging it.

Likely not all magnets are created equal, and some of those Nobidium $$ and exotic magnets conceivably can lift your car off the ground, while the cheapo ones you can blow off with a child's breath.

The MFJ dual band 20" whip with mag mount likes to blow off. Bumping anything, like a willow, and off she comes. For grandma driving her Subaru here in town, never going out of town, should be OK. I recommend otherwise.

Look at the Diamond mag mount shown on the left. A bit spendy, but that magnet is huge and will not pull off under most circumstances.

The Advantage of the MagMount is that you drill NO HOLES in the car. The disadvantage, besides coming off, is that you have to thread this coax cable INTO the car. This is often done thru the window, which pretty well damages your coax the minute you crank the window up.

A better location is to bring the cable in thru the door opening. I have found that a dab of silicone here and there will keep things orderly. Tie wraps, little clamps and dabs of silicone and the coax is in.

It simply gets squooshed between the door and the rubber weatherseal in the car body and seems to live happily there with no air leaks and no permanent damage to the coax or the weatherseal. Recommended.

Some mounts come with a short length of RG174 cable, which is very thin (under 1/8"_ to go into the car, then it transitions to the more normal quarter inch RG-58 cable. I have not tried one, but these are probably OK.

(2) Drilling a hole in your car.

OK, not for everyone, but it will give you the BEST radio performance if you stick it smack in the center of your roof.

No two cars are the same, but some tricks are:

(a) take off the dome light. Can you see the roof from in there? If you can, it simplifies things. Make sure you are seeing the real roof. Many roofs, like Ford Pickups, have a double layer roof, so what you see is not the roof itself, but a weldend on double liner which is VERY hard to deal with.

(b) Just drill blind from the outside with a 3/4" hole saw for the NMO, or the right hole for the UHF. See the comment above for the double roof liner, because if you drill your nice hole into the double cavity in the roof you will cry and cry. Trust me on this, I still have tear tracks on my face!

Again, removing dome lights etc is a good way to ascertain the construction of your roof. Even double roofed cars often have areas that are single layered.

(c) Invest in old metal coathangers to fish coax cables from the middle of the roof to a door pillar or someplace you can continue your coax run to the radio.

You will CRY when you find out how hard it is to fish that wire to the pillar and then take half the car interior apart to bring that coax to the radio.

For a complete installation of this type, allow 2 - 3 hours minimum to do the antenna in a workman like manner. If your 1975 rustbucket Chevvy pickup looks like the remnants of Katrina on the interior, then you do not have to be so picky. Twenty minutes will do it. Until your dog snakes his head into a coax cable loop and brings it all off.

Doug Doyle, KL0HH of Council, AK fame has done lots of body work and he suggest the round tire patches to cover up an unwanted hole.

(3) Various types of patent mounts.

There are some that simply clip on the edges of your roof, door, window, trunk lip, etc. I have found most of these simply do not perform well as far as radio goes, but with some antennas they might do well.

The typical one goes into the gap between the door and the body, and the clamp part grips the rain gutter. I think most of these adjust to various brands of cars.

Some are simple, some have a knob that you can loosen and lay the antenna down (good for garages or RJ Auto), and some even have an electric MOTOR, for god's sake, to do the same thing. That way your Tuxedo and Spats do not get dirty or wet when you arrive at your garage. You also need a remote door opener.

You may notice my condescending tone when speaking of these mounts. Most of them place the antenna at the edge of the roof, which is not a very good radio location. More about this later. If you can avoid them, do so. But I mention them because under some circumstance (radio-wise or marital-wise), they may be just the ticket.

(4) Glass mounts of various types.

The ones that go between the window glass and the window frame are SCARY.

There are antenna mounts that GLUE on both sides of a windshield or rear glass. These are SCARY in that if something grabs your antenna you could take a chunk out of your windshield and get glass bits all over your face.

A further disadvantage of those "Thru the glass" antenna mounts is that about 90% of the radio signal is lost. I have been taken to task about this statement, but I stand behind what I say.

This type of antenna is suited to a big city, protected environment, and perhaps covert law enforcement. It has no application for up here or serious ham work. However, your wife may insist on it for her Lexus.

(5) Various brackets, etc.

My Jeep has one mounted between the tail light and the body. Nice, but see the next article on antennas and grounds.

Another Jeep mount goes in the front, to the front door hinges.

Jeeps have several other options. Reason is that Jeeps have plastic or cloth roofs.

If your car has a luggage rack above it, there are mounts that will clamp to those rails. See next article regarding grounds. Otherwise they seem OK, some are fixed, some have the laydown knob and some are Motorized, like the one shown at left. If you carry a canoe up there, it might interfere with the antenna placement, so think this thru.

CONCLUSION

Radio wise the center roof mounted is best, without question. An order of magnitude better than any other way. It will require you to drill holes and go thru a bunch of rigamarole, but worth it to some.

The simplest is to "throw a magmount" on the roof and be done with it.

It might surprise you to hear that a Magmount has some saving graces: It allows you to TEST your mobile setup, etc,, before you go drilling holes. It also means that when you SELL your car, they don't knock off $1,500 trade in value because your Lexus now has an ugly hole in the middle of the roof. Even with a tire patch!

It also allows you to switch your setup from car to car, or even a rental.

I recommend you start with a Magmount.

The next article will discuss various antennas and have some specific model recomendations.