Mobile Antennas, Part 3

by Ramon Gandia, AL7X 2/23/2013

See also Part 1 and Part 2

Okay, you have read the two previous articles and have more or less decided on some sort of mount. Now what antenna are you going to put on it?

Ground considerations. When you mount an antenna, what is directly under it is of vital importance.

Read the above paragraph ten times and take it to heart!

Most, like 99%, of mobile antennas require that the antenna be mounted on a metal roof. That serves three purposes:

  • Half of the antenna is comprised by the metal roof;
  • The antenna's radiation pattern requires the metal roof; and
  • The antenna will not match properly to your radio without it.

As you know, an antenna has two wires feeding it. Just like a light bulb that needs two wires to it. Electricity flows from source to sink, or from plus to minus, or in the case of AC from plus to minus at the instant of the ac cycle.

The simplest antenna of all is a half wave dipole, just a wire half inch long, cut in the middle and fed with coax cable. The coax cable has two conductors: the center conductor and the shield.

However in VHF mobile work, the horizontal mounted dipole so favorite of HF operators do not work. For one thing, it would be an ugly brute with two short towers on your car roof, and for another thing, we want vertical polarization.

Because of these reasons, automotive - and in fact - mobile and portable antennas are usually whips of some sort.

The typical whip antenna is a quarter wave long, about 19 to 20 inches on the two meter band. This whip is connected to the center conductor of your coax, and the coax shield to the car roof right next to it. Many antennas of this sort are in use worldwide. They are simple, unobtrusive and inexpesive. Besides, they work rather well ...

This type of antenna is a dipole that has the car roof as its other half.

And because of its appearance, most people think that the whip is the important part, and the roof is not. That could not be further from the truth!

If you were to mount this antenna on a plastic, wood, or fiberglass roof, it would not work. You will get a tiny percentage of your signal radiated, the rest would be reflected back to your radio. This would be indicated by a high swr, and could result in damaging your $$$ radio.

You can sort of improve on this antenna by installing one that has gain.

Let's talk about gain a bit. Gain is not like an amplifier. An antenna realizes gain by radiating more power in the desired direction and less in other directions.

In a vehicle, we usually want the antenna to radiate equally well in all directions around the car. But, for the most part we do not want the antenna to radiate upwards. In other words, we can concentrate the power towards the horizon and less towards the sky.

A notable exception are interstate truckers. They travel down these interstate roads, and they talk to other truckers along that road; thus they do not only want to radiate to the sky, but they also want their signal to go to the front and to the rear of the truck. They do this with two whips, side by side, typically mounted on the left and right mirrors of the semi. This type of configuration is called co-phased antennas.

It serves no purpose in ham radio, particularly in the Nome area where your repeater or contact could be in any direction from where your vehicle is pointing. Please do not ask me about co-phased antennas.

Now that we established what gain is ... how do we get gain in a mobile whip? Well, for one it is going to take a longer structure than a quarter wave whip. It could still be a single metal rod, up to a certain length ( 5/8 of a wave ), or it could be a stack of elements of rod, spaced by phasing sections or coils. Look at the Comet CSB-790A antenna at left.

As the antenna gets longer and longer, the more and more gain it has. This can be good or bad.

The good about antenna gain

With higher gain you will be stronger, reach farther and impress your friends more. Check out this article in the MARC website concerning the review of a 61 inch tall Comet CSB-790A antenna vs another Comet model but one that was only 50 inches tall.

Pretty impressive, right?

The bad things about antenna gain.

There are several disadvantages:

  • Expensive. More stuff, more dollars.
  • Mechanically difficult.
  • Propagation difficulties.

The mechanical aspect is that a 61" or taller whip is hard to hold up. As a rule, mag mounts will not hold them up reliably. If they tip over, they usually get bu$$ted up. The big magnet might come in thru your window, and if you go into a garage, under a sign, under a low wire or thru willows you will have all sorts of things break.

Sometimes what breaks is the roof of your car. Not only from hitting things, but also the continuous back and forth flexing of this whip will put a lot of torque on your roof, your mount and the antenna itself. Often installing a spring helps, like KL2ZF here in Nome has done on his snow trac. He has a big spring!

Not all antennas can sport a spring; most will be affected electrically by adding one, so don't count on "... i'll just add a spring if I have trouble."

Propagation means that your signal does not go in the intended direction or acts funny in several ways.

If you are going up a hill, or down a hill, that antenna that radiates beutifully on level ground now is pointing the signal at the sky or the ground. This drawback is magnified if you park your car in other than a level position. You may well park where the antenna has no useful signal towards where you want to talk to.

When the vehicle is moving at speed, that antenna will often bend in the wind. Where do you think your signal is going with the antenna leaning back 30 degrees? The worms and birds will be happy to receive a moment of warmth from your ham radio, but the signal will fade.

If you are on a bumpy road, the antenna will dance around like Mae West, or one of those Polynesian girls doing the hula. Where do you think your signal is going then?

Usually those problems are manifested by what is known as picket fencing.

Let's say the signal is not too strong where you are. If the antenna is perfectly vertical, the signal is okay, but if it leans, tilts or bends, the signal fades, either completely or partially. As the car moves, the signal comes in an out on a regular basis. If you have ever driven in the lower 48, on a rural road with farms having picket fences, and the windows down, you can often hear whoosh - whoosh - whoosh - whoosh as the car passes each fence post.

This is exactly what you will hear with this type of antenna.

"So," you ask, "why do a lot of people use them?"

Antennas, driving areas, cars, driving styles, terrain and just preferences is the main factor regarding which antenna you use. Think girls. Some like the dark, sultry ones with long black hair, and some like the lanky, tall blondes with short hair. They can both make terrible wives with no guarantee of which is better or worse. It is a matter of likes and preferences.

"So," Ramon, "which one do you like?"

The dark, sultry ones with long black hair! I joke! I like my antennas to be the shorter ones. I am not alone. You will hardly see a fire truck, a Police Car, a Taxi or most urban vehicles with anything but a quarter wave whip! Less trouble, less cost, less prone to damage, and better and more reliable communications at moderate ranges.

Back when SPARC came into being, I had this blue Chevvy pickup and my antenna was an 18 inch dual bander smack in the center of my roof. It worked real well. Until I took the Council road between Solomon and Council. Then, Colby Carter, KL0CR following me in his truck with a longer antenna could hit repeaters that I could not.

Then there is Don Erickson, KL2ZF, that has the most absolutely monstruous antennas on his truck and SnowTrac. He can regularly talk to KL9ABCD on the far side of the moon with it! But he has also had a series of mechanical failures, mounting difficulties, etc. etc. with his. He is quick to tell you how HE solved each problem as it arose, but trust me: when you get one of these monsters for your truck, you will go through the whole 12 stations of the cross in getting things to settle down. If they ever do. I fully expect KL2ZF to break his antennas still yet!

In the lower 48, with all paved roads, gentle hills, fair winds, and sultry maidens, much less abuse, these long antennas may well be worth it.

And, in that MARC article they also talk of Goldwing Motorcycles parked in a lot; they did not really talk about long-range performance in rough conditions. A word to the wise.

My car has a plastic, fiberglass or cloth roof! Waaah!

Oh, you mean like my Jeep?

First, close your eyes, and look at your car with your mind's eye. Imagine that all that plastic, fiberglass and cloth is not there. Gone, poof. Something will jump at you.

Without that invisible roof stuff, why do you insist on mounting your antenna atop a cloth roof? Makes no sense. You can mount the antenna on a fender, or even in your hood provided you don't mind going cross-eyed trying to look around one on the hood.

If your antenna requires a ground, you are going to find that mounting on a fender will give you skewed antenna propagation, and perhaps not a good ground at all.

In that case, consider an antenna that needs no ground.

Remember the half wave dipole at the start of this article? It is possible to feed a half wave dipole at the END, not the middle. It requires a coil or matching section to do so. When done with a mobile whip, the result is a half wave or 5/8 wave antenna that needs no ground.

But beware, just because an antenna is half wave or 5/8 wave does not mean that it is of the ground-not-required type. Most, in fact, will need a ground or will not tune up. But some are made to operate with or without a ground.

In my Jeep I have the antenna mounted on the left rear corner, on a mount made of steel plate that sandwiches between the tail light and the fender. Because of the unfavorable location, poor grounding, no metal area to speak of under the antenna, I chose the Diamond NR-770HB whip. This is about a yard tall or 40" long, dual band, and can operate with or without a ground.

I could have mounted it on my plastic roof, but thoughts of hitting a willow and making a big hole in the roof dissuaded me. And trust me, in a Jeep you hit willows often. If you don't, why did you get a Jeep in the first place?

Confessions of a short, groundless, Jeep antenna owner.

My antenna looks clean, spiff and neat. But it has a few shortcomings. For one, since it is mounted lower than the roof line, it does not reach out as far. Then, being mounted on a vehicle corner means it radiates better -believe it or not- diagonally across the car towards the front right. By a very noticeable amount!

But it sure looks spiff and clean, does it not?

Someone asked me if I could have put this antenna INSIDE my Jeep, after all the plastic body is transparent to radio, right? The answer is, yes, sure, but there are mechanical issues with that option. So I went the way I did. Your situation may be different, and in fact here is an axiom:

Each car presents a different set of issues and choices.

Another antenna option is one that has a SPRING built into it. The spring will absorb much of the abuse that an antenna takes, as well as gives to the car. The Comet ss-680SB shown at left is one such, and it is one rugged mother! It is 27 inches tall, has dual bands, usable gain on 2 meters and is virtually indestructible.

This particular one will fit a UHF mount; I believe they also have an NMO mount version. It does require a metal roof to operate properly.

Don, KL2ZF, has discovered that springs on an antenna make a big difference. His antennas are monsters. I fear that one of these days he will snag a powerline and will get incinerated ... but, the fact is that he was braking all sorts of things and tearing divots out of his truck and SnowTrac until he put in springs.

Don's springs are not for the faint of heart, but they do the job as his springs are scaled to the size antennas he uses. This particular one has a much more modest spring, but one suited to the antenna in question.

Here at left is a very plain, mag-mount antenna, also made by Comet. It has a small magnet, but one up to the task. This antenna is only 20 inches tall. If mounted on the roof of your car it will simply talk one or two orders of magnitude farther than the rubber duck on your HT handheld.

It comes in two styles: the M24BSJ has a coax with an SMA female, that will go into Kenwood, Puxing type of HT radios. The M24BS has an SMA male that will plug into the non-standard Yaesu HT's. If you get the antenna, and later switch radio brands, you can get an inexpesive "gender changer" type of adapter that changes male to female. You can also get other adapters to go to BNC or UHF or mini-UHF connectors as found on other HT's or mobile radios.

This little mag mount antenna is very, very nice and is one I highly recommend to newcomers that have an HT, and not much faith in drilling holes in car roofs, or getting welder, metal benders, big drills, macho men, etc. involved in the project. It has low impact on cars and wives, but remember: It needs a metal car foof, and you ought to put it as close to the center of the roof as you can. Make sure your roof is clean and flat where you place it. A bit of dust or grit will sharply reduce the magnetic effect of the mount, and off she goes!

An antenna for the firemen!

Firemen that are hams have a dilemma. They like to operate on the ham 2 meter band, but might also want an antenna that will work on the fire department frequency of 155 mHz. Comet makes an antenna for them. It has been very carefully engineered to work across both the ham and commercial VHF and UHF bands.

is the antenna in question. It has a low SWR from 144 to 160 mHz on VHF, and from 435 to 470 mHz on UHF. This antenna will work on ham 2 meters and Fire/police/taxi/marine etc up to 160 mHz, and on UHF will work on ham/taxi/GMRS and FRS family radio service channels. It stands 40 inches tall. If you are a Fireman you need or else. Else what? Well, if you buy a ham antenna, then on the Fire channel you are throwing out 95% of your watts and risk possibly burning out your radio. If you have a Fire antenna, then the operation on the ham channels will be equally challenged.

Before you operate an antenna over 20 to 24 inches long on a mag mount, check carefully the specs on the mag mount and make sure you get a mount that will carry your proposed antenna. And once on the roof, keep an eye on it make sure it does not drift across your roof, or pop off. A good way to do this is to put a tiny, postage stamp sized duct tape alongside your mag mount, and see if the antenna moves away in relation to it.

That concludes this series on Mobile Antennas. Watch out for future articles on Mobile radios, and HT in various tactical scenarios. I hope you enjoyed reading my rants as much as I enjoyed writing them.

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Copyright © 2013, Ramon Gandia, all rights reserved.