Contents: Caveats; Basics; Far Removed Ground Planes; Mag Mounts; Ballmounts; Bed Mount; Clamps; Foldover Mounts; Glass Mounts; Home Brew Mounts; Miscellaneous Brackets; Quick Disconnects; Pocket Mounts; Post Mounts; Springs & Guys; Trailer Hitch Mounts; Trunk Lip Mounts; Whips & Masts; Odd & Ends;
There is one very important caveat to keep in mind when reading this article. The roof of a vehicle is a very good place to mount an antenna. However, more and more new vehicles are coming equipped with side curtain SRS devices (air bags). They typically are mounted along the edges of the headliner including the rear seat area if there is one. The wiring to these devices may be routed through any one (or more) of the roof pillars. Extra care is required when installing antennas in vehicles so equipped. If you are the least bit apprehensive about your installation, seek professional help from your dealer or a qualified installer.
There are many reasons to permanently install any mobile antenna. Aside from maximizing performance by minimizing ground and shunt capacitance losses, there is also a safety issue, and an insurance issue. Remember this one very important point; some insurance companies will not cover antennas which are not permanently installed. This fact precludes the use of mag mounted antennas. What's more, if you fail to inform your insurance company of antennas and radios, you might not be covered at all!
A lot of misinformed folks use clip and mag mounts because they're convinced they don't depreciate the value of their vehicle. Look closely at the left photo (click to enlarge). While a drilled hole can be easily plugged, and go unnoticed, there is no way this type of damage can be ignored.
By the way, the antenna shown is an ATAS-120; one of the lossiest antennas money can buy, all $450+ of it. It requires an SO239 type mount, which is suspect at best. It is also very prone to common mode currents, a fact noted in the FAQs about the product. What's more, it can not be controlled by any other brand of radio, and no one makes an interface for it, except for a manual one.
One very important point needs to be made here. A vehicle is not a ground plane, but rather a capacitor between the antenna and the surface under the vehicle which acts as the ground plane. Since the surface in question is a poor conductor of RF, ground losses occur. The term ground plane in the following text is therefore a bit of a misnomer, but is used to differentiate it from DC and RF grounds.
First entry in the excuse book? My wife won't let me drill holes in my car.
Antenna Efficiency is a balancing act between minimizing ground and shunt capacitance losses, and maximizing length and radiation resistance (Rr). Keep these factors in mind while reading this article.
A decent HF mobile antenna is not a inexpensive commodity. While some HF monobanders are as cheap as $20, their performance, no matter how they are mounted, is substandard. A high Q, excellent quality, HF mobile antenna can cost upwards of $1,200, although most are about half this amount. Assuming you spent this much on yours, why would you mount it in an inefficient manner?
When it comes to HF antennas, there are so many different configurations, it is all but impossible to write a how-to. There are, however, a few common threads which need to be followed for various reasons, not the least of which is safety.
What ever method you choose for mounting your HF antenna, the first rule is it must be sturdy enough to hold the weight of the antenna without too much flexing. For example, the Lakeview mount shown at left won't hold much more than their Hamstick®.
It should be attached in such a way to maximize what little ground plane a vehicle represents. The key phrase here is, it's the metal mass directly under the antenna, no along side that counts! More on this below.
Another often overlooked requirement is which side of the vehicle the antenna should be mounted on. Here in America, we drive on the right and steer from the left, so we should choose the left side. This can be very important especially when you live in an area with low bridges and overhanging trees, as there is more clearance towards the center of the street. Further, it is easier to see the antenna in the rear view as opposed to craning your neck. It's also less likely to be "twanged" when you're parallel parked. If you've chosen front mounting as a lot of folks who pull trailers do, then it should be mounted to the right into your peripheral vision to avoid distraction.
Vans and motorhomes present a special case. Except for front mounting, the body of these vehicles shadow a large portion of the antenna. This causes tuning problems and reduces efficiency which is already poor at best. If shadowing cannot be avoided, then make sure the coil is as far away from metal as possible. I suggest readers look at the article "Mobile and Loaded Antennas" on Tom Rauch's, W8JI, web site for further information.
I mentioned earlier about maximizing what little ground plane a vehicle offers. My article on Bonding covers part of the solution. The aforementioned shadowing is also important, and there's another one, mounting height. As we raise an HF antenna from low on the bumper towards the roof, the resonance frequency increases. Part of this increase is due to a decrease in ground losses, and part to less capacitive coupling between the antenna and the surface under the vehicle. The input impedance decreases too, which is a good thing. Remember, reduced ground loss equates to increased antenna efficiency, even though the unmatched SWR might increase.
Digressing for a moment. Ground loss is in series with the other antenna losses, including radiation resistance, and it is these combined losses which make up the input impedance. Therefore, any increase or decrease in one or more of the other losses, will also effect ground loss, and the current which flow through that ground loss. As a result, you cannot assume that some change in one (or more) of the other losses making up the input impedance is a positive (or negative) one, without considering all of the other factors as well.
There is drawback to increased height, as it raises the tip of our antenna to perhaps unmanageable heights. From a safety and practical standpoint, the maximum height is from 11 feet to 16 feet depending on your area of the country. Here in the desert southwest, I have little trouble with the tip of my antenna extending to 16 feet. If I lived in the New England area, I'd be hard pressed with just 11 or 12 feet.
Most HF mobile antennas are already length-compromised and shortening them isn't really a viable option if you're seeking good performance. You could use a cap hat (capacity hat) which will allow you to shorten the antenna and maintain some level of efficiency, but you have the extra wind load and complexity to contend with. This height vs. efficiency vs. practicality conundrum leads to an obvious compromise. How much of a compromise depends on the type of antenna you use and where you choose to mount it. The only rule of thumb is, mount it as high as your terrain and antenna length will allow up to the point of striking low-hanging wires, branches, bridges, and parking garages.
There are many different manufacturers, lengths, and styles of remotely tuned antennas, but they all have one thing in common; they're difficult to mount. They require both a coax feed and a power feed, and no one makes a universal mount for them. Due to the extra weight compared to a nominal monoband antenna, the mounting medium must be extra strong. As a result, most folks opt for a bumper or trailer hitch mount which reduces their efficiency. Additionally, most have short masts making it difficult to get the coil clear of the bodywork. With a large mass of metal close to the coil, they are much more of a compromise than most folks realize. Visit the Photo Gallery for ideas on how to mount yours.
The type of mount is dictated by several conditions. These include a yes/no on drilling holes, the size, weight, and length of the antenna, and to a lessor degree (in my eyes) aesthetics and spousal approval. If you're into daily HF mobile operation, you've already drilled the necessary holes. If you're not, you could stay with VHF. Or, if your level of satisfaction is low, then a license plate mount and Hamstick® might be enough.
I have heard just about every excuse for not drilling holes to mount antennas, and about 50 more you've never thought of. It isn't this fact which surprises me, but the lengths some amateurs will go to avoid doing so. Part of the problem is, too many folks don't know what constitutes a ground plane. Or the difference between DC and an RF ground. Instead, they believe a simple ground strap will solve their problems, regardless of the cause. Nothing could be further from the truth!
A vehicle is an inadequate ground plane for any HF antenna. Mounting the antenna atop a long stalk (as shown left) adds insult to injury. How much injury depends on the installation. Remembering a nominal HF mobile antenna will have an unmatched input impedance of about 25 ohms, if yours measures 35 ohms or more, the bottom line is, your antenna is lossy, your mounting scheme is poor, or both.
Let's look at this another way. Wherever the coax connects to the antenna, is where the ground plane should begin. In other words, if there isn't a mass of metal directly under this point, then ground plane losses increase. What's more, this causes RF to flow over the outside of the coax (common mode currents), and on what ever control lines are present. While RF chokes can reduce the amount, they are no more of a cure than a ground strap is.
Compare the coax feed points (blue circles) in the two drawings. The post mounting method shown, is equivalent to installing a dipole with the elements parallel to one another; an obvious faux pas. Again, it is the metal mass directly under the antenna, not along side, that counts.
In the case of the mount shown at left, the ground plane losses could easily exceed 30 ohms, even on 20 meters! This photo, taken during the 2008 Dayton Hamvention®, is the worse example of poor mounting technique I've ever seen.
I often get ask how much difference there really is? The answer depends on a lot of factors which vary from installation to installation. From personal experience, the difference between a trailer hitch mount, and a quarter panel mount is always more than 10 dB. In some cases, as much as 20 db! There is more information about this subject in my Antenna Efficiency article.
If you still can't comprehend the importance of an adequate ground plane, you should read this article.
I've placed mag mounts first for a good reason; they're not safe! The one shown in the left photo is a typical unit. When they're demonstrated in retail outlets, they're stuck to a thick chunk of metal. Since the amount of force is reliant on the thickness of the material it is stuck to, the actual force will be much less when it's sitting atop your vehicle. And it doesn't make much difference how many magnets they have, or how big they are. The fact remains they will become flying missiles in the event of a crash. There has been one confirmed death, and possibly a second, caused by a mag mount dislodging in a crash. Distracters say that's good odds, but it isn't. If you haven't already, read my Insurance article.
At a hamfest I attended, I ask the owner why his three magnet mount was tied down with a couple of bungie cords. He said he tied it down after it came off while he was on the interstate. Aren't you glad you weren't behind him?
There is another problem with them; they increase ground losses. If you've read my Antenna Efficiency article, you'll know why minimizing ground loss is so important. If you think attaching a ground strap to the nearest hard point on your vehicle will help negate this situation, you're wrong! By the way, it doesn't make any difference how many magnets they have, they're still lossier than direct mounting.
There are two more problems users should be aware of. First, they collect road debris (mainly metallic brake dust) which eventually gets between the magnet and the sheet metal. Secondly, the base cover, usually a rubberized plastic, has an affinity for clear coat vehicle finishes. It is a given that the surface under the mag mount will become both scratched and/or discolored over time. In some cases, less than a week!
The bottom line is, if you have to resort to a mag mount for your antenna, you haven't thought long enough about other, safer, more efficient, mounting techniques.
Nowadays, most ballmounts aren't worth the effort, and the one in the right photo is worst of the lot. I don't know the actual manufacturer, but Motorola®, Wilson®, Antenna Specialists®, Hustler®, and others currently sell the exact same one. It consists of two, stamped, stainless steel half spheres, separated with a rolled steel ferrule, replete with a cheap plastic insulator and thin backing plate.
Note the teeth at the bottom of the ball. They're suppose to keep it from rotating. They don't. If you tighten the cross bolt enough to hold the antenna upright, the ferrule will unroll, and your antenna will flop over. If your antenna weights more than 2 pounds, forget about this mount! Worse, the various pot metal ones found in CB shops won't reliably support a Hamstick®.
If you can find one, the old cast iron GE Master ballmount is an excellent choice. They're probably the strongest ones ever made. The photo at left shows my GE unit with a home brew Delrin® insulator. I'm sure there are plenty of these ballmounts out there languishing in attics and garages.
The insulator (3 7/8 inches in diameter) was made from a one inch thick sheet of Delrin®. I used a combination of holes saws and Forstner bits to drill the holes, and counter sink the ball itself. There are press fit steel ferrules inserted in all of the drilled holes, and the ball is pinned in place to keep it from rotating. With a 8 x 12 x 1/4 inch backing plate, it easily held up a 14 pound, 16 foot long, HF antenna.
There is one all brass ballmount available (right photo) made by Breedlove Machine Shop. They come with and without the quick disconnect shown. The QD is also available as a stand-alone unit.
One model sports larger end bosses, rendering it sturdy enough for most screwdriver and bug catcher antennas. However, its small backing plate negates its use on body panels. It sells for about $100, and considering the quality of workmanship, its worth every penny.
They recently introduced their large ballmount, and it is indeed a work of art. As you can see from the left photo, it has a large aluminum mounting plate, and under ideal conditions will hold about any antenna. The ball itself is pinned to prevent rotation, making for a very secure fit. The unit pictured in the upper left of the photo, is an SO-239 built as part of the center stud attachment bolt. This makes the coax connection very secure.
If you do use a ballmount for body mounting, you should endeavor to attach it near a major crease in the sheet metal to minimize flexing. You can just see the crease under the insulator in the upper left photo. If your antenna is a heavy one, internal bracing may be necessary.
If you own a Honda Ridgeline, you might be interested in this mount (click on the photo for an expanded view). It's made by REP Designs. It fits all years, and includes spacers to raise it above the factory bed cover if so equipped. It bolts onto the existing upper-front ties downs, so no holes are required. Be advised that the tie downs are covered with a very thick layer of paint. As a result, they do not make adequate contact with the body of the vehicle. The use of heavy-duty star washers under the heads of the bolts is a prerequisite. In any case, HF antennas should be mounted at one end of the bar (not the middle) for stability, and low ground losses.
Since I personally own a Ridgeline, I've been asked numerous times how to get coax and cables into the bed area. There are two ways, but you have to drill holes. If you remove the front bed cover, there are two flow-through air vents which can be used. Further, if you remove either side panel, you can use one of the existing grommets. The left one will be used if the optional rear camera is installed. However, there is not enough clearance for wiring to come out from behind the panels. Most owners either drill the front cover, and use grommets. If you use a bed cover, then route the wires up to the location of the cargo lights, and drill the holes there. The panel which holds the cargo lights costs about $40, and can always be replaced in the future. Just for the record, the space between the bed floor and the trunk area is about 1/8 inch, and even then fishing a wire through the gap is nearly impossible.
Here's an interesting tidbit about Ridgelines, albeit off the subject. If you look closely in the trunk toward the passenger side, you'll notice a large grating. This is a vent which allows airflow in and out of the trunk. If you live in a clime where slushy snow accumulates under the vehicle, it can also accumulate over this vent sealing it off. If you then slam the trunk lid, one or more of the plastic plugs in the bottom of the bed will pop up. These plugs cover the holes for the trunk lid hinges, and the rear suspension mounts.
This mount is similar to a clamp, but designed to mount in the side rails of Nissan and Toyota small pickups. It's also made by REP Designs, and is one of many they supply. Personally, I don't like the insulation material used, and if it were mine, I'd use a block of Delrin®. Remember, any metal which clamps around the antenna mast adds shunt capacity to the lower portion of the antenna, and is not recommended.
The best mounting location along the rail is in the far rear. This keeps the antenna away from the back of the cab, and in the clear.
Again, while off the subject, look at the split bead applied to the motor leads (click to enlarge the photo). This is inadequate choking, even for a manual controller. Assuming the bead is a mix 31, the impedance of the choke is less than 150 ohms, and should be a minimum of 5,000 ohms! For more information on the requisite choke, read my Antenna Controllers article.
