Contents: Caveats; Basics; Far Removed Image Planes; Mag Mounts; Ballmounts; 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 stray 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!
Antenna Efficiency is a balancing act between minimizing ground losses, minimizing stray capacitance losses, maximizing length, and maximizing radiation resistance. Keep these factors in mind while reading this article.
A decent HF mobile antenna is not a cheap 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 $1,000, 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 has. 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 and input impedance go down, and the efficiency goes up. These effects are a result of reduced capacitive coupling to the vehicle and to ground. The drawback is 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. HiQ makes some of the strongest mounting brackets, but again mounting can be a problem. If you visit HiQ's site, you'll find several photos of how others have mounted their screwdriver antennas and some are quite clever. At least it will give you an idea or two, as will my Installations article.
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 image plane (commonly, but incorrectly referred to, as 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 image 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, or your mounting scheme is poor.
Let's look at this another way. Wherever the coax connects to the antenna, is where the image plane should begin. In other words, if there isn't a mass of metal directly under this point, then image 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.
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 in Signal plus Noise to Noise ratio (S+N/N) may be as much as 25 dB, perhaps more in some cases. 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. Distractors say that's good odds, but it isn't. If you haven't already, read my Insurance article.
At the last 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.
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 actually 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 workmanship, 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 left photo. If your antenna is a heavy one, internal bracing may be necessary.
Almost every screwdriver manufacturer supplies a clamp mount like the Tarheel one shown at left. The supplied insulator isn't very robust. On other manufacturer's models, the top clamp is just a U-bolt, and a piece of rubber is supplied to wrap around the mast, and act as an insulator. During inclement weather, these mounting schemes can cause problems as they have a tendency to arc over. One solution is to coat the assembly with high voltage lacquer available at most hardware and home improvement stores. One thing you don't want to do, is coat them with car polish; the film that's left is conductive!
On some cheaper models of antennas, these clamps are an absolute necessity as the bottom 3/8x24 thread is not adequate to support the weight of the antenna. This fact makes one wonder if the rest of the antenna is similarly constructed.
The one in the right photo is a Diamond NMO mount. They also make them in 3/8 x 24, and SO239 styles. Most of them are designed to clamp on like trunk lip mounts. Unless you're using a ground-independent VHF antenna, I wouldn't use an angle bracket.
If you need just the angle adjustment, then look at the Breedlove unit shown at left. It's very sturdy, and will hold all but the largest of antennas. Visit their web site for more information.
Glass mounts probably shouldn't be listed here, because they are actually a VHF antenna. However, because they are worst of the worst, I just couldn't help myself. Glass mount antennas do not have a ground plane under them. This causes the return currents to flow on the outside of the coax. In fact, the coax does the majority of the radiation. If your vehicle has passivated glass (almost all do nowadays), the efficiency is near 0%. The best advice I can give anyone is this; don't!
When you just can't find what you need or want, one way out is to home brew your own. The mounts shown here are two of many I have constructed from Delrin® plastic. There are high res photos of most of them in the Photo Gallery.
The nice thing about Delrin® it's RF transparent up to the SHF region. Shy away from the various colors, especially black as they are not as RF transparent. One drawback, is cost. Current prices for 1 inch material is about $100 per square foot. The mount shown at right is 4 inches by about 6 inches, or about $35 worth.
It can be sawed, drilled, tapped, planed, and sanded with standard hand tools. In some cases, I have press fit ferrules into the material, but this really isn't necessary. Delrin® is very slick. In fact, it is almost as slick as Teflon®. If you need to keep a through bolt from turning (see ballmount photo, upper left), I suggest you place I/O star washers on both sides of the mount.
If you need to drill holes larger than 3/8 inch or so, use a Forstner or spade bit. Make sure you clamp your work to keep it from binding, and take your time to avoid melting the material.
The proliferation of CB radio use by OTR truck drivers, has resulted in literally thousands of different brackets. Angled ones, clamp on ones, bolt on ones, you name it and you can probably buy it. Some are rather husky, others aren't much more than junk.
While the bracket themselves may not be junk, the places some of them are use certainly are. For example, using one to mount an antenna to a mirror bracket. The left photo is an example. The simple question to ask yourself is, where's the ground plane? There isn't any!
Sometimes, there isn't much choice about where we mount an antenna, efficiency notwithstanding. If this is your plight, take a quick trip to your local CB radio shop. You just might find the exact bracket you need.
If you own a pickup truck, Geo Tool and Breedlove Machine Shop have the solution.
Geo Tool make a variety of stake pocket mounts to fit all of the popular makes. They're sturdy enough for just about any antenna. Fact is, they are stronger than most stake pockets! The unit shown in the left photo is their 3/8 x 24 style which is the one to use for all HF mobile installations.
Besides a 3/8" x 24 threaded style, Geo Tool also have NMO and SO239 styles. By the way, and SO239 type mounts is not the stuff of champions. Anything bigger than a VHF 1/4 wave and you're going to have problems. Fact is, you're better off with the NMO style. They also make a special Delrin® insulator for use with auto-coupler based installations.
The pocket mount in the right photo is a Breedlove unit designed for the Border Patrol. Note the solid brass base adapter. This is just the ticket for those antenna which do not have a built in base assembly. Higher res photos of all of the Breedlove mounts are in the Photo Gallery.
There maybe others who make stake pocket mounts. However, most of them utilize a rubber block that is squeezed by a mounting bolt. Thus they rely on friction to hold them in place. I wouldn't trust them to hold anything more than a small VHF antenna.
It is not uncommon to see antennas mounted atop a long post, like the Tarheel model shown at right. The consciences of opinion is, that by raising the antenna to the top of the post will increase efficiency by unshadowing the mast and coil. As alluded to above (Far Removed Ground Planes), this premise is incorrect as the post raises the antenna above its ground plane. This increases losses. Regardless of their popularity, trailer hitch and post mounts should be the last resort.
To be honest here, this particular Tarheel post is a variation of the foldover one shown below right. The antenna bracket clamps to this post with U-bolts.
There are several styles of quick disconnects. Some are made for the whip itself (like the aforementioned Breedlove units), and some are made sturdy enough to be used at the base. The ones in the left photo are from HiQ Antennas. The heavy-duty base mounted ones are on the left, the lighter duty ones on the right.
Hustler and High Sierra both sell decent units. Both screw on and bayonetted ones are available. Some are plated brass, and some are stainless steel. The few that are made of pot metal aren't worth the effort.
Incidentally, HiQ now has a heavy-duty, base-mounted foldover, as shown. It's sturdy enough to hold the largest of antennas, HiQ or otherwise.
When using a body-mounted ballmount, springs are virtually a prerequisite. The spring helps prevent damage from low-hanging branches, and the ever-present antenna twangers who can't resist playing with your favorite toy. Most amateurs don't use springs at the base of their antenna as they believe is causes the SWR to vary while underway. If this is the case, chances are you have your antenna mounted too low and/or too close to the body. The other misconception is the inner braid fails. I used an old HyGain heavy-duty unit I have owned for some 30+ years. The braid is still bright and shiny.
Although I've never guyed an HF mobile antenna, doing so has some safety benefits, and allows the use of a lighter spring. However, let me add this; guying is not a substitute for proper mounting! In other words, if you have to resort to guying to keep your antenna in place, you haven't properly installed your antenna!
Although fishing line seems to be the preferred medium for guying, I think it stretches too much. Phillystrand® is a better option. Choosing how to anchor one end to the body is a personal choice. On the antenna end it is best to attach the guy below the coil where the voltage is less. Remember one thing about guys. Regardless of the material you use, moisture can collect on it or in it. Transmitting when it is wet will net you a burned guy, high SWR, or both.
If you plan on using a trailer hitch as an antenna mount, here's a suggestion. Have a second hitch ball receiver welded on the left end of the hitch. Then a short chunk of square tubing may be used as the actual mount, and it's easy to remove when necessary.
The trailer hitch receiver mount shown at right is one of several models sold by Tarheel. They are sturdily made, and will hold just about any mobile antenna. The mount slips into the hitch's receiver, and is secured by a bolt. This particular model features a folder over hinge, which might not be suitable for every installation.
The mount shown at left is one from REP Designs. It's a bit more complicated than some of its competition, but otherwise appears sturdy enough for just about any HF mobile antenna.
One very important thing to remember about trailer hitch mounting; in terms of efficiency it is the least desirable location. And contrary to popular opinion, running a ground strap to the frame will not negate this fact. If you doubt this premise, read my Antenna Efficiency article. Further, due to the extra ground losses involved, you have to use extraordinary decoupling to keep the RF off of the control and coax leads.
I do not like trunk lip mounts for several reasons. First, the coax feed to such mounts requires a sharp bend in the coax which is a potential problem area. They all are held in place by some form of set screw, and every single one I have ever seen was loose. Unless you're using a ground-independent VHF antenna, and have no other mounting choice, I wouldn't use one. That's why there isn't any photos of them here.
Whips, sometimes called stingers, are the flexible part of a mobile antenna extending from the top of the coil. Every amateur radio supplier stocks at least one brand. Typically sold without any ferrule, they vary in length from 24 inches to about 72 inches. Folks who need a longer whip usually use an 8 foot CB whip and cut it down if needed. If you do this, don't forget to add a corona ball.
If you need something a lot longer, say 12 feet (right photo), Array Solutions might be your best bet. While some of the pieces they sell can be purchased elsewhere, having all the needed parts in one bag has merit, and you save a little money too.
Their heavy duty mount shown at left, will support any practical mobile antenna. There is a price list on their site with a breakdown of the individual pieces. If you wanted to, you could use the individual pieces to build a stationary mobile antenna long enough for 20 meters without any loading coil (17 feet), and have no support worries. Incidentally, the heavy duty spring they sell is as rugged as the old HyGain unit which hasn't been available for many years.
Some amateurs use bronze welding rods for whips, and to build cap hats. Bronze works well, but it's easier to kink than stainless steel. You'll need a ferrule, and you can purchase them from Sauder Electronics in three different diameters (.1, .125, and .2), all with a 3/8x24 threads. MFJ also has a variety of mounting accessories.
While it is possible to make your own mast, some of the most rugged ones money can buy are from GLA Systems, better known as Texas Bug Catcher. Just about any length from 1 foot to 10 feet can be ordered. They're made from 5/8 inch stainless steel tubing, and are nearly indestructible.
The 3/8 x 24 studs attaching the various parts of our antennas together are another problem area. Several companies supply stainless steel ones which aren't much stronger than mild steel ones. I've had several pull apart over the years so I found a solution. I use 3/8" x 24 x 2" grade 8 or 9 bolts and cut their heads off with a Dremel® cutoff tool and then polish the threads using a small grinding wheel. Ace hardware and most Caterpillar dealers sell these high-grade bolts. I can almost guarantee your quarter panel will be ripped off before the stud fails!
Did you ever wonder why the threads on a mobile mount are 3/8 x 24? I've read several accounts, and heard from several OTs, and no two agreed. A few weeks ago, while reminiscing with an old friend, he said something that keyed my memory.
Once upon a time, standard threaded bolts were standard strength, or grade 1. While fine threaded bolts were usually grade 3 (or higher), and somewhat stronger. One of the reasons was; for any given length wrench, you could tighten down a fine thread must tighter than a course thread (the ramp principle if you've forgotten your high school physics), so you needed to have the higher strength. Given that was the case a long time ago (not true today), if you wanted a stronger bolt, you had to go to a fine threaded one. The question remains, why 3/8 inch? Well, that I can't answer.