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. 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.
Without doubt, one of the most important aspects of installing mobile antennas is having a good understanding of what is, and what isn't, ground. Ground, in this context includes ground (Image) plane, DC ground, RF ground, and the ground surface under the vehicle the antenna is mounted on. Because of the importance of this subject, I have dedicated a separate article to it. Before you plan, or complete, any antenna installation, do yourself a favor, and read my Grounds, RF & DC article.
The next article you need to read is Antenna Mounts. It's a little known fact, but an otherwise decent antenna can be made nearly useless by poor mounting techniques and/or location.
If your antenna doesn't need matching to achieve a low SWR, then you either need a better antenna or mounting position, or both!
Bonding is also very important. Bonding maximizes the performance of any antenna system, and minimizes the chances of of RFI ingress and egress.
If you have RFI issues, and you don't know what they are, read my Noise ID article.
If you're plagued with static, you might want to read my Static Control article.
If you are planning on using a remote controller for your antenna, read my Antenna Controllers article.
I have been taken to task several times for panning and/or praising certain antennas presented in this article. If I have overly panned or overly praised an antenna, there is probably a good reason. Most of the time, it is because I have first-hand knowledge.
In some cases, the basic design of the antenna is so poorly executed, it doesn't take much knowledge of such things to know they're nearly worthless. This also applies to junk science claims some antenna manufacturers tout in their literature, especially imported ones. On the other hand, some antennas are so superior in form and function that even an untrained eye can spot the difference. I might add, the selling price isn't an indicator of quality, longevity, or efficiency.
One unfortunate aspect, are the many questionable articles on the subject, in print and on line. One recent article published in the November 2007 issue of QST, would have the reader draw the conclusion that spirally wound antennas are better than any of the other types; an obvious misrepresentation.
Remotely controlled (motorized) antennas are commonly referred to as screwdriver antennas. They're called that because the first examples utilized a stripped down Black & Decker rechargeable screwdriver assembly to adjust the resonant frequency of the antenna. The motor turned a threaded rod in and out of a nut attached to the bottom of the coil. This in turn moves the coil in and out of the mast. Contacts at the top of the mast slide on the outside of the coil, thus adjusting the resonant point. Nowadays, calling them screwdrivers is a bit of a misnomer as the infamous B&D motor have been replaced with much more reliable gear motors.
Incidentally, Don Johnson, W6AAQ, is credited by many as the father of screwdriver antenna. His was, in fact, not the first motorized antenna, nor was it the first to hide the unused portion of the loading coil inside the mast.
I've placed this section here for a very good reason, which will become glaringly evident once you read it.
There are at least two commercially made mobile antennas that are nothing more than a 50 ohm, non-inductive, resistor connected across the coax feed, with a radiating (?) element attached to the center conductor. These antennas put new meaning in the term, dummy load on a stick! The advertising hype associated with these antennas is more than deceiving, it's down right fraudulent!
One of these is the Maxx-Com. It's advertised to have automatic matching with an input SWR of 1.2:1 or less from 1 MHZ to 54 MHz. The $400, 200 watt unit consists of a small box (about 3 inches square), an insulated output port, an SO239 input port, and a mounting bracket. The instructions state the whip can be any length, and the auto-tuning feature will compensate. This is pure, unadulterated, bovine fecal matter. The truth is, about 99% of the input power does nothing more than heat up the resistor.
Comet recently introduced a version of their own, the HA-750BL, which has a street price of $269 (you can buy a decent screwdriver for that amount). It too is just a 50 ohm resistor with a radiator attached to the center conductor of the coax. What's more, it mounts atop an SO239; a very poor mounting method.
Some manufacturers would have you believe winding a 5/8 wave length wire around a 4 foot mast will give the antenna gain. The facts are, no matter how much wire you wrap around an antenna mast, the antenna's effective length is the length of the mast. Undoubtedly, the worse offender of this type of deceptive advertizing is Firestik.
Do these dummy load antennas work? Sure, if you don't mind your signals being 50+ dB down compared with those of a decent mobile antenna. Please! Do yourself a favor, and steer clear of these sucker-born-every-minute antennas.
Basics
There are many reasons to permanently install any mobile antenna. Aside from minimizing ground and stray capacitance losses, there is also a safety issue, and an insurance issue. Further, convincing the casual mobile operator to drill a hole in his precious lease vehicle’s left quarter panel is an act in futility. What follows is a best foot forward approach. If your plans are less stringent, at least pay attention to the ramifications of temporary mounting schemes for radios and antennas.
Every amateur has options depending upon his/her tastes, cost factors, efficiency, and mounting possibilities. One minimal example is the Lakeview license mount and Hamstick® shown at left. The Yaesu ATAS120® shown right, is another. You just have to decide what factor is most important to you. One thing every reader needs to keep in mind is just how serious you are about mobile operation. Seriousness usually dictates which type of antenna you choose, and to a lessor degree how you mount it. However, safety is still the prime directive when installing or operating amateur mobile radio, which may eliminate some configurations. Mag mounts are one example.
High frequency mobile antennas come in about every size and configuration you can imagine and every single one of them is a compromise. Length matters and it would be nice if we could drive around with 20 foot long whips, but alas we cannot. Shortening an antenna lowers its input impedance, and poor ground planes and coil losses raise it. The end result being the input impedance is closer to 25 ohms than 50 ohms. In other words, an SWR of 2:1. This over simplification is explained later on in this article.
The Cost Factor
Again, the street price you pay is not an indication of quality, ruggedness, or efficiency. In fact, some of the highest priced ones are some of the worst in all these categories. The aforementioned ATAS is among them.
The further fact it is the second highest selling HF mobile antenna (High Sierra has the top honor), means little as well. They break easily, and when it comes to efficiency, it's at the bottom of the barrel. I'm convinced one of its major selling points is its ease of integration; you only need the coax, through which the DC control is routed. It's street price is $400 with a clamp-type mounting bracket. About its only good attribute is its ability to whet an appetite for something better.
There are a lot of back-yard manufacturers of HF mobile antennas, most of which are screwdriver designs. The biggest problem here is, yes you save money up front, but what happens when is breaks, and the company is long-gone?
So, before you spend your hard earned money, do a little research, and ask questions. Is there a warranty, and what does it cover? How long has the company been in business? How many antennas have they sold? What else will I need to use it? What sort of mounting is included, if any? How's it controlled? There are more to be sure.
Just keep in mind that buying an HF mobile antenna is a lot like buying a vehicle. It isn't the up front cost which counts, it's the long-term maintenance cost as well.
This is a good place to bring up an important point. All else being equal, increasing the over all length of an antenna by 50% will double it's efficiency. Obviously there is a limit to how long (height wise) an HF mobile antenna can safely be. Just keep these thoughts in mind; short, compact, small, light, and flimsy antennas are universally lossy!
Another important consideration is the power rating. I believe that HF antennas with a power limit aren't worth considering. The facts are, most manufacturers over-state the capabilities of their antennas. What's more, most don't tell you if their rating is CW or PEP. If you plan on adding an amplifier at a later date, it's best to consider it now, not later.
I do not like spirally wound antennas like the Hamstick® Outbacker®, and Opek (to name a few). They're very low Q (10 to 50), and very inefficient (the antenna article in the November 2007 issue of QST, notwithstanding). This is especially true of the shortened versions, some of which are only 3 feet long.
Their only attributes are, light weight, low wind loading (some models), and low cost. This means they can be attached by just about any type of mount, some of which add to their overall losses (i.e.: license plate mount). On a scale of 1 to 10, they're a 1 and possibly a 2 at the higher frequencies. Efficiencies range in the .3% to 20% (80 through 10 meters), and they typically don't need matching as the system losses bring the input impedance to near 50 ohms.
The one purported advantage of helically-wound antennas it that the coil radiates, which supposedly increases efficiency. This is pure bunk! I suggest readers look at the article "Mobile and Loaded Antennas" on Tom Rauch's (W8JI) web site.
Adding insult, almost all spirally wound antennas are hollow which allows the whip to slide inside the mast far enough to affect the inductance of the loading coil. This fact makes them nearly impossible to tune (especially on 80 meters) without an antenna analyzer like the MFJ 259B. Secondly, even if you get a good match, the proximity of the tail of the whip to the coil further reduces their already poor Q factor. Some of the cheaper imports are made with an inferior grade of fiberglass which weathers badly. After just a few months use they get very stiff and brittle, and often snap off.
I might add, that one manufacturer who markets both spirally wound and bug catcher style antennas, openly states on their web site that their bug catcher design (a 2 inch coil isn't what I would call a bug catcher) is 2 S units stronger than their spirally wound antenna. That's 12 dB of difference, and a lot of food for thought!
There is a fairly new import antenna supplier from China called Opek. They make both HF and VHF mobile antennas aimed at the amateur market. To call them antennas is a stretch of the truth, as they are some of the lossiest HF antennas on the market. Their HVT-400 (shown at right) has a retail price of $60 (its only attribute), is less that 6 feet in length, and mounts via a PL259 connector. Rugged and efficient, it is not. It really should be listed under the Don't Be A Sucker listing above.
One of the Most Popular Antennas
Hustler® (made by New-Tronics Antenna Corporation) is one of the oldest suppliers of mobile HF antennas in the world (the left photo depicts their standard RM-75 meter coil). On a scale of 1 to 10, they range from 2 to 4. You can even make them multi banded by using their "spider" mount to hold additional coils. If you go this route, read the last paragraph in this section.
They promote their super coils as being the ultimate. They're not. The first-addition addendum in Dr. Jerry Sevick's (W2FMI) booklet Building and Using Baluns and Ununs (now out of print) contained charts and loss tables on both the standard and super coils. In short, it is the large metal end caps on the super coil which makes them lossier than the standard ones. In other words, don't spend your money on the larger coils hoping for more efficiency. Besides the extra wind loading, their power handling capability isn't any better than the smaller ones, advertising hype notwithstanding.
Hustler® antennas have at least two additional drawbacks. One is moisture ingress. Water tends to get under the vinyl heat-shrink tubing covering the coil assemblies. This will detune the antenna and possibly cause arcing between the coil turns. This is the reason some authors suggest removing the vinyl sleeve which allows quicker drying. This is not a good idea as it allows road grime to build up on the coil windings which causes more problems than it solves.
If water ingress happens to you, here is a solution. Remove the whip assembly and place the coil in an oven at about 120° for 20 minutes or so. Don't leave them in too long, or use any higher temperature as you'll melt the heat-shrink. Once they cool down they'll be usable again. You can use a hair dryer for this operation, but be careful not to over heat the vinyl.
The another drawback is their masts. You often see examples of their fold over ones with a braided copper jumper across the hinge assembly to maintain a good connection. Unless you just have to use one, you're better off with their solid mast. In either case, you should use a base spring to minimize stress on the mast. If you notice any looseness (on this or any other mobile antenna), replace or tighten the part(s) before they come off while you're driving down the freeway!
Here is something to be very cognisant of. Hustler coils are secured to the mast by about five threads. Further, the 3/8x24 stud is pressed into the mast. The coil's 3/8x24 ferrule the mast screws into is also a press fit. It makes little difference if the coil unscrews, or the mast fails, or the ferrule fails, the coil will fly off! Over tightening them isn't the answer either, as this just hastens the connection's failure mode. If you use a cap hat, it behooves you to tether (guy) the antenna, as the extra wind load will hasten the loosening problem.
Remote Controlled Antennas
Quality wise, remotely controlled antennas run the gamut from poor to excellent, and as I stated above, price isn't always an indicator. Some of the better brands are High Sierra, HiQ, Scorpion, and Tarheel. Further, they come in several different overall lengths. Keep in mind, the shorter ones are less efficient, but this can be partially offset by mounting them higher up on the vehicle.
If you are in the market to buy one, read the reviews on eham.net and/or QRZ.com for more information. The reason is simply this; the last time I did a Google search, there were 52 different manufacturers of screwdriver antennas, and some models aren't worth the effort.
From a performance standpoint, their average rating is about a 5 with better models up to an 8. The Yaesu ATAS series is the exception. It's about a 1 at best.
A lot of the commercial antennas use beryllium copper as a contact material. It wears very well, and provides a secure connection when properly implemented. Some manufacturers would have you believe this is a dangerous practice. They cite the sluffing off of beryllium particles during normal operation. The facts are, the amount of beryllium dust sluffed off is almost nil, and so is the danger.
Let me clarify a point or two. With few exceptions, every commercially available, remotely tuned antenna, changes length as the resonant frequency changes. This is because the coil slides in and out of the base section (mast if you will) as the resonant frequency is adjusted. This makes weather sealing a top priority if you use your HF antenna year-around.
While the weather sealing of some models are admittedly better sealed than average, dirt and moisture can eventually take their toll. This fact exacerbates the wear on the finger stock that contacts the coil, and the other various rotating and sliding parts. If you operate on the lower bands (80 or 160 meters as the case may be), the coil is extended nearly its full length. This not only lessens the strength of the antenna, it exacerbates the weather sealing problem.
The High Sierra 1800 is arguably the most popular of the remote controlled antenna designs (left photo), followed distantly by the Tarheel, and Scorpion. All of these models are better weather sealed than the majority.
The one item to keep in mind when buying any HF mobile antenna, is the warranty. Do yourself a favor, and read very carefully, even if it takes reading between lines.
While I'm on the subject of High Sierra antennas, I want to dispel a common myth associated with them, and that's their susceptibility to interstate speeds. The fact is, it isn't any more (or less) suspectable than any other brand of HF mobile antenna. If you mount your antenna correctly, and you keep it clean of road debris, anything short of a collision won't shorten its life.
Scorpion is one of the newer screwdriver antenna manufacturers. I have to admit, the machining is first rate. They use 304 stainless steel to make the major parts of their antennas, and the build quality is excellent. Weather sealing is above average, and with 3 inch coils, efficiency is about as good as it gets. Several different models are available, so visit their web site for more details. When you do, you'll also find they're very competitive, price wise. That's their 680 model is shown above right.
There is one remotely tuned antenna which doesn't change length when it is tuned, and that's the HiQ (left photo). It is about as efficient as you can muster, and rates a 7 or 8. If you opt for the military or marine models with their gold-plated coils, they rate a 9. They come in 2.5, 3, 4, 5, and 6 inch models, with and without 160 meter coverage. There is even a special order, all stainless steel one, but you'll need to dig deep as it isn't inexpensive.
I purchased a HiQ 5/80RTM with a gold plated coil 3 years ago. I can say without reservation that it's worth every penny of its premium price. They're made from aircraft-quality materials, and they have an excellent warranty. If they have a drawback, it's weight.
It is a heavy antenna, so it was necessary to bolster the backing plate for the ballmount, as well as install a home brew Delrin® brace as shown in the right photo. Obviously I'm not bashful about drilling holes. If the antenna were mounted on a frame or trailer hitch mount, the extra bracing wouldn't be necessary. The black objects you see just below the ballmount are the toroid cores used to suppress the RF on the control lines which pass through the ballmount insulator. The lines are also bypassed with .01 caps inside the trunk as is standard amateur practice.
Although I cover this several places in this article, please note the coil location in the photos above. It is located well above any surrounding metal. If you want the best performance from any antenna, this is an absolute necessity. Besides reducing the power robbing stray capacitance, it minimizes the directivity bumper mounted antennas tend to have.
As I just mentioned, I'm not bashful about drilling holes. The Acura 3.2CL pictured had eight holes from 1/4 inch to 2 inches. I recently traded the car in on a new Honda Ridgeline (right photo). The trade in I received was $1,850 more than the Bluebook value. This, after I received an additional $4,200 off the list price of the Ridgeline. So much for worrying about a few holes.
Bug catcher antennas are big, ugly, cumbersome, costly, and a nuisance to retune. Sporting high Q coils, and typically mounted with care, it is not uncommon for these types to reach a 7 or 8 on our 1 to 10 scale. Some of the better ones are made by GLA Systems as is the coil shown in the photo. Although they don't list them on their web site, HiQ does make several different monoband models. Overall lengths vary, but masts are available up to 8 feet in length, and whips to 102 inches (standard CB style). Coil diameters vary from 3 inch to 8 inch. Some caution should be exercised as larger coils do not necessarily have higher Qs, regardless of the hype. Further, their low self-resonant points may negate their use on the higher bands. I cover Q factors in depth here. On the lower frequencies, additional performance can be garnered by adding a cap hat which raises the radiation resistance while bringing added drag and complexity to the formula.
Cap hats
are a mixed bag of tricks. Although there are several companies making them, they're not created equal. DX Engineering makes one of the better ones around, but even it has some drawbacks which I'll touch on in a moment.
As their name implies, cap hats add capacitance to the top portion of the antenna. This fact raises the radiation resistance, but only in that part of the antenna above the coil. Tom Rauch's, W8JI, site has a good treatise on radiation_resistance which explains how they work. A longer whip will do the same, but there is a limit to how long an HF mobile antenna can be. By adding a cap hat, the overall length can remain the same, while radiation resistance increases. There are several reasons for this, as Tom's article points out. This is the good part.
The bad part is, they increase wind loading and complexity. The increase in wind loading is not something to take lightly, if you'll excuse the pun. Further, I garage my vehicle at night, and I store the whip section in the bed (or in the trunk). If had a large cap hat, this would be impossible.
They work better on the lower bands where antennas are electrically short, and radiation resistance low. On the upper bands, their use is perhaps moot. The one thing they don't do is increase capture area. The term Capture Area is often misused, and misguided in application. Using it in association with cap hats is inane!
The real measure of any antenna (on receive) is the ultimate signal to noise plus noise ratio (S/N+N) the receiver generates. Contrary to a recently published article, there is no S/N+N difference between an antenna with a cap hat, and one with just a whip (all else being equal). Even if the coil losses are slightly less, the difference would be very difficult to measure. It's best to remember, the prime limiting factors are ground loss, and the egressed RFI we all have to deal with. I have more information on S/N+N in my Antenna Efficiency article.
Cap hats must be mounted away from the coil. Not under it, not over it, but way over it. A good rule of thumb is at least twice the length of the coil, and for best performance, at the very top of the whip!
There is another drawback which I've learned the hard way, and that's corona discharge from the tips of the cap hat. At the tip of an HF mobile antenna, the RF voltage can easily exceed several kilovolts. To minimize losses, and reduce the possibility of corona discharge, the use corona balls is a requirement. If you look at the photo to the upper left, you'll notice the rods are tipped with little plastic protectors. The rods themselves are polished round, but because of their small diameter, it is possible to have corona discharge off the tips of the rods, especially if you run high power as I do. To eliminate this problem, I purchased the 48 inch optional rods, and bent them into circles as shown in the upper right photo. There is no discernible difference in performance.
What isn't shown is the 4 foot extension the cap hat is attached to, and the one foot extension and corona ball that goes on top of the assembly. Even at full legal limit power, there is no discernible corona from this setup. Incidentally, the cap hat available from HiQ uses curved wires (not pointed ones) as shown in the left photo.
It should be noted, for maximum benefit, the cap hat should have the individual spokes electrically connected at their tips. Since this is a little difficult with the straight rods, bending them into circles is a good alternative.