Last Modified: November 18, 2011
Contents: Basics; Dummy Loads; Short, Stubby Antennas; Standard Sized Antennas; An Esoteric Flaws; Spirally Wound Antennas; Monoband Antennas; Bug Catcher Antennas; Cap Hats; End Caps; Notes On Whips; Odds & Ends;
The most important choice, any mobile operator will ever make, is their antenna. It pays to be very picky!
High frequency mobile antennas come in every imaginable configuration (as the right photo proves). Their efficiency, overall length, quality, design, sturdiness, ease of mounting, and selling price all vary. Choosing any one (or more) of these specific attributes, is no guarantee you're getting another (or more) needed attribute(s) included. The very unfortunate truth is; far to many mobile operators, choose their antennas based on their cuteness factor, spousal acceptance, and/or no holes mounting. This situation is indeed, a no win scenario!
The only way to avoid the aforementioned scenario, is to know more about the subject at hand. Toward the goal of helping the neophyte mobile operator to make a wise selection, the following sections explain the more important attributes, and why they're important.
The most important aspect of installing mobile antennas is having a good understanding of what is, and what isn't, ground. Ground, in this context includes ground 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 the Grounds, RF & DC article.
One very important point needs to be made here. A vehicle is not a ground plane, but rather it acts like a capacitor between it, and the surface under the vehicle which is the true 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.
Ground plane losses are (typically) the single largest factor in the efficiency equation. If you don't understand the importance of a good ground plane, then the Ground Plane Notes article is for you. Remember this; Excessive ground plane losses will render the best of antennas nearly worthless.
The coupling between the super structure of any vehicle, and the surface under it, is not consistent. As a result, there will always be standing waves between them. These standing waves are, in essence, the main cause of the ground losses in the first place. Please note, we're not talking about the standing wave ratio (SWR) of the antenna! It should also be noted that you cannot measure these standing waves directly. Therefore, although they are represented as part of the input impedance of the antenna in question, changes therein cannot be assumed to be a reduction, or increase, in either these standing waves and/or in ground losses, without a thorough understanding of the other parameters involved. Field strength measurements will give you a better comparison of the changes, but here too they have to be carried out in a scientific (all factors normalized) manner, or the results will be just as ambiguous as any input impedance measurement. There is more to this subject in the efficiency article. If you want the best out of whatever antenna you choose, please read the article.
Proper bonding is also important. Bonding aids in the performance of any antenna system by maximizing what little ground plane a vehicle represents. Bonding also minimizes the chances of of RFI ingress and egress. If you still have RFI issues, and you don't know what they are, read the Noise ID article. You might also want to read the Static Control article.
There are many reasons to permanently install your HF mobile antenna. Aside from minimizing ground and shunt capacitance losses, there is also a safety issue, and an insurance issue. As noted above, an otherwise decent antenna can be made nearly useless by poor mounting schemes and/or location. If you're not into drilling holes in sheet metal, at least pay attention to the ramifications temporary mounting schemes have on efficiency. Temporary mounting schemes include mag mounts, lip mounts, and clamps. Each one of these increases ground losses which are already too high in the best of installations.
Very few amateurs realize how important overall length is. Radiation resistance is based on (electrical) length, and how the RF current flows over that length. A full-sized 1/4 wave antenna has a radiation resistance (Rr) of ≈36 ohms. If we halve the length (1/8 wave), the radiation resistance drops to 9 ohms! Halve it again (1/16 wave), and the radiation resistance is just over 2 ohms! If the resistive losses were fixed at 10 ohms, simple math will tell you the efficiency rating. If you haven't read the efficiency article yet, the calculated efficiencies would be; 22%, 2%, and just over 1% respectively, assuming there are no other losses present. There will be, of course, because the shorter the overall length, the larger the loading coil inductance has to be, hence the greater the Q losses (Rc) will be. Let's look a little closer at loading coils, and their Q ratings.
The input impedance of any antenna shorter than a 1/4 wave length will exhibit capacitive reactance (-j). In order to cancel out this negative reactance, we incorporate a loading coil with equal, but opposite reactance (+j). The shorter the antenna (in terms of wavelength), the larger the coil's reactance must be, along with a corresponding increase in coil losses, all else being equal. The table at right (curtesy of the ARRL) illustrates the effects. Note the efficiencies levels. Further, an antenna loading coil is a lumped constant. On-line sources which state that a loading coil replaces a given physical length of an antenna, are incorrect!
The term Q stands for Quality. It is determined by dividing the inductive reactance in ohms, by the resistive losses in the coil in ohms, at the operating frequency. Therefore, you cannot measure the Q of a coil with a DVM (digital voltmeter). You need a Q meter for that. Suffice to say, the higher the Q, the better the efficiency, at least to a point. Depending on the length of the antenna in question, the ground losses present, and a few minor points, once the coil Q exceeds about 250 to 275, we reach a point of diminishing returns. In other words, even if we could double the Q to say 450, the increase in field strength would be minimal. Remember, ground losses (typically) dominate the efficiency equation.
Contrary to popular belief, it is very difficult to design a loading coil with a Q over 500, and once it becomes part of the antenna, maintaining a Q of 300 is just as difficult, especially over a wide frequency range (i.e.: remotely-tuned antenna). The leading question then is, what is the coil Q of an average HF mobile antenna? That is a hard to define parameter, difficult to measure, and one that varies a lot over an antenna's bandwidth. However, if you go through the requisite mathematical iterations, you'll discover loading coil Q factors are considerably less than those assumed, or advertised, to be! Fact is, the average coil Q for a spirally wound antenna is less than 50, and may be as low as 10 (!) for a 75 meter model. Screwdriver type antennas are all over the board Q wise, but the average is typically less than 150. There are a few that surpass the 300 mark, and the amount you pay isn't always an indication.
Some antenna manufacturers stake their claim of superiority in the Q ratings of their coils. Once again, it is important to note, that measuring the Q of a coil, once it is installed as part of the antenna, is both difficult, and inaccurate. As a result, the claims are typically based on work-bench, or static Q measurements. Once the coil is mounted as part of the antenna, Q ratings plummet by 50% or more. It should also be noted that large diameter coils (4, 5 or even 6 inches) do not necessarily have higher Q ratings. This is particularly true of coil assemblies with large metal end caps, and shorting bars. Antennas so assembled should be avoided.
One more point about design. There are a lot of copycat antenna manufactures, primarily copying one another's screwdriver designs. In far too many cases, these manufacturers are clever machinists, but poor antenna designers. Here's a question which will separate the men from the boys. Ask them if their antenna requires a matching coil to obtain a low SWR. If they say no, don't buy it on both accounts!
Several screwdriver designs have a base matching coil (of fixed inductance) machined in as part of the lower mast assembly. Since ground, and stray capacitance losses vary with the installation, the match may not be close enough for most solid state transceivers. If the match is good, it just means the coil losses are excessive, or you're just lucky.
While we're on the subject of inferior antenna designs, here is another consideration. Several screwdriver antenna designs incorporate a slip clutch (or worse a gap in the all-thread drive screw) to avoid motor damage once the coil reaches its end of travel (up or down). This negates the use of any automatic antenna controller, whether it be a turns counter unit, or one which measures end-of-travel stall current. These antenna types should be avoided.
There is one more point which needs to be made. Purchasing an antenna with a high Q loading coil, dictates that you take the necessary steps to properly mount the antenna, or any advantage it might give you over one with a lessor coil Q, will be masked by the dominance of ground losses.
There are actually antennas sold which are nothing more than a 50 Ω resistor with a radiating element (?) attached to one end. One of these was the Maxx-Com, now thankfully out of business. At $400 it is as costly as some screwdriver antennas.
The Comet HA-750BL is similar, but uses a very lossy, 6:1 impedance transformer, It is even worse than a dummy load if that's possible. It appears Comet has come to their senses, as the antenna is no longer listed on their web site.
Comet's UHF 6, shown at right, isn't any better. At less than five feet overall (72 inches with 80 meter coil), coils wound with #26 wire, it is the epitome of a dummy load on a stick.
The Diamond HV7A isn't a dummy load per sé, but it might as well be. Besides UHF and VHF, it covers 6 and 10 meters, plus one other HF band (40 and above). Its overall length is just 50 inches, and the coils are not just small, they're miniscule. The optional 40 meter coil is wound with what appears to be size #26 wire.
The Opek®, shown at left, shouldn't be called an antenna. With just 100 watts of power, the coil gets very warm after just a minute or two of operation.
If you buy one of these antennas, you're throwing your money away.
One may argue that some antenna is better than no antenna. If you subscribe to that adage, then your expectations aren't as high as they should be. The real bottom line, is putting enough SNR in the front end of the party you're attempting communicate with so he can hear you (it makes no difference if you can hear him!). When band conditions are marginal, even a 1 dB increase it ERP, is enough to make a big change in the SNR on the other end.
Short, stubby, remotely controlled, HF mobile antennas have seemingly become the rage, lead by the Yaesu ATAS-120 shown at right. They're popularity is in part due to their diminutive size (less than 7 feet overall), light weight, and apparent ease of mounting. They're not necessarily less expensive than their stable mates.
Yaesu's ATAS antenna is the least efficient, remotely controllable, HF mobile antenna, money can buy! The list price is 0ver $460, with the street price hovering around $375, not including the mount. It uses an SO239 type mount which is one of its major drawbacks. Its mechanical and electrical aspects are also questionable, as is its operating methodology. The driving transceiver sends a DC voltage (<8.3 or >9.7) causing the antenna to move to a lower or higher frequency respectively. The SWR detection has two levels; over 2:1, and under 2:1. There is no choke in the control lead (coax) which makes common mode current a given problem exacerbated by the type of mounting usually employed. These facts, coupled with its high cost of ownership, makes the ATAS a very poor choice indeed. Yet, it is the highest selling, single model mobile antenna in the world! An unexplainable conundrum.
Owners typically resort to trunk lip mounts (K400, click on left photo to enlarge), luggage rack clamp mounts, and even mag mounts. All of these add to the overall losses, including the mini-sized coax most of these mounts utilize. These losses are seldom noticed because owners have nothing to compare them to. If they can make contacts, especially DX ones, that's all they seem to care about.
No matter the brand, all short, stubby antennas have an excessive level of RF flowing on their control leads, and coax cables too (common mode currents). Thus choking off common mode current is especially important if you're using (or plan to use) an automatic controller. The requisite chokes need to be mounted outside the vehicle, and as close to the base of the antenna as possible. This requirement is very difficult to accomplish when using a clip mount; a fact which should be considered before purchase.
None of the current batch of these antennas requires any impedance matching (save one, see below), because their inherent resistive losses bring their input impedance close to 50 ohms. This means, the antennas are not DC grounded, and therefore tend to be noisier than their big brother counterparts, especially in rain and/or dusty conditions.
If you're intent on using one, keep the aforementioned inherent problems (and shortcomings) in mind. You'll need to purchase additional split beads (RF chokes) to tame the common mode currents, and other maladies which will occur.
I'm often asked how much difference there is, between a short, stubby antenna, and a full-sized one, all else being equal. In almost every case, the difference is at least 15 dB, and can be as much as 25 dB. You typically don't notice the difference if the band is wide open. However, when conditions are marginal, even a 1 dB difference in signal strength can make a huge difference in the S+N/N ratio (SNR) generated in the front of a transceiver. Remember, you have to have enough signal strength on the opposite end of the propagation path, or you won't be heard. It is this latter aspect where minimal antennas suffer the most. Unfortunately, the situation is often justified by citing band conditions rather than blame the antenna, which is the true cause.
The aforementioned statement is often disputed by those who trust the results of field strength measurements, taken during antenna shootouts. If you read the article, you'll get a grasp of why shootouts are a worthless exercise, designed to boost one's ego, but prove very little!
There is one antenna, advertised by its manufacturer as short, and that's the Scorpion 680SA. Calling it short, or stubby, is a bit of a misnomer. It is 28 to 36 inches long (less the whip) depending on the frequency, and weighs 13 pounds! This shortened version of the Scorpion 680 is almost as big as the competitors full-sized models, and outclasses most of them with its 3 inch diameter coil (#14, 10 tpi). It does require proper matching (included), and a heavy-duty mounting scheme.
Short, stubby antennas really aren't base loaded, yet you can't quite call them center loaded either, especially if the whip length is over about 6 feet long. The reason is, their coil assembly is typically a 18 to 24 inches in length. And, remembering that radiation resistance (Rr) of a vertical antenna is a function of the overall (electrical) length, and the current distribution along that linear length, short, stubby antennas exhibit an esoteric phenomena. If you do all of the calculations, you'll discover that increasing the whip length over about 6 feet, actually reduces their overall efficiency albeit slightly. It is a very good example of a Catch-22 scenario. Looking at this from a different angle; based loaded antennas are about one half the efficiency of a center loaded one. So, as you increase the whip length, the coil moves closer to the base, ratio-wise, which reduces efficiency, while the increased length does its best to increase efficiency. The frequency at which point this occurs, depends on the ground losses present, which are always higher in most short, stubby installations (poor mounting, mainly).
There really isn't a standard-sized HF mobile antenna, so calling them that is a bit of a misnomer. On average, they're about 9 to 11 feet in overall length (including the whip), with some of them extending to 13 feet.
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 Pittman® gear motors (with several exceptions).
Don Johnson, W6AAQ (sk), is credited by many as the father of the 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 of a sleeve (mast). He certainly popularize it, and perhaps that's all that counts.
The last time I did a Google search, there were 79 different manufacturers of screwdriver antennas. Some are copies, some improve on the basic idea, some are marketed better, some try to be something they're not, and some aren't worth the effort. Although there are exceptions, most on-line reviews need to be taken with a grain of salt, especially if they mention working lots of DX. Remember, a log sheet might be a measurement of bragging rights, but it sure isn't a comprehensive measurement of antenna efficiency or performance.
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 GS (at lower right), Scorpion (at left), and Tarheel (right). Further, they all come in several different overall lengths, power ratings, color finishes, and mounting configurations.
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 sluffed off is almost nil, and so is the danger. Want to know the real reason they don't use beryllium finger stock? Cost!
Let me clarify a point or two. With few exceptions (the GS is one, shown at lower right), every commercially available, remotely tuned HF 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 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. This brings up an important caveat about screwdriver antennas.
The mechanism which raises, and lowers the coil assembly is more or less the same, in almost every remotely adjustable HF antenna. Typically, a piece of 1/4x20 all thread is rotated by a gear motor of some sort. A threaded boss or nut is attached to the coil assembly, or plunger as the case may be. When the all thread turns, the coil moves up or down.
Some of the lessor models have very little coil support, especially on 80 meters when the coil is fully exposed. Hit a low hanging limb, and the all thread will bend with obvious results. This condition is exacerbated on models with small diameter coils (<2 inches in diameter).
This is a good point to mention current draw. The average motor run current for the majority of remotely tuned HF antennas is between 250 and 350 mils, depending on the model. Stall currents average from 4 to 5 times running current. This fact negates powering them from the accessory sockets of most mobile transceivers! I cover this in more detail in my controller article. The article also covers the requisite chokes as mentioned above. While some manufacturers supply motor lead chokes with their antennas, some are the wrong mix and/or the wrong size. It is always best to follow the controller manufacturer's recommendations.
Antennas equipped with slip clutches, like the GS antenna shown at right, are not compatible with most automatic antenna controllers. Garry Stookey, the manufacturer, is in the process of redesigning his antenna (summer 2011) to use a PolyFuse® to protect the motor, rather than a slip clutch. The Alpine series made in Alpine, California, also uses a slip clutch.
There are two screwdriver models which incorporate a heavy-duty drill motor which draws several amps in run, and a whole lot more on stall! As a result, care must to be taken in feeding them power, and setting up most automatic controllers.
Scorpion is one of the newer screwdriver antenna manufacturers. The machining, and build quality has to be seen to be believed! No other HF mobile antenna even comes close! Weather sealing is above average, and with 3 inch coils (#10, 6 tpi), efficiency is about as good as it gets. Several different models are available, so visit their web site for more details.
Their 680 model is shown upper left. It sells for $700 to $800 depending on finish, and is worth every penny of it. If you order one, make sure you ask about lead times, as it is becoming a very popular choice. I've been using a Scorpion 680 since June 2009, and it has lived up to my expectations, and then some.
Incidentally, the Scorpion 680, with its 3 inch coil, wound with #10 silver-plated wire, and 6 turns per inch, has one of the highest average Qs of any commercially available, HF mobile antennas. It achieves this by using a liberal amount of Delrin® and Lexan®, and a minimal amount of metal, in close proximity of the coil. For those who think Q isn't important, you need to review the efficiency article.
They have another unique feature, and that's the built-in, base quick disconnect at the base of the antenna. The bottom of this page explains the construction. It has other unique features which make it a standout among all of the other screwdrivers.
A few antenna manufacturers supply a factory-wound matching coil. All too often, the coil is mounted inside, and very close to the mounting hardware. This fact make impedance matching very difficult. If you follow the directions in my Antenna Matching article, you'll have much better luck.
Some remotely controlled HF mobile antennas, are available with 160 meter coverage. The inductance required to resonant a 160 meter, 8 foot long mobile antenna, is nearly 5 times greater than that required to resonant an 8 foot, 80 meter antenna. On average, this more than doubles the coil losses, which brings the input impedance very close to 50 ohms, and sometimes over it. This adds a level of complexity to proper input matching, and reduces efficiency to sub 1%! Further, due to coil design requirements, antennas with 160 meter through 10 meter coverage, will always be less efficient on the upper bands (>80 meters) when compared to an 80 through 10 meter one. Unless you're dead set on having 160 coverage along with its special needs, you're much better off with a 80 through 10 model.
With the above in mind, if you must have 160 meter coverage, here's an alternative. Scorpion offers an add-on 160 meter coil for their 680 model which sells for $149. It should work with any screwdriver antenna, with a few caveats.
First, as mentioned, matching can be a problem due to the higher input impedance on 160 meters vs. that of 80 meters, and above. So besides the coil, you'll need some easy solution to change the shunt matching coil. This can be done with plug-in coils like those sold by Scorpion, or by using a remote-controlled relay setup. The other issue is antenna strength. Lessor models of the screwdriver are not sturdy enough to take the extra strain imposed by the coil. This extra strain is caused by the added length, weight, wind loading, and the leverage caused by the wind loading. Scorpion also offers a three loop cap hat, just like the one shown above. It retails for $75, however, a coil and cap hat package is just $199.
I mention this in the Antenna Matching article, but it bears repeating here. Several commercial versions of the screwdriver antenna have machined-in matching coils, which is fixed in value, and therefore cannot provide an ideal match over the entire resonant frequency span of the antenna. In fact, it is often suggested that a specific length of coax be used to feed the antenna, in order to provide a low SWR. Doing so only masks the real problem; an incorrectly sized (impedance value) shunt coil. Although these antennas can be modified to use an adjustable coil, it's best to avoid the buying the problem in the first place.
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. Oh! And don't forget to read the instructions. In most cases, they can be downloaded before purchase. Fact is, after reading some of them, you just might want to rethink your purchase!
Screwdriver antennas dominate the remote controlled market, and for good reason. There is a lot to be said about changing resonance while under way. However, besides the aforementioned end cap problem, there is a lessor known ones which needs to be mentioned.
At least one model has a weather-sealed coil assembly. In some extreme weather conditions, water condenses in the inside of the coil, and mast. It collects in the bottom of the mast, and eventually reaches the motor assembly. The results are obvious. The manufacturer denies the problem exists.
Some models have relatively short masts (≈2 feet). In some installations this places the coil very close to body sheet metal. There is often an accessory masts available which raises the coil 1 to 2 feet. Doing so drastically raises the RF level on the motor leads which requires extra care in choking the RF. It is best to avoid antennas designed this way.
There is at least one monoband manufacturer that uses 6 foot masts, and a short 1 foot aluminum rod above the coil. Their literature claims this decreases losses commonly seen in antennas which use 17-7 stainless steel whips. However, nothing is said about the rather high Q losses in the loading coil. It is important to remember, the optimal position of the coil is dependent on ground losses, and is never higher than 60% of the overall length.
The resonant frequency of a screwdriver antenna is adjusted by changing the length of the coil above a contact assembly, mounted at the top end of the mast. The unused portion of the coil, stored inside the mast, is typically not grounded except in rare cases. Regardless of the design, the unused portion still has a fair amount of circulating currents flowing through it. How much detriment this has on coil Q depends on a lot of factors. However, one requisite design attribute is to minimize the space (clearance as it were) between the inside surface of the mast, and the outside of the coil. If the space is excessive, the circulating currents can be high enough to cause arcing between the mast and the coil, with predictable results. In one case, the mast is 2 1/8 inches ID, but the coil OD is only 1 1/2 inches. It is quite common to see examples with burn evidence toward the bottom of the coil assembly. While the problem is rare, if the diameter difference appears to be more than 1/8 inch, you might want to rethink your purchase.
As stated above, excessive circulating currents can destroy a screwdriver antenna's loading coil. Besides the excess clearance problem, using a short whip can also be a coil killer! For example, some manufacturers suggest using a 36 inch whip to work 10, and 6 meters, especially when using models which cover 160 as well. Remember, the higher up in the antenna the coil is mounted, the higher the RF voltage expressed across the coil, and between the coil's turns. If the voltage gets high enough, the coil's dielectric strength will be exceeded with obvious results. As a general rule, the whip should be at least as long as the mast, and coil assembly combined, if not longer. It should also be noted that trying to work 160 meters (or even 80) through 6 meters with the same antenna is fraught with problems, and another good reason to buy a separate 6 meter antenna, preferably a horizontally-polarized loop.
There are a bunch of spirally wound antennas on the market. Some are fairly good, and some aren't worth the effort. 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!
The popular ones tend to be the Hamstick®, and the various copycats. All are fairly low in efficiency, as their Qs are about 50 or less. A few of the 80 meter models have Qs less than 10! If you want a really lossy antenna, use one of the stubby 3 foot long versions!
Their only attributes are, light weight, low wind loading (some models), and low cost (≈$20USD, less mount). 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). 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.
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.
Monoband antennas are limiting in so many ways, they're not a wise first-purchase choice. Aside from the various hamsticks mentioned above, there isn't much choice in the market place nowadays with the exception of the Hustler® series. You can occasionally find older monoband coils made by Master Mobile, HyGain, and Mosley, but you're on your own for masts mounts, and whips.
The 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). You can even make them multi banded by using their "spider" mount to hold additional coils.
Hustler® promotes their super coils as being the ultimate. They're not. The first-addition addendum in Dr. Jerry Sevick's (W2FMI-sk) 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.
Recent manufactured models no longer have the compression type fitting to secure the whip; it has been replaced with a set screw. Whether this is a step backwards is perhaps moot, but the fact remains it is not as secure.
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!
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.
Big ugly bug catcher antennas have fallen out of favor, perhaps for obvious reasons. Besides being big, they're ugly, cumbersome, costly, a nuisance to tune, have excessive wind loading, and can't be used in inclement weather. They can, however, be the most efficient of mobile antennas if they're mounted correctly, and the correct-sized coil is selected.
Some of the better ones were made by GLA Systems (see photo), and sold by Main Trading Company. 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 when selecting a loading coil, as large coils do not necessarily have higher Qs than smaller ones, advertising hype to the contrary. Just for the record, it is all but impossible to achieve a loading coil Q of over ≈350. Further, the low self-resonant points of the larger coils may negate their use on the higher bands. Information on Main's web site about negating this issue by using several shorting jumpers is technically incorrect. The Antenna Efficiency article covers Q factors in depth.
If you visit Main's web site, you'll see two other technical errors. First, all of the cap hats are shown mounted too close to their respective loading coils. There is more information on this in the next section, and in the Antenna Cap Hats article.
The other error is the mounting location of their base matching coil. It is specifically designed to mounted around the mast (the mast goes through its center). This fact all but destroys its Q, which increases matching losses.
Cap hats, sometimes referred to a top hats, are a mixed bag of tricks. As their name implies, cap hats add capacitance to the top portion of the antenna (the whip), which has the same affect as increasing its electrical length.
The left photo depicts a cap hat incorrectly installed. So installed, the input impedance and bandwidth increase, however, the changes are due to increased coil losses, and not by the increase in radiation resistance (Rr). In other words, in order to be effective, cap hats must be mounted away from the coil; not under it, not on top of 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!
Cap hats have a drawback which might not be apparent, and that is the requisite mounting methodology. For example, the cloverleaf-shaped cap hat shown below right, has a solid 4 foot extension below it, and a solid one foot extension above it with a one inch corona ball on top. No matter how well an antenna is built, it can be destroyed if you smack it hard enough, and it doesn't take a big limb in most cases. This is especially true if the mast holding the cap hat is solid (not flexible like a whip). It is your basic, high school physics, lever law. Overcoming this problem isn't always simple.
Correctly applied, cap hats will increase efficiency by raising the radiation resistance, at the expense of wind loading and complexity. It is a trade off which may or may not be beneficial. Read the Antenna Cap Hats for more details.
Any metal structure placed within the field of a loading coil, will have a detrimental affect on coil Q. Several current designs incorporate a large aluminum or stainless steel end cap which supports the whip and/or cap hat assembly. In at least two cases, there is also a metal shorting plunger which slides up and down. This motor-driven plunger shorts out the unused portion of the coil thus reducing its inductance. These metal masses can cause the coil to operate above, or close to, its self resonant point which also increases coil losses (reduces Q).
At some point, these Q reducing factors add up, and the Q effectively becomes zero. At that point, the coil starts acting more like a rather lossy capacitor (operating above self resonance) than an inductor. At which point this occurs in any given antenna design, depends on a lot of factors, and ones which aren't easy to measure or calculate. They aren't easy to comprehend either, unless you have a fair understanding of how loading coils behave.
Alan Payne, G3RBJ, wrote an article for QEX (May/June 2011, page 39), covering a new theory about self-resonance in coils. He explains that at higher frequencies, coils act more like transmission lines. The theory better explains why coils act the way they do as they approach their self-resonant point, and is required reading for those interested in inductor theory. When reading the article, pay attention to the reduction in Q as a coil nears its self-resonant frequency.
Whips, called stingers by the CB crowd, come is all lengths, and sizes. If there is an average, it is 72 inches (6 feet). A standard CB whip is 102 inches long (8.5 feet). At one time you could also purchase 108 inch, and 120 inch whips. Almost without exception, they're all made from bare 17-7 stainless steel wire, .200 inches in diameter, and topped with a brass tip which is too small to be called a corona ball. The wire is hammer straightened, and taper ground to .100 inches, starting at about 60 inches from the base.
Some smaller length whips (<48 inches), notably from Larsen, are copper plated, and covered with powder coated black paint. On the upper bands, the assumed advantages are more cosmetic, than real. On the lower bands (40 and below), a case can be made for using copper plated whips, but the field strength difference is minimal, perhaps one dB or so. The cost of copper plating a 17-7 stainless steel whip is about $50.
A standard CB whip, sans the mount, has a resonance point of about 27.5 MHz. If you use a standard ballmount, and spring (overall ≈110 inches), the resonant point will be very close to 25.5 MHz. As a result, you cannot use one on 10 meters without trimming its length. Further, it is not quite long enough for 12 meters (24.95 MHz). However, you can add a small shunt coil (≈.5 uH), which will lower the resonant point, and raise the input impedance on 12 meter just enough for most solid state transceivers.
Full-length, stainless steel whips (102 inches overall) can be lossy, especially at lower frequencies. For example, on 40 meters, they represent an additional loss of ≈.5 dB. This increases to ≈1.25 dB on 80, and just over 3 dB on 160 meters. Finding a suitable replacement isn't easy. This is why knowledgable amateurs use aluminum rods (masts by any other name), topped with a cap hat, to reduce resistive losses (skin effect losses in this case), increase the radiation resistance (up to 4 times), and just as important, to reduce overall length.
Antenna weight can be a major purchasing factor. This is why most full-sized remotely controlled antennas end up being mounted on a trailer hitch type mount. As I alluded to above, spending your hard-earned cash on a quality antenna, and then mounting it poorly, is counter productive. Remember, it is the mass directly under the antenna that counts, not what's along side.
There are some drawbacks to large diameter loading coils besides the extra wind loading. First, the ratio between coil length and diameter are major factors in determining coil Q. Other factors include the dielectric material the coil is made out of; the size, pitch, and coating (if any) of the wire; and the resulting inter and intra capacitance between all of the parts making up the coil. These factors also limit the optimal diameter of the coil. While large diameter coils are preferred on the lower bands, and smaller ones on the upper bands, as a general rule coils much larger than 3 inch actually have inferior Q ratings in comparison.
Looking at this another way, the optimal L/D (length to diameter ratio) changes with the inductance of the coil. The ratio tends to be closer to 1:1 on the upper bands, and as much as 4:1 on the lower bands. However, a rule of thumb cannot be applied without knowing a lot more about the construction than mentioned here. In other words, don't justify your antenna purchase by the size of the coil alone.
I've stated many times that the most important point you need to establish is your personal level of satisfaction. The old proverb, one man's treasure is another man's trash, says it all. If your HF mobile setup is a treasure to you, then don't let someone tell you it isn't. If it is trash, then hopefully some of what you've read here will aid you in your quest for a better mobile station. Whatever you do, don't tell me you like it because you were able to work some rare DX. All that proves is your gullibility.
Once you make your purchase, if you haven't already, and you just cannot get it to work, read the Antenna Problems article.