Last Modified: July 1, 2014
Contents: The Most Important Purchase You'll Ever Make; Basics; Power Ratings; Short, Stubby Antennas; A Few Notes On Motors; Standard Sized Antennas; Esoteric Flaws; Spirally Wound Antennas; Monoband Antennas; Bug Catcher Antennas; Cap Hats; Built In Controllers; Notes On Whips; Odds & Ends;
The Most Important Purchase You'll Ever make!
It is the antenna which determines the lowest common denominator in any installation!
There are not enough adjectives in the English language to adequately describe the importance of your antenna choice. Not just the receive or transmit performance, but to the enjoyment of the hobby. Yes, you can get by with less, and you might even be satisfied. However, the longer you're in the hobby, the more you'll realize that an antenna is the single, most important purchase you'll ever make, bar none, your transceiver choice notwithstanding! This is especially true of mobile antennas. Since this site is about putting your best foot forward, here are a few tips toward that goal:
Don't buy any antenna that attaches with a clip, lip, or mag mount.
Don't buy any antenna with large metal end caps. While the coil Q may be advertised as higher than the competition, once those metal end caps are installed, the coil Q drops drastically, to perhaps 30% or less of its static (advertised) value.
Don't buy any antenna with a coil larger than 4 inches. While they appear to operate well on the lower bands, large coils have more distributed capacitance, and hence lower Q on the higher bands especially when short tapped! The optimal coil size for overall efficiency is about 3 inches.
Don't buy any antenna with a coil TPI (turns per inch) greater than 10.
Don't buy any antenna with a coil wound with anything smaller than #14, and preferably #12, or better yet, #10.
Don't buy any antenna with a whip (top element) under 36 inches long.
Don't buy any antenna that requires an external antenna tuner to match the input impedance.
If you just have to have 160 meters, here is something to keep in mind. An antenna capable of operating on 160, will require an inductive reactance nearly 5 times that required for 80 meters. This means they have specific matching concerns which need to be addressed. By the way, 160 add-on coils are available for most sturdily-built, 80 through 10 meter antennas.
Sturdiness does count! One of the best ways to maximize efficiency is to install a large cap hat at the very top of a mast, typically 3 to 4 feet long. Lessor made antennas cannot support the weight, or wind loading.
Be very leery of antennas advertised to cover 80 through 6 meters, or worse, 160 through 6 meters! There is no practical way to accomplish this feat, without additional losses occurring.
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!
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 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.
Yet, some antenna manufacturers stake their claim of superiority on 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, and without the end caps installed! 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 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 if efficiency is to be maintained.
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 are a few things to look out for.
If an antenna does not require a matching coil to obtain a low SWR, it means the overall resistive losses are high. This said, 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 some solid state transceivers. If the match is good, it just means the coil losses are excessive, or you're just lucky.
Several screwdriver antenna designs incorporate a slip clutch or gap in the all-thread drive screw assembly 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. In other words, only manual control is possible, and that is a major drawback.
Unfortunately, there are a few fraudulent antenna companies preying on the unsuspecting newbie mobile operator. One of those contains a 50Ω resistor, with a whip attached to the center conductor. Yet other designs use a 6:1 or 9:1 voltage balun at the base. One even has a multi-tapped, parallel tuned, LC circuit. All of these designs have two things in common: They're very lossy in terms of efficiency, and all are over-hyped. Remember, if is sounds to good to be true, is probably isn't!
Too many antenna manufacturers overrate the power handling capabilities of their various models, but that's not the whole story. Part of the efficiency equation is the magnitude of ground losses, which is power absorbed heating the air, and not being fed to the antenna! For example, the Comet UHF-6 (shown right) is rated at 120 watt SSB (PEP). Its manual clearly states The UHV-6 was designed to be lip mounted using the CP-5M or HD-5M. If you were to correctly mount one by drilling a hole, a 100 watt transceiver could easily destroy the antenna!
There are several screwdriver antennas rated at 200 watts PEP. Although they get warm during normal operation due to the rather high resistive coil losses (low Q), typically there's no permanent damage done. However, driving one with much more than 25 watts during tuning will damage the coil assembly beyond repair! As above, the scenario is exacerbated by proper mounting (reduction of ground losses).
Here's a bit of advice from the Amplifier Care and Feeding article, which is apropos even if you don't run an amplifier. Since far too many manufacturers stretch the truth, here are the antennas to avoid: Any vinyl covered one especially those with large metal end caps; Any screwdriver antenna with more than 10 turns per inch, or smaller than 2 inches in diameter, or wound with less than size 14 awg wire. This includes stubby screwdrivers (except the 680S Scorpion), any Hamstick®, any Hustler®, the Opek®, and any antenna where the loading coil is mounted higher than 60% of its length.
Short, Stubby Antennas
Short, stubby, remotely controlled, HF mobile antennas (like the Yaesu ATAS-120 shown at right) seemingly have become all the rage. They're popularity is in part due to their diminutive size (less than 7 feet overall), light weight, apparent ease of mounting, and presumedly their spousal approval rating. They're not necessarily less expensive than their stable mates, or anywhere near as efficient. In fact, the Yaesu 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. This fact is even alluded to in the Owner's Manual! These facts, coupled with its high cost of ownership, makes the ATAS a very poor choice indeed. Yet, until recently, it was the highest selling, single model mobile antenna in the world! An unexplainable conundrum.
Owners of short, stubby antennas 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 for the Scorpion 680SA, 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.
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.
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, due in part by the abbreviated mounting schemes most often used. 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.
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 square function of the overall (electrical) length, and the current distribution along that length, short, stubby antennas exhibit an esoteric phenomena. If you do all of the math 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; a based loaded antenna are about one half the efficiency of a center loaded one, all else being equal. 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 (due to poor mounting, mainly).
A Few Notes On Motors
All remotely-controlled mobile antennas require some sort of motor to resonant the coil assembly. It is, by any definition, a DC powered, reversible, gear motor, but there ends the similarity. For example, early screwdriver antenna designs utilized a stripped down Black & Decker® rechargeable screwdriver assembly, hence the name screwdriver antenna. The truth is, the plastic-geared B&D® motor is barely up to the task, albeit some late-model designs still use it. This fact has lead most manufacturers to adapt a Pittman® gearmotor to handle the task. It is a wise choice, as Pittman® makes a series of rugged, and well-designed gearmotors to fit almost any application. Unfortunately, some cheaper models avoid the expense, and rely on questionable gearmotors. This fact brings up several esoteric design problems.
Some gearmotors are designed to operate only in one direction due to requisite torque demands. Thus, when they are operated in their unnatural direction, the torque is not only lessened, but their rotational speed is greatly reduced. Therefore, it is not uncommon for 80 through 10, versus 10 through 80 transition times to vary by a factor of 2 or more. While the problem may sound trivial, if you QSY a lot, it isn't!
Pittman® gearmotors, and all other makes for that matter, come in a variety of styles. Some compact models use planetary gear sets wherein the motor shaft is concentric with the output shaft. Other models (typically less expensive) use straight gears wherein the motor shaft is not concentric with the output shaft. Both work equally well in operating a screwdriver, but there is a penalty to be paid when using a non-concentric design—motor space!
At least two commercial screwdriver antenna models use an non-concentric design in their smallest models. Read that as short, and stubby. By necessity, the resulting gearmotor is rather small is size—miniscule actually. While the current draw is low, the longevity of the gearmotor is suspect, especially when subject to stall conditions (end of travel). They are not, by any description, rugged! Unfortunately, the price you pay is no indication of what kind of gearmotor is being used, or its quality, or its ruggedness.
On the other end of the scale, are adapted motor designs. Several antenna manufacturers use gearmotor assemblies adapted from rechargeable hand drills. While rugged, their use is a screwdriver antenna is suspect from several angles, not the least of which is current draw. While a nominal Pittman® gearmotor may draw 200 mils on run, and perhaps 850 to 1,000 mils on stall, compare this to 2.5 amps run, and 8 amps on stall for the typical rechargeable screwdriver motor. This is higher current than most automatic antenna controllers can handle. In any case, power for any remotely tuned antenna should not be drawn from the transceiver's accessory port(s).
There are at least two antenna manufacturers who use a stepper motor design. This allows for precise repositioning of the resonant point, and decreases the length of time required to do so. However, the only controllers available to drive stepper motors are proprietary and rather expensive, often costing twice as much as the antenna itself!
Standard Sized Antennas
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.
All remotely tuned antennas, contain a gearmotor which turns a threaded rod (typically 1/4x20 all-thread) in and out of a nut or threaded boss 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.
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. Perhaps unfortunately, and contrary to popular belief, he never patented or copyrighted his design.
There are about 75 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 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.
Most 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.
Some of the cheaper models have very little coil support, especially on 80 meters when the coil is fully exposed. Hit a low hanging limb, and the antenna is toast! This condition is exacerbated on models with small diameter coils (<2 inches in diameter).
Scorpion is one of the newer screwdriver antenna manufacturers. Their 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 ≈$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.
The Scorpion 680, 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 to the coil. For those who think Q isn't important, you need to review the efficiency article.
Scorpion is the only HF antenna company in the world using N connectors as a standard feature. When properly installed, they assure that no moisture will seep into the coaxial connection. They have another unique feature, and that's the built-in, base quick disconnect at the base of the antenna. It has other unique features which make it a standout among all of the other screwdriver antennas.
Some models of 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%, or even less if the coil diameter is less than 3 inches.
If you own an 80 through 10 meter model, and you want to have 160 meter coverage, here's an alternative to buying a new antenna. Scorpion offers an add-on 160 meter coil for their 680 model which sells for $250. 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. Cheaper 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, similar to the one shown above. It retails for $90.
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!
Antennas equipped with interrupted threads, or slip clutches, like the earlier model GS antenna shown above right, are not compatible with automatic antenna controllers. Garry Stookey, the manufacturer, has redesigned his antenna to use a PolyFuse® to protect the motor. The Alpine series, made in Alpine, California, still uses a slip clutch, and there may be others that do as well. The best advice is to avoid them.
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 are lesser known ones which needs to be mentioned.
At least one model has a weather-sealed coil assembly. In some 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 are accessory masts available which raises the coil 1 to 2 feet. Even without a lower mast extension, some screwdriver designs place the motor as much as 20 inches above the base of the antenna. Either mounting scheme drastically raises the RF level imposed on the motor leads. It is best to avoid antennas designed this way.
As mentioned above, some screwdriver antennas use a PolyFuse® or similar product to protect the motor during prolonged stall conditions. The self-resetting device (it looks like a small ceramic capacitor) increases in resistance when the current exceeds its rating. Once the load is removed, the device cools, and the resistance drops back to near zero. At least one manufacturer uses a PolyFuse® rated exactly equal in value to the average run current. As a result, long run times (tuning from 80 to 10 meters for example) cause the PolyFuse® to do its job, albeit prematurely. If you notice such problems with your antenna, the solution is to replace the PolyFuse® with a larger amperage unit, but one not larger than 60% of the motor's stall current.
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 resistive (Q) losses in the loading coil. It is important to remember, the optimal position of the coil is dependent on ground losses as depicted in the chart at right (thanks to the ARRL). Since we really don't know with certainty what the magnitude of the ground losses are, selecting the optimal position is difficult at best. Suffice to say, however, any antenna with the coil mounted within the bottom 1/3, or top 1/3 of its overall length, will be less than stellar performers.
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 surface 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 predictable results. As a general rule, the (electrical) length of 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 overall length antenna is fraught with problems, and another good reason to buy a separate 6 meter antenna, preferably a horizontally-polarized loop.
Spirally Wound Antennas
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!
All spirally-wound are 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.
Mono Band Antennas
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.
Bug Catcher Antennas
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). This has the same affect as increasing its electrical length, but they also increase the radiation resistance by moving the current node further away from the base of the antenna—but only if they're installed correctly!
The left photo depicts a cap hat incorrectly installed. So installed, the input impedance and bandwidth will 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 antenna assembly!
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 at right, has a solid 4 foot extension below it. 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.
Lastly, cap hats do have a lot of advantages, but 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.
Built In Controllers
Over the years, several companies have devised antennas with built-in controllers. Don Johnson, W6AAQ (sk) of DK fame even tried the idea as evidenced by the photo. Although perhaps a novel idea, in the real world it is very difficult to keep RF out of the electronics. This is the main reason Don abandoned the idea.
Codan®, an Australian manufacturer, used to made a series of automatic tuning antennas for the military with the electronics built into the base. They are now made by United Arab Emirates-based Stealth Telecom®. Although the model numbers have changed, the mechanism is essentially identical to the original Codan® models. They have several drawbacks besides their high purchase cost, not the least of which is their low power and duty cycle rating (200 watts PEP, 6 minutes maximum). They're also advertised as an NVIS antenna, which no vertical can achieve.
Notes On Whips
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.
Odds & Ends
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 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.
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.