Maximizing Efficiency in HF Mobile Antennas

L. B. Cebik, W4RNL (sk); "It is a fascinating arena of trying to squeeze the last ounce of available efficiency from largely undersized antennas."

Those of you who subscribe to the ARRL publication QEX, will probably recall the series of articles by Rudy Severns, N6LF. The articles contained a lot of empirical data with respect to vertical antennas, and their requisite ground plane requirements. If you haven't read the articles, you should, as the data is rather enlightening. Copies of the articles may be downloaded from Rudy's web site.

These articles were not aimed at the mobile operator, but the data does explain the ramifications of an inadequate ground plane under a vertical antenna. Suffice to say, the lossier the ground plane, the lower the efficiency, and that's exactly what we have in a mobile installation; a very lossy ground plane. I should point out that any vehicle is an inadequate ground plane at HF frequencies. Fact is, the body of the vehicle acts as a capacitance to the surface under the vehicle, which acts as the ground plane, albeit a very lossy one.

An important point needs to be made here. The body of the vehicle is a much better conductor of RF, than the surface under the vehicle. When we mount the antenna low, on a trailer hitch mount for example, a goodly portion of the return current is made to flow through the surface under the vehicle which increases ground losses. If we mount the antenna higher, atop the quarter panel say, more current returns through the body, so ground losses are reduced. Even so, ground losses in a mobile installation are much higher than those encountered in a typical base station installation.

Adding a lot of insult to mobile mounted verticals, is the typically low Q of the loading coils, and short lengths. Fact is, few amateurs really understand just how inefficient an HF mobile antenna system is. In the worst of cases, efficiencies are less than 1% (80 meters), and in the best of cases, about 80% (10 meters). It seems the only specific attributes which count are low SWR, short length, and ease of mounting. When they're lucky enough to work a few DX stations, then the worth of their choice is confirmed, and any discussion about efficiency is summarily dismissed.

The July/August 2009 issue of QEX, contains a follow up article written by Bob Zavel, W7SX, entitled Maximizing Radiation Resistance in Vertical Antennas. Part of the article covers top loading. Top loading is a methodology which increases radiation resistance, hence efficiency, even if the ground plane is substandard; seemingly a ubiquitous vertical antenna shortcoming. This article is also a must read especially if your urban bound!

The conclusions at the end of Bob's article are well founded. Of specific importance are the following points. To paraphrase: The radiation resistance (Rr) of a vertical antenna is a function of the physical height (overall length), and the current distribution along that linear height; The efficiency of a fixed-height antenna can be optimized by orientating the maximum current point at the half way point (height) of the antenna; Series and parallel losses (ground losses and stray coupling losses respectively) are always present, with series losses the most severe; Lowering of ground (series) losses, and raising radiation resistance will result in higher efficiency, but the latter is easier to accomplish.

These conclusions support the thought that reducing ground losses, and maximizing radiation resistance are the two paramount objectives in achieving maximum performance from a base station vertical. Or from an HF mobile antenna!

Let's look at the things we can do to maximize efficiency in a HF mobile antenna.

Ground losses dominate the efficiency formula in any vertical installation, but it's of particular importance in a mobile environment. Unlike a base station, we don't have the luxury to add more radials. What's more, the notion that adding ground straps to the mounting hardware will somehow replace, enhance, or rectify the ground losses are anecdotal. It is true there must be a solid connection for the currents to return to the source, but beyond bonding and a proper ground connection for the coax shield, there is little we can do.

Digressing for a moment. Whether the encountered losses are serial or parallel, they appear as part of the resistive portion of the input impedance. As such, they cannot be measured qualitatively (broken down into their individual parts), but we certainly can measure them as a whole. All it takes is a relatively inexpensive antenna analyzer. To make things simple, we're combining the serial and parallel resistive losses together, and calling them ground loss.

Further, the resistive portion of the input impedance also contains the radiation resistance; the only good loss an antenna has! Here too, we can't separate it out from the other losses encountered. As a result, we have to be careful about making assumptions based solely on changes in the resistive component of the input impedance. This point will become glaringly evident later on.

As alluded to above, mounting the antenna higher up on the vehicle will reduce, but not eliminate, ground losses. The problem is, few amateurs are willing to drill holes, make custom brackets, and the other prerequisites necessary for minimizing ground losses. Perhaps if they had a better understanding, they might think differently. Certainly the aforementioned articles are a very good place to start. Here's some additional food for thought.

The calculated ground losses for an average vehicle vary from about 2 ohms on 10 meters, to about 10 ohms on 80 meters. However, the real world losses can easily be double this amount. The factors which cause the loss have already been discussed. The only alternative we have, is to move the antenna as high as we can on the vehicle, albeit we have to contend with the localized conditions (low trees, wires, etc.). This said, we also have to keep as much metal mass under the antenna as possible, and we have to keep the antenna close to this mass. That is to say, mounting antennas atop long brackets is counterproductive.

The question remains, why is ground loss so important? If you've read the aforementioned articles, you'd already have an idea of the answer. From this author's empirical experience, the noted difference between low mounting (trailer hitch, bumper, etc.), and high mounting (atop a quarter panel, bed rail, etc.) is typically 4 to 5 dB, or about what you would achieve by adding a mobile amplifier.

But the truth is, you can't make a pat statement unless you consider all of the variables. In a mobile scenario, any specific calculation has to include the antenna's coil losses (Q), the location of the coil (base, center, etc.), and even the conduciveness of the surface under the vehicle.

Digressing again. As stated above, ground losses dominate the efficiency formula. However, in some cases coil losses become dominate. A case in point are the various, short and stubby, HF mobile antennas which have become all the rage; their coils have rather low Q ratings.

If you design an HF mobile antenna carefully, you can achieve a coil Q averaging about 300 as mounted in (on) the antenna. On 80 meters, a well-designed, center-loaded coil with a Q of 300 will have a resistive loss of about ≈12 ohms. Combine this with an overall length of about 7 feet, and a ground loss of 12 ohms, and the radiation efficiency is about 3%. That's 100 watts in, just 3 watts out!

One with a coil Q of 50 will have a resistive loss of ≈72 ohms! Combine this with an overall length of about 7 feet, and a ground loss of 12 ohms, and the radiation efficiency is just .7 %. That's 100 watts in, just 7/10 of a watt out!

Incidentally, these figures are straight out of the ARRL Antenna Handbook.

Few amateurs have a grasp of radiation resistance, but Bob Zavel's, W7SX, article does a good job explaining the factors involved. We'll take a more simplistic approach, and say; the effective radiation resistance of an HF mobile antenna is directly related to its overall length. A length, incidentally, which is fixed by practicality in our mobile-in-motive environment. From that standpoint, an overall length of 13 feet is about the maximum, with just 10 feet being the mean average.

Based on this maximum length, and remembering our paraphrasing above; The efficiency of a fixed-height antenna can be optimized by orientating the maximum current point at the half way point (height) of the antenna, we're left with moving the current maxima up towards the top of the antenna. One way to do this is to use center loading, rather than base loading, but there is more we can do.

In his article, Bob discusses using supporting guys to top load a vertical antenna, which will indeed move the current maxima up. Unfortunately, like adding radials, this isn't a luxury we have in a mobile environment. We do, however, have an alternative, albeit with a few of its own drawbacks. Enter the Cap Hat.

Properly placed, capacitive hats, sometimes referred to as top hats, increase the capacitance of that part of the antenna above the loading coil, thus moving the current maxima point towards the center of the antenna which in turn raises the overall radiation resistance. It does this without other adverse effects save one (wind loading), but only if it is positioned correctly.

Digressing once again. Whatever capacitance any given cap hat adds, is the same no matter where it is placed. However, whether or not a cap hat increases the effective length and/or increases the radiation resistance and/or increases overall losses, depends on where (how high above the coil) the cap hat is placed. For example, when placed too close to the loading coil, the capacitance can have a detrimental effect on the coil's Q, and will indeed produce an increase in the measured input impedance. For example, the left photo depicts a cap hat incorrectly installed. The input impedance and bandwidth will indeed increase in this example. However the changes are due to increased coil losses, and not by an increase in radiation resistance (Rr). Therefore, the following assumes the cap hat is mounted at the very top of the antenna, and thus the noted increase in input impedance is a positive one, not a negative one.

I have owned, and used, four commercially-designed cap hats. Every one of them has had a major drawback, besides the acknowledged wind loading. Universally, they're too small to be truly effective; they're all designed to be mounted too close to the coil; their solid mounting hardware, albeit short, can have a negative effect on an antenna's structural integrity; and some of them are too expensive. I wanted something better.

Any design concept should have clearly stated objectives. In this case, there were criteria which needed to be satisfied. An increase in the radiation resistance was the prime goal, followed closely by wind loading, weight, and ruggedness. There was one more criterion, and that was the ability to operate from 80 meters, through 17 meters. The reasoning will become apparent as we continue.

Every commercially available cap hat, is designed to be supported atop a short, solid shaft. Since the prime goal was to increase radiation resistance as much as possible, the cap hat would have to be mounted at the very top of its support structure. Using a solid shaft as a support would put the antenna in peril should the cap hat hit a low-hanging limb. This meant the support had to be flexible. Hence, the winding loading and physical weight become a critical factor.

The incorrectly-installed cap hat above left, consists of just spokes, with no outer rim, while the drawing at right shows one with a rim. Based on empirical testing, cap hats without this peripheral connection, have an effective length approximately 60% of their diameter, depending on the mounting height above the coil. In comparison, a cap hat with the peripheral connection, has an effective length nearly twice the cap hat's diameter. Here too, the maximum effect depends on where (how high above the coil) it is placed. Put another way, adding the peripheral wire increases the cap hat's effectiveness by nearly 4 times, but only when properly mounted.

There's a hidden factor at play, and that's the frequency of operation criterion. As mentioned above, a properly implemented cap hat, including its support structure, will increase the effective electrical length of an antenna. If the effective length is too great, the maximum usable frequency criterion (17 meters in this case) won't be met. As anyone can clearly see, it's a mixed bag of tricks, with clear limitations.

With the help of Ken Muggli, KØHL (an excellent machinist and draftsman), several different designs were tried, and compared. One design was cone shaped, and another was shaped like a wire-framed flying saucer (I live in Roswell, NM after all). Both of these designs were rejected for various reasons, primarily wind loading and weight. After about a dozen different attempts, the design shown in the left photo was settled on, due in part to its relatively low wind loading, and overall light weight. The rest of the story is truly serendipitous.

The loops are made from 1/8 inch, 17-7 stainless steel wire, purchased from Small Parts. Their standard length is 60 inches overall. These were inserted into a hub laid out, and machined by Ken, as shown at right. Plans for the hub may be downloaded here. The support structure, a stainless steel whip actually, requires an explanation.

There is just one supplier of 102 inch, 17-7 stainless steel whips, no matter where you buy one. They start out life as rolled wire, about .210 inches in diameter. The wire is straightened, and ground into the common size, and shape we all know. Starting at approximately 60 inches from the base, the wire is taper ground so the tip is .100 inches in diameter. A swaged brass 3/8x24 fitting is attached to the bottom, a small chromed, brass tip is added at the top end, and your standard 102 inch whip is born.

Strictly by accident (I really hate to admit that), the optimal position along the whip, where the cap hat is mounted, is exactly 60 inches! All of the criteria was met: The maximum usable frequency coverage included 17 meters; Wind loading was slightly more than the whip alone, and the assembly doesn't oscillate in the slip like bare whips tend to do; The total weight of the cap hat, and hub is 10.5 ounces; And it did increase the apparent radiation resistance.

Incidentally, during empirical testing, the effect of any whip protruding above the cap hat, was rather small. In view of this, and in effort to keep the height low, it was eliminated. And for the record, the total combined weight (cap hat and shortened whip) is 23 ounces (7 ounces more than the 102 inch whip).

The antenna in question is a Scorpion 680, mounted in the bed of my Honda Ridgeline. Photos are located here. The empirical testing was done by comparing the cap hat design against an MFJ-1956; a 12 foot, telescoping whip.

As we know, a full quarter wave vertical antenna (no loading coil), mounted on a vehicle, should have an input impedance, at resonance, of 36 ohms plus whatever ground, capacitive, and resistive losses are present. Using the aforementioned whip, it is possible to resonant the Scorpion 680 on both 20 and 17 meters, with the coil fully collapsed (fully shorted out). So resonated, the unmatched input impedance on 20 meters was 40 ohms, and on 17 meters, 39 ohms. These measured figures, using an MFJ-259B antenna analyzer, are very close to the theoretical input impedance, plus the calculated ground loss using the formulas published in the ARRL Antenna Handbook.

Here's how the (80 through 17 meters) comparisons were done. Once the antenna was at resonance (X=Ø), and the unmatched input impedance measured, the cap hat was removed, the whip installed, and then extended to the exact same resonant point. In all cases, the R value was slightly higher (two to four ohms) with the cap hat installed, when compared with the whip. This is close to the accuracy fuzz of the MFJ-259B. One might then argue that the difference was capacitive loading to the body of the vehicle (extra loss), or a slight increase in radiation resistance (a little gain). Either argument is moot, perhaps. What isn't moot, is the 42 inch reduction in the overall length (height) of the antenna. The height is now under 13.5 feet except for 80 meters, which for most folks is a worthy goal.

As stated above, the cap hat, when mounted 60 inches above the fully-collapsed coil, resonates the Scorpion antenna on 17 meters. The unmatched input impedance measures 43 ohms, or 4 ohms better than the equivalent whip. The reader can draw his/her own conclusions.

Alan, KØBG

www.k0bg.com

Member Comments: