Last Modified: March 26, 2013
Contents: Basics; The Cap Hat; Building A Cap Hat; Conclusions;
Cap hats are a mixed bag of tricks to be sure, but they can be effective in more ways than one.
Cap hats are not a new innovation, and have been used in one form or another almost since the dawn of radio. Perhaps their use atop HF mobile antennas is a bit newer, but not by much. Photos in early issues (circa 1930s) of QST, the monthly journal of the ARRL, illustrate cap hat use well before WWII. In fact, they date back to 1898, and to Sir Oliver Joseph Lodge. He was a British physicist who was involved in the development of early wireless telegraphy. One of those inventions was the tuned radio system (US patent #609,154). Part of that invention, was a dipole comprised of conically-shaped elements. The abstract of the patent reads in part:
"...As charged surfaces... I prefer, for the purpose of combining low resistance with great electrostatic capacity, cones or triangles or other such diverging surfaces with the vertices adjoining and their larger areas spreading out into space..."
The reason he chose the design was to reduce the Q of the antenna system, thus increasing its bandwidth. Based on his comments from the patent, he also realized that the input impedance would increase, and the overall length at resonance would decrease! This is exactly what cap hats do! Proving, of course, that capacitive loading has been with us a very long time indeed.
The other reason I bring up the history of cap hats, is that I have been accused several times of plagiarizing existing (so called commercial) cap hat designs. If you click on the photo montage to enlarge it, you'll notice the cap hat in the third row down, second from left, is a cap hat exactly like the one I use. This montage dates back to 1961, and perhaps much earlier! While there are a whole bunch of different configurations shown, they all do essentially the same thing; they add capacitance to that portion of the antenna above the coil. As you will see, application is much more important, than the basic design.
About 1954, the ARRL published their first Mobile Manual. It was a compilation of the various mobile articles which had appeared in QST since mobile operation was reinstated in 1948 (see left). The forth, and final addition (unfortunately) was published a few years later. In that second addition, were two cap hat examples. Both, incidentally, incorrectly installed directly atop the coil.
Then in the July, 1961 issue of QST, there appeared a short article which covered the second-annual California Mobilecade and Field Trials. That article was highlighted by a photo montage above right of 16 contestants entered into the field trail (click to open in new window). All but two of those sported cap hats!
It should be noted, that cap hats increase the wind loading, and bending moment stresses significantly more than a whip. Obviously, the antenna in question has to be built sturdy enough to withstand these extra stresses. I would not recommend using one on any light duty antenna, no matter how well it is mounted.
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 lossy.
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 just like they are when we use elevated radials versus ground laid radials. Even so, ground losses in a mobile installation are much higher than those encountered in a typical base station installation.
A later article, written by Bob Zavel, W7SX, appeared in the July/August 2009 of QEX, 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!
What follows is a methodology to increase the (series) capacitance of that portion of the antenna above the loading coil a mobile antenna. This causes the current node to move further up the antenna's length, which increases the radiation resistance, and the efficiency along with it. It should be pointed out, however, that we can also add (parallel) capacitance to the base (low mounting for example). Doing so reduces efficiency. This fact brings up another important subject; where to add length (capacitive reactance) to a mobile antenna to increase efficiency; below the coil, or above the coil? Generally speaking, as the ground losses increase, so does the optimal height of the coil. Thus, if the ground losses are excessive, one might see a slight improvement in field strength (you can't use the input impedance as a true indication) by increasing the mast length. If ground losses are relatively low, the best improvement in field strength will always occur by adding length (capacitance) to the whip, either by lengthening it, or by adding a cap hat to it.
The Cap Hat
The mobile equivalent to top loading is the cap hat. As their name implies, cap hats add capacitance to the top portion of the antenna (the whip). Whatever capacitance any given cap hat adds, is the same no matter where it is placed. However, whether a cap hat increases or decreases 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 affect on the coil's Q, and will indeed produce an increase in the measured input impedance. For example, the right photo depicts a cap hat incorrectly installed. The input impedance and bandwidth will 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. Incidentally, the rule of thumb is, the cap hat should be mounted at least its radius above the coil, and preferably more!
Because the effective electrical length is increased, the radiation resistance also increases. Physically increasing the whip length will do the same, but there is a practical limit to how long an HF mobile antenna can be (height above ground). By adding a cap hat, the overall length can remain the same (or less if the cap hat is large enough), while radiation resistance increases. That's the good part! The bad part is, they increase wind loading and complexity.
The actual design of the cap hat is important too. If the cap hat is enclosed like the wheel at right, or the loops shown below, the effective length is increased approximately twice the cap hat's diameter. Cap hats without the peripheral connection, like the DX Engineering unit shown at above left, are equivalent to approximately 60% of their diameter. However, cap hats without the peripheral wire are less prone to snagging on low hanging tree limbs. For some operators, this fact may be the deciding factor of which kind to use—with or without the outer rim. One item to keep in mind when making this decision, however, is that cap hats with the rim (everything else being equal), will result in the antenna being shorter (in length) overall than those without the rim, and thus will be less prone to low hanging limbs.
Cap hats typically increase bandwidth. The amount of the increase is based on a myriad of factors, most of which you cannot measure directly. Here too, you have to be careful about applying a change in input impedance (SWR over any given bandwidth), as a positive indication. In fact, the bandwidth of an antenna with incorrectly mounted cap hat (directly atop the coil), will be greater than a correctly mounted one, and with far less efficiency as well!
There's another drawback which might not be apparent. 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, and not flexible like a whip. It is your basic, high school physics, lever law. Yet, every currently sold cap hat is designed to mount atop a solid mast.
There's one potential problem depending on the design, and that's corona discharge from the tips of the cap hat. At the tip of an HF mobile antenna, the RF voltage can easily exceed 10 kilovolts. The upper left photo shows a stock DX Engineering cap hat. You'll notice the rods are tipped with little plastic protectors. The rod ends are rounded, but because of their small diameter, it is possible to have corona discharge off the tips of the rods, especially if you run high power. You can get around this problem by purchasing three of the optional 48 inch rods, and bend them into circles as shown in the right photo. As mentioned above, this also increases the effectiveness (capacitance) of the cap hat, while making it more susceptible to snagging on an errant limb.
There are at least two antenna manufacturers who sell what amounts to a tubular cap hat. They're essentially a large diameter cage made of several spirally-wound wires like a round bird cage, or simply a large piece of tubular aluminum. Although solidly constructed, they're rather heavy, and have a great deal of wind loading. As noted above, they do indeed increase the capacitance above the coil. Unfortunately, a major portion of the added capacitance is within the coil's field. This fact reduces overall efficiency rather drastically. It is been shown by actual field strength measurements, that antennas equipped with spiral cap hats are no more efficient than ones using a whip of the same electrical length.
Building A Cap Hat
I have tried about two dozen designs trying to come up with the most effective design consistent with a minimum of wind loading, and of course light weight. I tried wheels, loops, cones, multiple shorting circles. One was in the shape of a baseball pendant. I even did two solid disks, but the wind loading for those beasts was enormous! Some were better than others, but most had one drawback or another. So, what you see in the left photo, is the culmination of all of those designs.
The current cap hat is a modified commercial one manufactured by Scorpion Antennas, and shown at left. The 6061T6 aluminum mast is 4 feet long (the stock one is 3 feet long), and the 60 inch 304 stainless steel loops have been replaced with 72 inch long ones. The effective diameter of the cap hat is ≈58 inches. The longer lengths (loops and mast) further increased the efficiency. The antenna will tune from about 3 MHz (coil fully extended), to 18.5 MHz (coil fully collapsed). The SWR is too high for use on 15 meters, as the antenna is almost fully collapsed on 17 meters.
The previous cap hat used a hub designed by Ken Muggli, KØHL. It is shown at right, and plans for the hub are in the Antenna Mounts album of the Photo Gallery. It used a cut down CB whip with an overall length of 60 inches. Even with aluminum wire for the loops, there was excessive swaying in the slip stream at highway speeds, which made tuning difficult, hence the switch over to the Scorpion unit. This said, it would be easy to increase the center hole of the hub to accommodate a larger diameter support. It could even be threaded 3/8x24 if need be.
The increase in efficiency cannot easily be measured without a bench-full of expensive equipment. But you can infer it in other ways. One of those is to count the number of coil turns showing above the contact ring on a screwdriver antennas. Here's how the antenna stacks up with the cap hat, versus a standard 102 inch, 17-7, stainless steel whip.
On 80 meters with the modified Scorpion cap hat installed, ≈55 turns show above the contact ring. Using the whip, ≈80 show. On 40 meters, ≈12 turns show above the contact ring. Using the whip, ≈20 show. On 20 meters, ≈3.0 turns show above the contact ring. Using the whip, ≈6.75 show. The other good part is, the top of the antenna is under 13 feet, even when tuned to 80 meters. In some areas of the country, this is all but a prerequisite!
Note there is no whip showing above the cap hat, and this is by design. During the empirical testing, it was discovered that any additional whip showing above the (properly mounted) cap hat had very little affect on the resonant point, or the input impedance. There is no corona ball either, as the effective end(s) of the antenna are the rounded loops. To date, no corona discharge has been observed, even at near legal limit power (≈1,300 watts PEP).
Cap hats aren't for everyone, but if you want the best performance (read that as the highest efficiency) out of an HF mobile antenna, they are the only way to accomplish the goal, yet maintain some modicum of practicality. They have some drawbacks as any system will have, but I am convinced the pluses outweigh the minuses.
For those who might ask, I did do several whip length comparisons as mentioned above. I used an MFJ-1956, 12 foot, telescoping whip. Once the antenna was at resonance, and the input impedance measured, I removed the cap hat, installed the whip, and extended it to the exact same resonant point. In all cases, the R value was slightly higher (two to five ohms) with the cap hat installed, when compared with the whip. The original measurements utilizing an MFJ-259B, have since been confirmed using a calibrated MiniVNA Pro.
One might 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 all but moot. What isn't moot, is the reduction in overall length, which for most folks is a worthy goal.
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, or 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 are very close to the theoretical input impedance, plus the calculated ground loss using the formulas published in the ARRL Antenna Handbook.