Last Modified: August 21, 2014
Contents: Introduction; Basics; Design Considerations; Building A Cap Hat; Conclusions;
The most effective way to increase efficiency and bandwidth in an HF mobile antenna, is the correct application of a cap hat.
Cap hats are not new innovations, and have been used in one form or another almost since the dawn of radio. The first patent covering them (US patent #609,154). was issued to Sir Oliver Joseph Lodge, a British physicist, in 1898. His 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, if installed correctly.
Based on photos appearing in QST, cap hats atop HF mobile antennas date to early 1930s. In 1954, the ARRL published their first Mobile Manual which was a compilation of the various mobile articles which had appeared previously in QST. Several cap hat examples were included, but unfortunately they were shown 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 shown right of 16 contestants entered into the field trail (click to open in new window). David Evans, WA6JJG, won that competition, and was rewarded with a kiss! Take note of the three loop, cap hat design directly above David's head.
The radiation resistance of a vertical antenna is a function of its electrical length, and how the current flows over that length. However, in a mobile scenario, we don't have the luxury to increase the overall height to much more than 13 feet, or about 11 feet in physical length. And we can't reduce the ground losses significantly, bonding notwithstanding. Thus the only way we can increase efficiency is to increase the radiation resistance. A good example is the difference in radiation resistance between a base loaded antenna, and a center loaded one.
The highest current point in a base loaded antenna, is just above the loading coil. If we relocate the loading coil to the center of the antenna, the current flow from the base to the bottom of the coil is almost linear. This fact doubles the radiation resistance. It would double the efficiency too (all else being equal), but there is a bit more coil (Q) losses as center loading doubles the required reactance for resonance. Moving the coil higher isn't the answer either, as the increasing Q losses soon offset the increase in radiation resistance. This fact is illustrated in the ground losses vs. optimal coil position chart shown at right.
We can raise the current node even higher, by top loading the antenna capacitively—hence the term cap hat. Generally speaking, the more capacitance, the higher the radiation resistance. However, cap hats increase the wind loading and bending moment stresses significantly more than a whip. Therefore, there is a physical limit which must be weighed against the increase in efficiency.
Cap haps come in a variety of designs, but some of them perform better than others. For example, a spoke design like the one shown at right, don't work nearly as well as those with an enclosing rim, like the one shown below. The reason is, for a given overall diameter, the outer rim almost doubles the effective capacitance. This has to be weighted against the slightly increased wind loading, and a greater chance the cap hat could catch on an errant tree limb.
There is also a practical limit to the minimum and maximum diameter. Too small and they're not worth the effort. Too large and the overhang will be excessive. There is a limit to the length of the mast supporting them too. Too short, and there will be excessive capacitive loading between the cap hat and the body of the vehicle. Too long, and even the sturdiest of antennas will fail due to the excessive bending moment cause by wind loading.
Weight is another important factor. One commercial design weighs in at just over 6 pounds. So, not only must the antenna hold it up, it must be capable of withstanding the bending moment of acceleration and deceleration, irrespective of the wind loading.
The position of the cap hat within the antenna's length is also very important. For maximum efficiency, it must be placed at the very top of the antenna. Ones mounted atop the coil as shown at left, will indeed increase bandwidth, but at the expense of drastically-reduced efficiency!
If one considers all of the above factors, you'll come to the conclusion that the three loop design shown left, offers the best overall performance. In this specific case, the mast if 4 feet in length, and the loops are 72 inches long. The effective diameter is approximately 5 feet. Mounted atop a Scorpion 680, the antenna can be tuned from 80 through 17 meters. The stock cap hat from Scorpion uses a 3 foot mast, and 60 inch wires, and allows the inclusion of 15 meters.
Four loops could be used, but this increases wind loading by 1/3, with only a very small increase in effectiveness. A longer mast isn't viable, as this would place the cap hat above the nominal legal limit of 13.6 feet in most states.
There are several commercial designs on the market which use what would best be described as globular wire cages. Besides the excess wind loading they represent, they place the cap hat too close to the body of the vehicle, and in some cases, too close to the loading coil. The resulting stray capacitance loading is a detriment to both efficiency and bandwidth.
There is one question which remains, and that is the actual wind loading. It is easy enough to calculate the loading on the mast (≈3 pounds at 60 mph), but the wires are dynamic (blowing in the wind) making calculations difficult. However, this is a way to estimate the wind loading close enough. Here's how it was done.
A small whip spring was temporarily installed in place of the QD (quick disconnect). A piece of Spectra, 50 pound test fishing line was attached to one of the loops. The other end was brought into the cab through the moon roof. With the help of a passenger, the vehicle was driven at 60 mph, and the line let out just to the point of bowing in the wind. A reference point was marked on the fishing line. Back in the driveway, a second piece of Spectra was attached to the hub, and to a hand held scale. Enough pull was exerted to move the first line to its reference point, and the scale was read. Surprisingly enough, the scale read just 6 pounds. Obviously, this non scientific exercise isn't perfect, but it does give one an indication of the bending-moment forces applied to the top of the antenna.
Building A Cap Hat
If you decide to build you own cap hat, rather than buy one, you need to read this section thoroughly, starting with the hub. Ken Muggli, KØHL, helped design and machine several cap hat hub designs. The one shown at right is very similar to the one Scorpion sells, except for the mast design. In this version, the mast was a cut down 102 inch whip. That idea was soon abandoned because it swung around too much. However, the design is such that a larger mast hole could be drilled. All considering, a 1/2 inch mast is the minimum size. The plans for the hub are in the Antenna Mounts album of the Photo Gallery.
The wires are just as important, and should be no larger than .125 inches due to wind loading. Whatever you do, don't use aluminum! For starters, they sing in the wind like a banshee on steroids, and eventually snap off right at the hub. The best material turns out to be 304 stainless steel. One very good source is Speedy Metals.
Speedy Metals also sell aluminum rod in various lengths and diameters. The best material for the mast is 6061T6. Whatever you use, flats should be machined just about the bottom threaded portion. This allows the mast to be easily tightened, and provides a break point should you hit an immovable object. Like your garage door for instance!
You'll need a lot of set screws, and Micro Fasteners is one of the best suppliers. Those shown are 10x32x.25, and are ideal.
You can use the cap hat hub sold by DX Engineering, but here's a tip or two. The longest wires DXE sells are 48 inches in length. If you use them as shown here, it will be more effective than just the straight wires. They holes are .125 ID too, so longer wires could be used. However, they'll still look sort of like a clover leaf due to the design of the hub. Incidentally, the set screws are somewhat miniscule, and easily stripped, so care is warranted.
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 the pluses outweigh the minuses.
Unfortunately, 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!