Basics

Cap hats are a mixed bag of tricks to be sure, but they can be effective in more ways than one.

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 very 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.

Adding a lot of insult, is the typical 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 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!

Important Points

The overall length of most commercial HF mobile antennas, varies between 4 feet and about 13 feet, overall. Obviously, the shorter length the less efficient the antenna will be (all else being equal). To what extent length plays in determining an antenna's Rr (radiation resistance), and efficiency isn't well known within the amateur community. One very important point to keep in mind is simply this; Rr is proportional to the square of the effective electrical length! That is to say, an 8 foot antenna will have four times the Rr that a 4 foot one will have. Or, a 12 foot antenna will have twice the Rr of an 8 foot one. To say length matters is an understatement!

The maximum length (height at the tip) for any given installation depends on the antenna itself (structural integrity), its base mounting height above the surface under the vehicle (i.e.: bumper, quarter panel, roof), and the area of the country one travels through. Since Rr is directly related to the physical length (hence electrical length) of the antenna, and the current distribution along that length, we need to have a methodology to increase Rr, without excessive physical length. Enter the Cap Hat.

Ground losses also play a big roll in maximizing efficiency. Suffice to say, if you're going to the trouble of installing a cap hat to maximize efficiency, you have already properly mounted your antenna in the most efficient manner. This precludes low mounting (trailer hitch, bumper, etc.), and typically requires drilling holes for mounting hardware. Yes, you can still install a cap hat to increase efficiency no matter where you've mounted your antenna, but the amount of increase will be mediated by the abundance of ground losses.

It is also important to reduce the other resistive losses, primarily coil losses due to low Q. Installing a cap hat on a short, stubby antenna will improve its performance, but not to any great extent. Remember, ground losses dominate in any HF mobile antenna installation, but if the coil Q is low enough (less than 100 say), the coil's resistive losses could be more than the ground losses! In other words, adding a cap hat to an overly-lossy antenna is counter productive.

The input impedance of an HF mobile antenna consists of all of the losses, good and bad. Whether the resistive losses are in series or parallel, they show up in the input impedance. Since you can't measure (separate) the individual resistive components, you have to be very careful about applying a change in input impedance to an increase in efficiency. Incorrect cap hat placement is a prime example; a caveat covered below.

Cap Hats

Cap hats, sometimes referred to as 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). Properly placed, they can effectively increase the electrical length, by moving the current maxima toward the center of the antenna.

Digressing for a moment. 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.

Because the effective electrical length is increased, the radiation resistance also increases. Physically increasing the whip length will do the same, but as noted above, there is a practical limit to how long an HF mobile antenna can be. 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 shown at right, 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 left, are equivalent to approximately 60% of their diameter.

Cap hats typically increase bandwidth. The amount of the increase is based on a myriad of factors, most of which you can't 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.

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's your basic, high school physics, lever law. Yet, every currently sold cap hat is designed to mount atop a solid mast. And, they're typically mounted too close to the coil to have a positive effect.

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 rods themselves are polished round, 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 left photo. This also increases the effectiveness (capacitance) of the cap hat.

Digressing again. There are at least two antenna manufacturers who sell what amounts to a tubular cap hat. They're essentially a large diameter whip, although solidly constructed, and rather heavy. 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's been shown by actual field strength measurements, that antennas equipped with tubular cap hats are less efficient than ones with just a whip of the same length.

Efficiency Increase

The increase in antenna efficiency garnered for any given cap hat installation is both difficult to measure or calculate, as the resistive losses (series and parallel) will be different for every installation. Further, the overall size of the cap hat, its height above the coil, and whether its ends are electrically connected, all play a part. While not definitive as a properly conducted field strength measurement, the following is at least an indication of increased efficiency.

The antenna in question is a Scorpion 680, mounted in the bed of my Honda Ridgeline, directly atop the main cross member which passes just behind the fuel tank. The 102 inch whip is your basic CB item. Adding in the QD, and the corona ball, its overall length 105 inches. The Scorpion's 3 inch diameter coil is wound with #10 silver plated wire, at 6 turns per inch.

The cap hat consists of three loops made from .125 by 60 inch, 17-7 stainless steel wire. They're mounted atop a shortened .200 by 60 inch, 17-7 stainless steel whip (standard 102 inch). The effective outer diameter is ≈42 inches. Mounted as shown atop the whip—the correct position for any cap hat—its equivalent electrical length is ≈120 inches depending on the frequency of operation. The tip of the cap hat remains below 13 feet, 6 inches except on 80 meters where it's just under 14 feet. By the way, it is properly matched with a base shunt coil. The SWR on any frequency from 80 meters, to the top of 17 meters (the highest I can go with a full 8 foot whip or the cap hat), is less than 1.3:1.

On 80 meters with the cap hat installed, ≈70 turns show above the contact ring. With the whip, ≈80 show; On 40 meters with the cap hat installed, ≈16 turns show above the contact ring. With the whip, ≈20 show; On 20 meters with the cap hat installed, ≈4 turns show above the contact ring. With the whip ≈6.75 show; On 17 meters with the cap hat installed, Ø turns show above the contact ring (antenna fully collapsed). With the whip ≈4 turns show.

There are corresponding increases in the input impedance as well. Depending on the band, the unmatched input impedance increases 2 to 4 ohms. Obviously part of the increase is caused by the slight decrease in coil losses, but for the most part one can assume the radiation resistance is the major portion of the increase. There is more information about this in the Cap Hat How To article.

Field Strength Measurements

The only real, concrete way to determine a change, positive of negative, between one antenna design, and another, is to measure a change in the far field signal strength. Doing so is beyond the means of most amateurs, if for no other reason than real estate! Of course, there's the requisite hardware requirements too, but fortunately, I have that covered in that I have two Motorola 2001A, calibrated service monitors at my disposal; one to generate the signal, and one to read the signal.

At the far end, the receiving antenna is a 102 inch whip, with four 102 whips for a ground plane. It's mounted atop a 10 foot mast. While not ideal, it is adequate for measuring the difference between one design, and another. The distance between the receiving, and transmitting sites, has varied between 1 and 90 miles, although the latter often required a lab amplifier to boost the transmit power to 4 watts. During some tests, the band noise was severe enough, that accurate measurements couldn't be made. At other times, 100 mW was enough to be heard clearly 90 miles away (near the peak of Capitan Mountain, ≈10,000 ft above sea level).

The measured receive signal strength difference between the cap hat configuration versus the whip, varied between ≈3 dB (80 meters), and ≈6 dB (20 and 17 meters). About 25 different measurements were made, over a 7 week period (early Summer 2009). I believe this is proof-positive that the cap hat design herein, does indeed increase efficiency significantly enough, to offset the mechanical aspects of the design, let alone the reduction in antenna tip height above ground.

Odds & Ends

One way to think of a cap hat is to ignore the increased radiation resistance (hence efficiency) it will produce, but as a way to reduce the overall length, yet maintain a given level of efficiency. This will allow the antenna to be mounted higher on the vehicle, decreasing the inherent ground losses, which gives you increased efficiency. In most cases, this will garner you better overall performance.

If you're reluctant to drill holes to mount antennas, perhaps you should forgo installing a cap hat. As alluded to above, when ground losses and/or coil losses are high, whatever changes are made to the electrical length of an antenna, will be swamped by those losses.

If you want to know more about cap hats, read Tom Rauch's, W8JI, treatise on radiation_resistance.