Needed - A New Mobile Antenna Paradigm

The typical HF mobile antenna is a vertical whip with either center loading or base loading, or some combination of the two.

Results are generally quite good when the physical radiator is at least one-eighth of a wavelength or greater. What this means is that an eight-foot radiator is reasonably efficient (around 50%)on 20M.

The problem is that once antennas become physically shorter than one-eighth of a wavelength, efficiency drops radically.

You can illustrate this fact yourself on a piece of graph paper. Draw a horizontal axis that is four squares long, each square representing one-fourth of the maximum value. For the vertical axis (starting at the leftmost point of the horizontal line, draw a line that is eight squares long. This vertical line represents a one-quarter wavelength vertical antenna.

At the top of the quarter wavelength, voltage is maximum and current is minimum (zero). At the bottom of the quarter wavelength, voltage is zero and current is maximum. If you draw one quarter of a sine wave to connect the voltage and current points at the top and bottom of the chart, you'll see that they cross at the vertical halfway point. This is the most efficient part of the antenna; the ends are the least (remember, power equals voltage times current.)

Looking at this plot, if you could magically take your eight feet of physical 20M vertical antenna out of the middle of a full quarter wave antenna, you would have an antenna with efficiencies exceeding 90%. This is precisely what Force 12 does with its verticals. The "T" bar on top, and the coil at the center of their vertical dipole serve as end loading for the half of the quarter wave dipole section, and they claim efficiencies of well over 90%.

Unfortunately, inductor loading cuts out some of the most efficient portions of an antenna. Center loaded antennas, such as Hustlers and Hamsticks, are essentially shortening the antenna in the most efficient radiating portion. Base loading shortens the antenna at the bottom half, also having an impact on efficiency. The loading coils, whether at the bottom or center of the element DO radiate slightly, but not with any degree of efficiency,

The question then becomes how can we emulate an end-loading scheme on a mobile antenna? The exercise becomes a game of elimination, what we can and can't live with in a mobile operation.

We know that a capacity hat will shorten an antenna electrically, but capacity hats are unwieldy and probably require the antenna to be guyed. Verdict: Doable, not practical.

The Force 12 model with a "T" bar could possibly be done with rabbit-ear style elements attached to the top of a whip, but would still probably require some guying. A tuner or small inductor could provide the end loading at the base. Verdict: Doable, somewhat impractical.

A "stinger" wire attached at the top of a generic eight-foot whip could be tied down at some point on the car body. For example, a 4-5' wire attached to a whip and combined with a small amount of base loading would be very efficient, and still retain mostly vertical polarization. The wire could be tied to a distant part of the car body, extended with clear fishing line or something similar. One scenario I contemplated is tying lightweight line to the car antenna, thus keeping the stinger in as much of an inverted L configuration as possible. Verdict: Doable, fairly practical, wind loading characteristics might have to be tested.

The double stinger "T" bar configuration is a great alternative to a regular "T" bar. By putting two stinger wires off the tip of a whip, (in as much of a "T" configuration as possible) you can emulate the "T" bar concept, while also using the wires for guying the whip. A small amount of base inductance would result in a good match and a very efficient antenna. Verdict: Doable, and quite practical

Another consideration is to replace a center loading coil with a matching section made of ladder line. This hairpin match could also be tied off somewhere on the vehicle (keeping it as horizontal as possible) and would more efficient than a coil. There could be multiple places along the matching section for shorting bars for multi-band operations. Verdict: Doable, somewhat practical. A possible solution, though, as the matching section might serve as a guy, but may also be unwieldy.

The open/closed end loop is a possibility. With today's larger vehicles, a wire around the perimeter of the roof, supported by small vertical supports extending a foot or two above the roofline, could be used as an end fed random wire, or as a small horizontal loop with some effectiveness. The roof of my van is 5x8', so a 40' loop could be placed there and could be end-fed as a 40' rectangular wire, or loaded as a loop, and tuned in either scenario with a tuner. Verdict: Doable, somewhat impractical due to the number of supports needed, weird in appearance, not tested.

The last possibility is a random longwire. My present vehicle measures about 16' by 5'. With a bumper-mounted 8' whips at opposite corners of the vehicle connected with a wire, I could have a radiator that would be almost 40' long. With end loading by means of a tuner, it could be quite efficient, even on 80M. There could be various points along the wire where you could have shorting clips to lengthen or shorten the antenna for multi-band use, or use the tuner for all bands. Verdict: Doable, but practical only if you're willing to have two whips on your car. Wind loading at high speeds could be problematic, lightweight fishing rods may help here. Antenna is likely to be quite directional on the higher bands.

Conclusion: More and more hams are taking to the road, and with the antenna restrictions that many of us face, mobile activity is a great alternative to nothing at all. I've presented several alternatives to conventional HF mobile antennas, most of which will outperform the conventional alternatives. Are we willing to think outside the box and make some of these ideas work for us?

Paul K5AF

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