Practical_Antenna_Handbook_0071639586

382 P a r t V : H i g h - F r e q u e n c y A n t e n n a s f o r S p e c i a l i z e d U s e s
tor to get out of the vehicle in order to do a good job of retuning. In recent years more
and more mobile operators have switched to using a motor-driven variable inductor for
the loading coil. Today several manufacturers (High Sierra and Tarheel are two wellknown
and highly respected suppliers) offer both base-loaded and center-loaded coils
and entire antenna assemblies that are either motor-driven or relay-selectable to permit
frequency changing from inside the vehicle. Some models even feature a remote-control
tilt to allow driving into the garage!
Arguably the best enhancement that one can make to any short HF whip is a capacitive
top hat. Usually consisting of a half-dozen or more radial spokes emanating from
the top of the stinger to a circumferential support wire or tube, a top hat increases the
electrical length of the entire assembly, thus bringing more of the natural high-current
portion of the monopole out of the feedline and into the vertical element. Benefits include
overall antenna efficiency improvements and increased VSWR bandwidth. Top
hats for 21 MHz and below should be as large as you can make them; it’s unlikely you’ll
exceed the dimensions at which they become an electrical detriment! Mobile HF antennas
from some manufacturers have top hats with a 3- or 4-ft diameter; in many designs,
guying of the upper half of the antenna is required.
Tuning HF Mobile Antennas
Tuning a typical mobile HF antenna is not particularly difficult, since there’s very little
you can adjust. Here’s a sequence that works for center-loaded verticals with fixedinductance
loading coils (resonators):
• Initially set the whip or stinger to the manufacturer’s recommended length. If
you have no guidelines—or your mobile antenna is totally homebrew— set the
total length to the maximum you can within the practical constraints (overpasses,
utility lines, etc.) mentioned earlier.
• If your loading coils are not adjustable, attempt to find a length for the stinger
(the portion of the whip above the loading coil) that produces a low VSWR back
at the transceiver in the lower portion of your desired range of operating frequencies
on a given band. While transmitting briefly on spot frequencies (no
further apart than 25 kHz on 80 and 40) throughout the appropriate band, determine
the frequency of minimum VSWR, if possible. (Use a VSWR meter at
either end of the coaxial cable between the mobile rig and the antenna. Alternatively,
it is perfectly fine to use the VSWR meter built into many of today’s
mobile rigs. But do not have an antenna tuner in the line at this stage of the
process.) If there is not an obvious minimum VSWR somewhere in the desired
range of operating frequencies, try a different stinger length. If a minimum
VSWR can be found and it’s on the high side of your chosen center frequency,
lengthen the stinger slightly and remeasure. If the minimum VSWR is on the
low side of your desired center frequency, shorten the stinger and remeasure.
• Once you have obtained the lowest VSWR you can by adjusting the length of
the stinger, reintroduce any matching networks, starting with any at the base of
the vertical. As a last resort, use the antenna tuner in your mobile rig to minimize
the VSWR. Presumably this will coincide with maximum RF out of the
vertical if you have been monitoring the field strength meter mentioned earlier.
In short, about all you can do is attempt to match the antenna to the feedline