Practical_Antenna_Handbook_0071639586

CHAPTER 16
Mobile and Marine Antennas
Mobile operation of radio communications equipment dates back nearly as far
as “base station” operation. From the earliest days of wireless, radio buffs
have attempted to place radio communications equipment in vehicles. Unfortunately,
two-way radio was not terribly practical until the 1930s, when the first applications
were amateur radio and police radio (which used frequencies in the 1.7- to
2.0-MHz region). Over the years, land mobile operation has moved progressively
higher in frequency for practical reasons. The higher the frequency, the shorter the
wavelength and therefore the closer a mobile whip can get to being a full-size antenna.
On the 11-m citizens band, for example, a quarter-wavelength whip antenna is 102 in
long, and for the 10-m amateur band only 96 in long. At VHF and above, full l/4 mobile
antennas are practical. Partially as a result of this, much mobile activity takes place in
the VHF and UHF region.
Interestingly, even as land mobile moved higher in frequency with the passing decades,
amateur radio mobile activity has moved both higher and lower, with everincreasing
interest in overcoming the challenges that very long wavelengths present.
Today, enterprising amateurs in the United States are working transoceanic DX on
160 m during their daily commutes!
Mobile operation in the amateur 144-, 220-, and 440-MHz bands is extremely popular
because of several factors, not the least of which is the small size (and cost) of efficient
vehicle-mounted verticals (or whips) for these bands.
Mobile Antennas for VHF and UHF
At 2 m and above, the roof of even a small coupe has enough flat or nearly flat surface
area to act very much like an ideal ground plane—provided, however, that the antenna
is located near the middle of the roof, and that the shield, or grounded, conductor of the
coaxial feedline makes a clean, low-resistance connection to the roof at the base of the
antenna. If that is done, a simple 19-in piece of tinned copper wire or plated spring steel
will work as well as any store-bought whip on 2 m. Over the years, the author has had
2-m whips installed by a commercial two-way radio shop on more than one occasion,
but has also done it himself a few times. If you are comfortable with snaking RG-58 or
RG-8X cable between the roof and the liner of your vehicle and know how to install an
old-fashioned chassis-mount SO-239 (drill a pilot hole for a chassis punch of the appropriate
size, such as those made by Greenlee), you can have a 2-m mobile antenna that is
hard to beat! (Don’t forget to seal against rain around the connector. Use a rubberized
gasket or a caulking compound.)
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