Practical_Antenna_Handbook_0071639586

384 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
any other country whose coastal waters he enters. For radio regulatory purposes, the
world has been divided into three regions, and the rules—including, for instance, allowable
frequencies of operation—differ from region to region. Consequently, someone
planning on using amateur radio equipment outside the coastal waters of his or her
own country should be up to date on the pertinent regulations before embarking.
The problems of antennas on boats are the same as for shore installations but are
aggravated by certain factors. Space, for one thing, is less on a boat, so most HF antennas
are, like mobile antennas, quite short relative to the wavelength in use. Also, RF
grounds are harder to come by on a fiberglass or wooden boat, so additional grounds
must usually be provided.
A typical powerboat example is shown in Fig. 16.3. The radio is connected to a whip
antenna through a transmission line (and a tuner on HF), while also being grounded to
an externally provided ground plate. Over the years, shipboard radio grounds have
taken many different forms. For example, the ground might be copper or aluminum foil
cemented to the boat hull. Alternatively, it might be a bronze plate or hollow bronze
tube along the centerline of the boat or along the sides just below the waterline. The
ground is connected to the engine. Careful attention must be paid to the electrical system
of the boat when creating external grounds to prevent electrolytic corrosion from
inadvertent current flows.
On sailboats the whip antenna (especially VHF-FM) might be mast-mounted as
shown in Fig. 16.4, or it might be an aluminum mast itself. Alternatively, a stay might
serve double duty as an HF antenna. In any event, the same grounding schemes apply,
including the additional option of a metal keel or metal foil over a nonmetallic keel.
Antenna
Radio
Engine
Foil
Bronze plate
Figure 16.3 Motorboat grounding system.