Practical_Antenna_Handbook_0071639586

C h a p t e r 1 6 : M o b i l e a n d M a r i n e A n t e n n a s 383
and the transmitter, to maximize RF transfer and minimize any tendency by the
transmitter to reduce power in the face of VSWRs higher than 1.0:1.
• At some point in this procedure you may find you can’t make the stinger any
shorter because it has bottomed out in the clamp at the top of the loading coil.
If so, you may need to cut some of it off. Be careful! It’s hard to add length if you
cut it prematurely. In fact, if the minimum VSWR is not unreasonable (say, 1.5:1)
with the original stinger stuffed all the way into the clamp, be satisfied. Do any
further VSWR reduction with a matching network in the line or back at the
transceiver. In short: Learn when to leave well enough alone!
In other mobile whip installations, there may be an adjustable (or tap-selectable)
loading coil in addition to the adjustable stinger. Figure 16.2A takes this idea one step
further with an adjustable or tapped center-loading coil in series with a tapped impedance-matching
coil at the base. Alternatively, the upper coil is a fixed inductance, and
the only adjustment to the upper part of the vertical is to the length of the adjustable
resonator shaft (stinger). Ideally, a dip meter (see Chap. 27) is available to set the upper
inductance of the stinger so as to roughly resonate the vertical at the desired operating
frequency. Then—and only then—should the matching network at the base of the antenna
(or back at the transceiver) be adjusted. With this configuration, it may also be
possible to initially adjust the upper coil or stinger for maximum reading on a field
strength meter, then follow up with adjustment of the base matching network for minimum
VSWR on the transmission line back to the transceiver. However, tuning the
upper coil and/or stinger for maximum field strength can lead to ambiguous or even
erroneous results if the output power of the transceiver changes substantially in response
to the changing VSWR it sees as the stinger wire or upper coil is adjusted.
Marine Radio Antennas
From its inception, radio communication considerably lessened the dangers inherent in
sea travel—so much so, in fact, that the maritime industry took to the new “wireless
telegraphy” earlier than any other segment of society. In the early days of radio, a number
of exciting rescues occurred because of wireless. Even the infamous S.S. Titanic sinking
might have cost less in terms of human life if the wireless operator aboard a nearby
ship had been on duty. Today, shore stations and ships maintain 24-hour surveillance,
and vessels can be equipped with autoalarm devices to wake up those who are not on
duty. Even small pleasure boats are now equipped with radio communications, and
these are monitored around the clock by the U.S. Coast Guard.
When operating type-accepted radio equipment on marine frequencies, the small
shipboard operator can have a selection of either HF single-sideband communications
or VHF-FM communications. The general rule is that a station must have the VHF-FM
and can be licensed for the HF SSB mode only if VHF-FM is also aboard. The VHF-FM
radio is used in coastal waters, inland waters, and harbors. These radios are equipped
with a high/low power switch that permits the use of low power (1 W typically) in the
harbor, but higher power when under way. The HF SSB radios are more powerful
(100 W typically), and are used for offshore long-distance communications beyond the
line-of-radio-sight capability of the VHF-FM band. Of course, a radio amateur can usually
utilize the full panoply of modes, subject to the regulations of his home country and