28
The insulated backstay antenna requires two porcelains isolators. The coupler
must be placed as close as possible to the base of the backstay antenna. Proper
grounding of the coupler is very important. Connect the RF ground terminal
of the coupler to all metal parts or structures of the boat (keel, engine, etc.).
5
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Loop antennas can be used very effectively in small apartments, offices and
holding rooms. Radiation for a loop antenna is always efficient but highly
directive. Therefore, the orientation of the loop is very important. Generally,
the vertical loop antenna with horizontal radiation is much preferred for a
general application throughout the 1.8 to 30 MHz band. However, loop anten-
nas can be very effective when mounted horizontally to the ground to radiate
vertically and provide efficient short range communications on higher fre-
quencies. Generally HF connections are difficult to establish in the distance
range of 50 to 250 miles. In a large room (such as 20 x 30), we would recom-
mend the installation of a single wire loop antenna mounted on the ceiling
with the coupler against the wall directly below the antenna. The wire gauge
should not be less than 16 AWG with insulation.
When operation is for a small room, it is recommended that a small multiple
wire loop (6 loops) on a rectangular configuration 3 by 4 feet be installed (as
illustrated in the figure below).
This configuration allows operation from 3.0 to 30 MHz and the SG-235 will
tune and load all frequencies well below a ratio of 1.1:5 VSWR. Directivity will
be high from 3 to 12 dB depending upon the frequency. If this is a problem,
two loops at 90 degrees can be installed to communicate at 90 degrees off
direction of the first loop antenna. Loop antennas are low noise antennas.
However, because they are used in apartments and urban cities, high industri-
al noise can be expected. Avoid using neon of fluorescent lights within 50 feet
of the loop antenna, they may completely jam one or more frequency bands.
Loop antennas of much larger sizes can be used, however, please remember
that while the low frequency operation of such antennas may be quite good,
the larger antennas may not work well at very high frequencies when located
in a plane parallel to the earth. This is because large loop antennas generally
radiate their maximum lobe (field strength) at right angles to their plane.
This means that a large loop, say 60 feet on a side, mounted 20 feet above the
ground would radiate much of its energy upward. While during the day this
would work well for close in communications, longer distances would be
SGC Inc. SGC Building, 13737 S.E. 26th St. Bellevue, WA 98005 USA
P.O. Box 3526, 98009 Fax: 425-746-6384 Tel: 425- 746-6310 or 1-800-259 7331
E-mail: [email protected] Web site: http://www.sgcworld.com
© 1998 SGC Inc
SG-235 Manual