SAC SA100, Manual

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SOUTHERN AVIONICS COMPANY
Model SA100 Dual 3-1 Antennas
3 Antennas
The range of a beacon and antenna depends on many variables and cannot be guaranteed.
The field strength can be calculated if the ground conductivity is known, but the field strength
needed depends on the background environmental noise which depends on location. A field
strength of 70uV/m is generally adequate in the United States and Europe but may not be
sufficient in latitudes between 30 degrees N and 30 degrees S. Range figures given in the
following sections are based on average ground conductivity and a field strength of 70uV/m.
The resistance of the antenna is dependent on many conditions over which the manufacturer
has no control. The ground condition, for example, is extremely important. In a high
conductivity ground region, the resistance will be fairly low. This is especially true on offshore
oil rigs where the ground system consists of a large metal structure over salt water. On the
other hand, if the ground conductivity is low, the resistance can become quite large. For
example, permafrost in Alaska generally makes a very poor ground and in most cases a
special counterpoise system must be used. Without a special counterpoise system, the
antenna resistance may be as high as 50 Ohms and the antenna efficiency will be very low.
THE MAST ANTENNA, "H" ANTENNA, AND 50-FOOT GUYED MAST ANTENNA ARE HIGH
Q, NARROW BANDWIDTH ANTENNAS. THEY SHOULD NOT BE USED FOR VOICE
MODULATION. THE SYMMETRICAL "T" ANTENNA IS THE ONLY ANTENNA
RECOMMENDED FOR VOICE MODUATION.
3.1 Mast Antenna
The Mast Antenna is a short vertical monopole with capacitive top loading and inductive center
loading. It was designed for use where space is severely limited such as on offshore
structures. It is centerloaded to be resonant at approximately 500 kHz for operation between
l90 and 415 kHz and at approximately 900 kHz for operation between 415 and 625 kHz.
Tuning below the resonant frequency is accomplished with the antenna coupler. There are
two versions of this antenna; one designed to be mounted on land, the other on drilling
platforms or ships. Neither antenna requires guying. A capacitive hat at the top of the 34-foot
Mast Antenna consists of six 8-foot radials. These can be shortened if necessary for
clearance on drilling platforms. The counterpoise system used for land installations consists
of sixteen 60-foot radials made of #10 copper wire with 6-foot ground rods at each end and one
in the center. The offshore version of the Mast Antenna uses the platform structure and the
water for the ground system.
The range with the Mast Antenna and this transmitter depends on ground condition,
frequency, the ADF, and atmospheric noise which in turn depends on location and time. Range
for most locations in the United States with one of the lower priced ADF's is from 50 to 70 miles
for frequencies above 250 KHz except over low conductivity ground. SAC does not
recommend this antenna for land installations at frequencies below 250 kHz. The "T"
Antenna is far superior at all frequencies and should be used whenever space permits. The
50-foot Guyed Mast is recommended for land installations where the "T" cannot be used.
Calculated range over sea water with the Mast Antenna and this transmitter is 40 miles at 200
KHz, increasing to 100 miles at 400 KHz.