SOUTHERN AVIONICS COMPANY
SD Series Installation and Setup
9-15
Coupler Setup
9.4 Antenna Theory
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 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.
9.4.1 Symmetrical “T” Antenna
A Symmetrical “T” Antenna is recommended for use with a 500 - 1000 watt transmitter if
sufficient land is available. This antenna requires a plot of ground approximately 150 feet by 400
feet for a 60 foot high antenna to 150 feet by 500 feet for a 120 foot high antenna.
The standard Symmetrical “T” Antenna is supported by 60 foot above ground level towers or
poles placed 300 feet apart and has a 55 foot vertical radiator and a 2 wire 280 foot horizontal top
hat. Range depends on the field strength required, the ground condition, and the power radiated.
The power radiated increases with frequency, however, the propagation loss due to ground
condition decreases with frequency. For a 70 uV/m field strength, the range over “good” ground
varies from approximately 60 miles at 190 KHz to 70 miles at 535 KHz with a 50 watt transmitter
and approximately 80 miles at 190 KHz to 90 miles at 535 KHz with a 100 watt transmitter.
A Symmetrical “T” Antenna supported by 90 foot towers has a 85 foot vertical radiator and a 2
wire 280 foot horizontal top hat. Range with this antenna over “good” ground varies from
approximately 90 miles at 190 KHz to 95 miles at 535 KHz with a 50 watt transmitter and from
approximately 105 miles at 190 KHz to 115 miles at 535 KHz with a 100 watt transmitter.
9.4.2 Tower Antenna
A Tower Antenna can be used if sufficient land is not available for a Symmetrical “T” Antenna. A
portion of the top set of guy wires is used to form the top hat for this antenna. A Tower Antenna
is electrically inferior to a Symmetrical “T” Antenna of the same height due to the decreased top
loading. Top loading serves two purposes; decreasing the input reactance and consequently the
voltage on the antenna and increasing the effective height.
A 50 foot tower requires a plot of ground with a radius of 60 feet. Range over “good” ground
varies from approximately 40 miles at 190 KHz to 50 miles at 535 KHz with a 50 watt transmitter
and from approximately 60 miles at 190 KHz to 65 miles at 535 KHz with at 100 watt transmitter.
