Table of Contents
Advertisement
4 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 70~V/m is generally adequate in the United
States and Europe but may not be sufficient in latitudes between
30°N and 30°S. Range figures given in the following sections are
based on average ground conductivity and a field strength of 70~V/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.
4.1 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 190 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 #i0 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.
SS~800AV~
Southern Avionics
81
Company
Advertisement
Table of Contents
