Radar System Engineering

714 RADAR RELAY [SEC. 17.10
between fixed ground stations since they can be permanently oriented in
the correct direction. Very high, even microwave, frequencies are
desirable for such applications since directional antennas can then be
small, rugged, and easily constructed and installed. At least one micrw
wave equipment has been successfully used in experimental tests (Sees.
17.14 and 17.15).
When one or both stations are moving, on the other hand, it is necessary
either to use sufficiently wide antenna patterns to provide coverage
in all the necessary directions, or else to provide for automatic pointing
of the antenna, with the consequent added complexity and weight. (If
several receiving stations are involved the transmitting antenna must
cover them all. ) As a result, lower frequencies (100 to 900 Me/see) have
predominated in such applications since more power is available and
higher antenna gains can be obtained with a given pattern. In cases
involving transmission from aircraft, certain interference effects described
below are much ‘less troublesome at low frequencies than at very high
frequencies.
In systems tested at the Radiation Laboratory for the transmission
of data from an aircraft to the ground, elementary antenna arrays have
been used. One system, operating at 300 Me/see, had a vertically
polarized dipole mounted on the tail section of the aircraft and two vertically
stacked dipoles at the receiving station. The vertical gain of this
latter antenna improved somewhat the ratio of signal to inherent and to
local noise, but did not reduce interference appreciably since little interference
comes from high angles. In a test of a 100-M c/sec relay equipment,
the transmitting antenna was a quarter-wave vertical radiator
mounted on the skin of an aircraft and a corresponding vertical quarterwave
receiving antenna was used at the ground station.
When a link was established ‘at 800 to 900 Me/see, it was found very
desirable to increase the gain of the two antennas in order to extend the
usable range of the equipment. This was accomplished by using stacked
dipoles at both the transmitting and receiving stations. These arrays
had a uniform horizontal pattern and some gain in the vertical plane.
Associated or near-by equipments often constitute a serious source of
interference; consequently both antenna and power-line filters are desirable
at the receiving station. Frequently these must be designed to reject
a particular frequency being radiated by a near-by antenna. Conversely,
it is often necessary to filter the r-f output of the relay transmitter in
order to minimize the radiation of harmonics that interfere with near-by
receivers. The specific filters required for a given installation must be
designed to meet the operational requirements of a specific system involving
a given complement of radar, communication, and navigational
equipment. No general rules can be given.