Lesson l/Learning Event 4
contacting surfaces clean, bright, and firmly held together. Movable parts
should be bonded to the stationary frame with flexible straps of tin-copper
braid.
Such straps should be kept as short as possible to reduce their
g. Good electrostatic shielding of a receiver (or any noise-producing
equipment) can be insured by enclosing the receiver in a high-conductivity
metallic shield, such as solid copper, aluminum shielding, or copper
screening. All wires entering or leaving the equipment (except the antenna
leads) should be properly bypassed or filtered. Necessary holes or cracks
in a shielding should be kept as small as possible.
Shielding can be
improved by bonding the joints at closely spaced points.
The lower the
frequency, the thicker must be the shield.
Because of the strong
electromagnetic field that occurs at low frequencies, a sheet-iron shield
may be more effective than one made of copper or aluminum.
Learning Event 4: IMPROVING THE SIGNAL-TO-NOISE RATIO.
Circuit reliability in long-distance radio operation depends to a
large degree on the signal-to-noise ratio (S/N).
The higher this ratio
(high signal strength compared with noise level), the better are the chances
for reliable communication.
a. Selection of media.
Wherever several transmission media are
available, the technical controller should select the one that provides the
best S/N for important message traffic.
(1) Radio propagation predictions help the technical controller to
select the most reliable transmission frequencies. The predictions tell him
in advance which frequencies will most likely provide him with reliable
communication circuits. However, there is no guarantee that the predictions
will always hold true. Sudden ionospheric disturbances always add elements
of doubt to the success of HF long-distance radio communication. Of all the
radio frequencies in the spectrum, only those in the HF range offer long-
distance transmission at economical levels of transmitting output power
without resorting to frequent relay stations.
There are times when
communications in the HF band becomes nearly impossible.
For this reason
the Department of Defense has developed and installed a satellite
communication system that does not depend upon the ionosphere.
(2) Experience has shown that satellite communication systems
consistently provide the highest quality long-distance communication in
spite of the relatively low power used in the ground terminal transmitters.
This quality and reliability results from a combination of line-of-sight
transmission paths,
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