c. Long-distance strategic communications by HF propagation is not always as reliable as military
commanders feel is necessary in present world conditions. In addition to the variations in usable bandwidth that
occur daily, seasonally, and over the 11-year sunspot cycle; there are occasions when the entire HF band
becomes relatively useless (blackout) because of magnetic storms caused by solar flares. These latter effects
are particularly disrupting communications and from the polar areas.
2. VHF and UHF limitations.
band (300 to 3,000 MHz) may be accomplished by two basically different techniques. The first technique uses
numerous repeaters or radio relays spaced at or near the optical horizon. These systems are referred to as LOS
systems. Where VHF is employed, the relays may be somewhat beyond the optical horizon (that is, "fringe
area") due to refraction of VHF signals in the atmosphere. Because both VHF and UHF are essentially LOS
systems, the transmission path loss is relatively small and low transmission power provides satisfactory
b. While having many advantages over HF, the LOS systems do have limitations. Neither LOS nor
tropospheric scatter propagation is capable of spanning a large body of water where lack of islands prohibits the
installation of repeater stations, and ionospheric scatter is too limited in bandwidth to provide more than a
fraction of the service needed for the DCS. HF radio has therefore furnished our major overseas long-distance
links for years.
c. Terminal and repeater stations are vulnerable to both attack and sabotage. The vulnerability of LOS
systems is particularly great because of the large number of repeaters.
d. Jamming is possible with only moderate effort. Ground-based or airborne jammers can jam the side
3. Cable limitations.
a. The use of cables for long-distance communications is extremely expensive, especially with deep sea
cables. Cables are ordinarily used when radio techniques are not feasible or adequate.
b. Wideband communications, on the order of several megahertz, requires the use of RF transmission lines.
The most widely used RF transmission line is a coaxial cable. However, when a coaxial cable is used for long-
distance communication, it exhibits the characteristics of nonlinear attenuation, with effective cutoff at some