Section 7. FREQUENCY MANAGEMENT OF HF NETS USING AN/TRQ-35(V) EQUIPMENT

TM 11-5820-917-13

SECTION 7

FREQUENCY MANAGEMENT OF HF NETS

USING AN/TRQ-35(V) EQUIPMENT

INTRODUCTION

7-2. The TCS-4B Chirpsounder Transmitter, RCS-4B Chirp sounder Receiver, and

RSS-4 Spectrum Monitor form a complete equipment set, AN/TRQ-35(V), to manage

frequency selection for any HF circuit. Three TCS-4B's can be used with one

RCS-4B to manage a 3-circuit network. Experience has shown that one RSS-4

colocated with the RCS-4B provides channel occupancy data adequate for all sites

within several hundred kilometers. For long range circuits (greater than 2000

km) , use of separate RSS-4's at each end of the circuit is advisable.

7-3. The following discussion presents a brief summary of the essential elements

of HF propagation which need to be understood in order to perform network man-

agement. It then covers the basic principles of Chirp sounding, instructions for

interpreting the RCS-4B display, and procedures for frequency managing an HF

network.

7-4. HF radio energy can propagate for great distances through the interaction

of the electromagnetic waves and the free electrons of the earth's ionosphere.

This interaction causes the radio energy to be refracted or partially reflected

from a straight-line path. The amount of refraction depends on the radio wave-

length and the electron density in the ionosphere and can be sufficient to return

RF energy to the earth's surface on a "one-hop" basis at ranges up to about

4000 km.

7-5. The ionosphere is defined as that part of the earth's atmosphere in which

free electrons exist in sufficient quantities to affect propagation of radio waves,

Thus, the ionosphere can be thought of as existing from 40 to 50 km altitude

to a height of several earth radii.

7-6. Free electrons are produced primarily by the effects of solar radiation on

molecules at ionospheric altitudes. The production of electrons and their subse-

quent recombination with positive ions or attachment to neutral molecules are de-

pendent, among other factors, on altitude. There are three altitude regions of

interest:

Altitude range (approx)

50-90 km

90-140 km

above 140 km

7-7. The D-region is that portion of the ionosphere where electrons, agitated

by RF energy, more readily collide with molecules or are more rapidly absorbed

by recombination or attachment. This collision and absorption process signifi-

cantly reduces the RF signal propagation strength and is the primary contributor

to signal propagation path loss during daylight hours at lower HF frequencies.

Because the probability of electron collision and absorption increases with increas-

7-1