MANAGING HF CIRCUITS USING CHIRPSOUNDERS (Cont)

TM 11-5820-917-13

m. If the RCS-4B shows a frequency to be propagating and no contact can be

made, the cause will nearly always be found to be a comm equipment problem or im-

proper tuning. Remember that the Chirpsounder transmitter and receiver represent

a separate propagation "test set." If the sounder system operates on different

antennas than the communications system, there will be some disparity in path loss

between the two systems, but this will invariably be insufficient to cause a misreading

as to which assigned frequencies are propagating. (In fact, the opposite is generally

true: the usually higher power communications systems can operate marginally over

a greater frequency range than the sounder system indicates.)

n. The RCS-4B operator should anticipate potential frequency changes well be-

fore they become necessary. This is particularly true if an RSS-4 Spectrum Monitor

is unavailable for clear channel search. Candidate new frequencies should be moni-

tored as long as possible to determine if potential interference exists. Frequency

changes should be made in advance of potential outages and not just before total

outage is imminent.

o. Care should be taken in extrapolating propagation conditions on a sounded

circuit to an unsounded one. The differences in path length and azimuth (particu-

larly east vs west), and the potential propagation differences that result, must be

taken into account. Figure 7-15 can be used as a guide for extrapolating the effects

of range on the 1-hop F-mode MOF by taking the ratio of factors shown for ranges

of interest. For example, a daytime MOF on a 600-km circuit is measured at 9 MHz,

and it is desired to estimate the MOF for a 900-km circuit. From figure 7-15, the

extrapolation factors are 1.5 for 600 km and 2.0 for 900 km. The resulting estimated

In making such estimates keep in mind MOF's for circuits

1.5

to the west of the RCS-4B site will lag behind those observed to the east. Also,

avoid extrapolating Es conditions from sounded paths to unsounded paths unless

(a) the Es is clearly widespread and strongly established and (b) good fallback

procedures exist in case of sudden outage. In such cases, the upper curve in figure

7-15 can be used as a conservative guide for estimating Es MOF extrapolations.

7-34. Experience has shown that successful frequency management of a large

number of HF circuits is rather easily attained provided:

a. As many of the circuits are sounded as possible, and the sounded circuits

are carefully selected;

b. Care is taken in spectrum monitoring for clear channels by using the RSS-4

Spectrum Monitor;

c. A sufficiently large list of potentially usable channels exist (the more, the

better); and

d. Procedures for coping with frequency changes and unexpected loss of

communication are clearly understood and followed.

7-19