MANAGING HF CIRCUITS USING CHIRPSOUNDERS

TM 11-5820-917-13

e. Keep close track of the F-mode and Es MOF's for several days and nights

to learn their short term and long term fluctuations. Learn the periods around

dawn and sunset when major propagating frequency changes occur. Note the

strength and variability of the Es propagation. (The frequency cursor storage

feature of the RCS-4B display can be used to keep track of MOF variations.)

f. Select the best propagating band on the basis of the criteria given in

para. 7-20. If more than one band satisfies the criteria, the higher frequency

band is generally to be preferred even at some sacrifice of received power.

g. Before judging the suitability of an Es mode, observe its performance for

half an hour or so to determine if it is prone to serious short term fluctuations.

Watch for signs that the Es mode may be breaking up (e.g. falling MOF or gaps

or dots in the Es trace). Do not operate closer to the Es MOF than perhaps 30%

below the MOF.

7-27. Numerous tests have been run comparing HF communications quality with the

corresponding Chirpsounder ionograms. Figures 7-9 through 7-12 show examples.

In figure 7-9, one would expect best communication between 12 and 15 MHz. There

is a good solid Es mode with no multipath, good received signal power, and with

probable low noise levels. There is also good received signal power in the 6-11

MHz band; however, the traces show the presence of multiple modes, hence poten-

tial fading problems. It is, therefore, not surprising that comm at 13 MHz rated

better than at 9 MHz. Obviously, comm at 28 MHz, well above the MOF, would not

be expected.

7-28. Figure 7-10 shows another example of the best frequency band being one

utilizing Es propagation. The band from 12 to 16 MHz shows high signal level in a

low noise band with a single solid mode displayed. The reported comm on 15 MHz

was "excellent. "

7-29. Figure 7-11 shows the result of operating too near the Es MOF. Note the

weakening signal above 13 MHz. It is not surprising that communication was weak

but readable. Operation in the 6-9 MHz band would be expected to be excellent.

7-30. A similar situation to figure 7-11 is shown in 7-12. Communication on

11228 kHz, near the Es MOF, would be expected to be weak, while use of frequen-

cies in 7-10 MHz band would produce excellent results with signal levels 30-40 dB

higher than at 11 MHz.

7-31. Figure 7-13 shows another type of situation which can occur. Propaga-

tion is via F-mode in an ionosphere where the electron content is quite inhomo-

geneous. This phenomenon is termed "spread F" and can yield a weak, sometimes

fast-fading signal. The frequency band from 4 to 7 MHz is obviously the best

available, but, as the report at 6 MHz shows, the best leaves something to be

desired.

7-32. MANAGING HF CIRCUITS USING CHIRPSOUNDERS

7-33. Efficient frequency management of a set of HF circuits using Chirp sounders,

particularly in a tactical military operation can be demanding but yields results