Practical_Antenna_Handbook_0071639586

62 p a r t I I : F u n d a m e n t a l s
Two disadvantages of multihop modes to remember are that they are generally
weaker and more subject to fading than a single-hop path.
Most important, keep in mind that there is no one “best” propagation mode or antenna
radiation pattern. Some high-frequency bands (including the 11-m, or 27-MHz,
citizens band) exhibit predominantly high-angle “short skip” during the summer and
longer skip during the other months. Of course, when using higher angles of radiation
at the higher frequencies, there will be a critical angle wave and escape angle waves that
are not returned to the earth. These combinations are useless for terrestrial broadcasting
or two-way communications.
So what is the ideal angle of radiation? The answer is: It depends! To date, the best
answer to this question comes from the accumulated knowledge of an inquisitive band
of dedicated DXers and contesters in the amateur fraternity. Using highly directive antennas
at multiple heights in combination with sophisticated switching systems in their
90
80
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60
Radiation angle (degrees)
50
40
30
20
10
0 500 1000 1500 2000 2500 3000 3500 4000
Figure 2.34 Takeoff angle versus skip distance.
Distance to first reflection (miles)