Practical_Antenna_Handbook_0071639586

168 p a r t I I : F u n d a m e n t a l s
Example 5.2 An amateur radio operator’s favorite band for casual operating is 20 m,
but she also spends time DXing on 15 and 10 m when those bands are open. She is planning
to purchase a triband Yagi antenna but is trying to decide whether to put up a 35-ft
house-bracketed tower or a guyed 70-ft tower to support the antenna. From what we
know about the characteristics of those bands throughout most years of a typical sunspot
cycle, we don’t expect them to be open for short skip very often, so she should try
to select a height that favors elevation angles below 50 degrees on 20 m, somewhat
lower angles on 15, and even lower angles on 10. A height of l/2 on any band provides
a very broad lobe centered on 25 degrees—right in the middle of her desired range of
angles. On 20 m, l/2 corresponds to 35 ft, which is just about the practical minimum if
the antenna is to be above nearby obstructions. On 10 m, however, a triband beam at 35
ft is a full l high and the peak amplitude of the main lobe has moved lower in takeoff
angle even as a second lobe, slightly weaker than the main lobe, appears in the elevation
pattern, as seen in Fig. 5.9H. Of course, average ground will act more like a dielectric
and less like a perfect mirror at 28 MHz, so the nulls will partially fill in. Except at
times of extremely high sunspot activity or sporadic E-skip, it’s very unlikely that the
ionosphere will support high-angle F-layer paths at 28 MHz, so the most useful elevation
angles on 10 m are almost always lower than those of 15 or 20.
If our operator is interested primarily in working very long haul DX on 20 m, she
might want to consider heights closer to l (70 ft) at the risk of inserting some nulls at
useful angles in her 15- and 10-m elevation patterns. By raising her triband beam from
35 to 70 ft over average ground, she gains not only another decibel of forward gain at
the peak of the main lobe, but the elevation angle of that peak drops from 25 degrees to
14 degrees. At the low elevation angles that are so useful for long-haul DXing, the
resultant overall improvement from doubling the height of her beam from l/2 to l is
about 5 dB—equivalent to tripling her transmitter output power! In return, a null
appears at 30 degrees (reducing her signal for some short-haul elevation angles on
20 m) and the elevation patterns on 15 and 10 break up into multiple lobes, albeit with
lower peak elevation angles. In short, a height of 70 ft or as close to it as she can get will
put most of her output power in the right range of takeoff angles on the band where she
will need it the most—20 m—at a cost of some higher-angle nulls on all three bands.