Practical_Antenna_Handbook_0071639586

C h a p t e r 1 2 : T h e Y a g i - U d a B e a m A n t e n n a 311
the total gain at any given elevation and azimuth relative to some reference antenna,
whether dipole or isotropic radiator, is the pattern of the antenna multiplied by the
array pattern. In this case, the array pattern is formed in conjunction with the image Yagi
located an equal distance beneath the ground; thus, the patterns of these two figures are
those of a three-element Yagi multiplied by the ground reflection pattern for that specific
antenna height. The peak boost provided by the ground reflection pattern over
perfectly conducting ground would be 6 dB, occurring at an elevation angle that depends
on the Yagi’s height above ground; in general, the gain is somewhat less over real
ground.)
Which antenna height is better? Certainly for very long distance communication,
the upper Yagi has the benefit of slightly higher gain and noticeably lower takeoff angle
for its main lobe. But at midday on the HF bands, the lower Yagi might well be the superior
performer as DX signals shift to higher takeoff and arrival angles, especially if
both stations involved in the communications are in North America, are in Europe, or
even are attempting to communicate between eastern North America and western Europe.
In particular, the lower Yagi is near the peak of its response curve at elevations
around 30 degrees, where the upper Yagi has a deep null. The depth of the null will
depend on the actual nature of the ground within a few wavelengths from the base of
the tower (for flat terrain). Which antenna height is better? It depends. At one time or
another, either one is better.
Now that we have pattern data for the two antennas, let’s look at Fig. 12.10B to examine
the effects of splitting our transmitter power between them by feeding them in
phase (BIP) and out of phase (BOP). Here’s what we observe:
• The peak of the broken curve (BIP) in Fig. 12.10B is 13.2 dBd, or 1.3 dB higher
than the peak gain of the high Yagi of Fig. 12.10A. This stacking gain is less
than the theoretical maximum of 3 dB because of the close spacings and because
3 dB can be attained only when the vertical elevation patterns of the two
beams are identical (as in free space). Nonetheless, there are many communications
scenarios when an extra 1.3 dB means the difference between success
and failure.
• The elevation angle of the BIP main lobe (14 degrees) is the same as that of the
upper beam alone.
• Feeding the two beams out of phase (BOP, solid line) creates a single main lobe
similar to the main lobe of the lower Yagi alone but enhanced with 1.3 dB
stacking gain and centered on a higher elevation angle (42 versus 24 degrees).
As a typical day on the HF bands progresses, one might reasonably expect to
progress from BIP through Lower to BOP on most transoceanic paths. Upper
would be useful primarily for very long haul links at those installations where
the lower beam was fixed on a different azimuthal heading and not able to
contribute to forming the BIP pattern.
• Although the apparent benefit of stacking beams at l/2 and l above ground is
“only” 1.3 dB compared to the peak pattern response of the upper beam alone,
there are other benefits to stacking. BIP and BOP allow us to double the number
of elevation angle ranges we can select with two beams. And often more
important than stacking gain is the reduction in interference that comes from
eliminating the secondary (upper) lobe of the higher Yagi by using BIP mode.