Practical_Antenna_Handbook_0071639586

68 p a r t I I : F u n d a m e n t a l s
F 1 and F 2
XMTR 1
F 1
RCVR 1
AF 1
XMTR 2
F 2
RCVR 2
AF 2
F 2 F 1
F 2 F 1 F
Composite
audio
out
Figure 2.36 Frequency diversity reduces fading.
The spatial diversity system of Fig. 2.37 employs a single transmitter frequency but
two or more receiving antennas are used, typically spaced one half wavelength apart in
the direction of the transmitter. The theory is that the signal will fade at one antenna
while it grows stronger at the other. Diversity receivers utilize separate, but identical,
phase-locked receivers tuned by the same master local oscillator and each connected to a
separate antenna. Simple analog audio mixing or digital voting techniques based on the
relative strengths of the signals keeps the audio output relatively constant while the RF
signal at any one antenna fades in and out. In the past few years, use of diversity reception
on the 160- and 80-m bands with this approach has become popular with serious
weak-signal DXers, thanks to the Elecraft K3 transceiver, which can be outfitted with a
second, identical receiver that can be phase-locked to the main receiver. The audio outputs
of the two receivers can be combined in an electronic mixer or kept separate to feed
the left and right channels of stereo speakers or headphones. In the latter case, the operator’s
brain performs the final signal processing that results in enhanced ability to
pull weak, fading signals out of the band noise.
Polarization diversity reception (Fig. 2.38) uses both vertically and horizontally polarized
antennas to enhance reception. This form of diversity is based on the fact that
our “always undulating” ionosphere slowly and randomly shifts the polarization of the
transmitted signal as it refracts it back to the earth. As in the space diversity system, the
outputs of the vertical and horizontal receivers are combined to produce a constant
level output.
Using the Ionosphere
The refraction of MF, HF, and some lower VHF radio signals back to the earth via the
ionosphere is the backbone of long-distance terrestrial MF/HF radio communications.
As a direct consequence of the complex interplay between the three major ionospheric
layers and their individual reactions to bombardment by the sun, in conjunction with
path loss considerations, absorption, and the ever-changing position of the earth
with respect to the sun, determining the best frequencies and times for communicating