Practical_Antenna_Handbook_0071639586

C h a p t e r 9 : V e r t i c a l l y P o l a r i z e d A n t e n n a s 249
is true for most FM mobile and repeater operation on 2 m and above, whereas
weak signal SSB or CW DXing on those bands is accomplished with long-boom,
high-gain Yagis employing horizontal polarization. Almost all broadcast
television receivers were originally located in residential dwellings, so that
service began with horizontal polarization as the standard, today’s videoequipped
RVs and limousines notwithstanding. Today some FM broadcasting
is done with the transmitted signal split between horizontal and vertical
polarization, in recognition of the fact that substantial numbers of antennas in
both “flavors” are in use.
• What frequencies do you wish to use? What is your objective for a particular
antenna? On the HF bands there is generally a 6-dB ground reflection advantage
at each end of the path at higher elevation angles for horizontal antennas, but
below about 10 MHz verticals often outperform all but the highest dipoles and
beams at low radiation angles. Otherwise, the source polarization is immaterial
for skip propagation because the ionosphere tends to randomly rotate the
polarization of waves passing through it regardless of what was delivered to it.
If you are an inveterate DXer, you will most likely enjoy more success on the MF
and lower HF bands with a vertical than with a dipole, but if your principal
operating activity is a regional or local network of friends, a low horizontal
dipole or loop may be your best choice.
• Are you bothered by local noise sources, and, if so, do they favor one polarization
over the other? If you can effect an improvement in received signal-to-noise ratio
(SNR) through the proper choice of polarization, that may be the single most
important factor for your specific installation.
• What space or support limitations do you have? A treeless residential backyard
may be better suited for an 80-m ground-mounted l/4 vertical than for a l/2
dipole, while some college dormitory residents have had great success with a
simple dipole or end-fed longwire stretched from rooftop to rooftop across the
courtyard below.
Perhaps the best answer to this question is: “Put up at least one of each!” Some
years ago the author had both a dipole and an elevated ground-plane vertical for 40 m
behind his house. In actual use, there were “dipole nights” and “vertical nights” on 40,
with received signal strength differences on transatlantic paths shifting back and forth
between the two antennas from night to night by as much as 20 dB! No one antenna can
do it all; if you have the space and the time to put up two or more antennas.
Supporting the Vertical Antenna
Verticals for frequencies below about 5 MHz are substantial structures; their support
requirements are akin to those of 60-ft or greater towers. Therefore, information on supporting
verticals for the MF and lower HF bands is found in Chap. 29 (“Towers”).
Above 5 MHz, supporting a l/4 vertical is a much simpler task. For these antennas,
the techniques of Chap. 28 (“Supports for Wires and Verticals”) are typical.