Practical_Antenna_Handbook_0071639586

CHAPTER 18
Antennas for 160 Meters
One of the most explosive growth areas of amateur radio interest worldwide in
the past decade has been the 160-m band. Topband, as it is called by its aficionados,
presents today’s active amateur with extreme technical and operational
challenges and opportunities. Part of this comes from the physical dimensions required
for efficient antennas, part from the high noise levels often found at these low frequencies,
and part from the fact that there are aspects of 160-m propagation that remain an
enigma to even the experts.
Efficient transmitting antennas for the medium-frequency band (300 kHZ to 3 MHz)
pose certain difficulties for the user. Of course, the first thing that springs to mind is the
large size of those antennas. As explained in earlier chapters, the most efficient transmitting
antennas for the MF and HF bands approach a half-wavelength (horizontal dipoles)
or a quarter-wavelength (vertical monopoles) in their long dimension. For
instance, a half-wavelength dipole for 160 m is about 270 ft long, usually exceeding the
capabilities of a typical suburban lot. Of course, as the frequency decreases, a halfwavelength
increases, so it is not for nothing that towers for the AM broadcast band
(530 to 1700 kHz, just below the 160-m band) are usually hundreds of feet tall.
Vertical Monopoles
For the amateur blessed with lots of real estate, a generous budget, and no zoning
limitations, the transmit antenna of choice for 160 borrows from the best engineering
practices of the AM broadcast industry; it is almost always one or more l/4 groundmounted
verticals with anywhere from 12 to 60 radials originating at the base of each
tower.
But AM broadcast stations are usually concerned with maximizing their ground-wave
signal strength throughout their own local metropolitan area. Why would Âamateurs—
especially those interested in very long distance communication—adopt broadcast station
techniques? Why, for instance, wouldn’t a l/2 dipole for 160 m be a better choice?
There are at least two reasons:
• To radiate well at the low elevation angles that are most useful for long-haul
communications, a horizontally polarized antenna should be at least a halfwavelength
above ground. On 160 m, this suggests a minimum height of nearly
300 ft for the antenna supports!
• Although a low dipole will normally provide acceptable nighttime coverage on
160 m throughout a radius of a few hundred miles around the antenna, sunlight
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