Practical_Antenna_Handbook_0071639586

CHAPTER 19
VHF and UHF Antennas
The VHF spectrum is defined as extending from 30 MHz to 300 MHz, and the
UHF spectrum from 300 MHz to 3 GHz. In common parlance, however, the UHF
upper limit is sometimes taken to be 900 MHz, with frequencies above 900 MHz
loosely defined as microwave.
When designing, constructing, and using antennas for VHF and UHF, keep the following
points in mind:
• Shorter wavelengths translate to smaller element dimensions. Thus, VHF and
UHF antennas are often less expensive and simpler to construct than their HF
counterparts, and their supports don’t need to be as strong.
• Shorter wavelengths may allow the use of free-space design rules. A 2-m antenna
atop a 70-ft tower is 10 wavelengths above ground, far enough away to model
the behavior of the antenna as if it were in free space.
• Yagis and other antenna types that use aluminum tubing or rods for elements
tend to deliver performance over a wider percentage bandwidth than at HF
because the element diameter is much larger relative to wavelength.
• Directive antennas with more gain, narrower beamwidths, and better rejection
of signals off the sides and rear are relatively easy to implement. A 14-element
Yagi for 144 MHz is a lot easier to put up and keep up than even a 3-element
Yagi for 14 MHz.
That’s the “good” news. Now keep these points in mind, too:
• Smaller element dimensions mean that greater precision and accuracy in assembly
are required. Small hardware items, such as element-to-boom mounting
brackets, become a critical part of each element’s electrical characteristics and
can have a huge effect on how the assembled antenna performs compared with
predictions based on models or calculated values.
• Some of the propagation modes require aiming highly directive antennas above
the horizon, so two rotors (called the el-az mount) may be required.
• The higher the forward gain of the antenna, the narrower its main beam and the
more precise your ability to aim it must be.
• In contrast to the ionospheric skip of the HF bands, most VHF and UHF propagation
modes demand that your antennas be as high as possible, to attain true
line-of-sight paths extending as far from your site as possible before encountering
the first obstacle. Amateurs dedicated to VHF and UHF DXing live on hilltops
or, better yet, mountaintops. HF DXers don’t need to.
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