UHF Vs. VHF Antennaby Scott Knickelbine
In the days before digital broadcast television, local channels from two to 13 were broadcast in VHF (for "Very High Frequency"), and channels from 14 to 83 went out in UHF (for "Ultra High Frequency"). Today digital television is broadcast in the UHF band, but VHF is still used for FM radio and marine-band radio, and amateur radio enthusiasts still broadcast on both bands. Because of the different characteristics of the two frequency bands, the optimal designs for UHF and VHF antennas are significantly different.
The VHF frequency band runs from 50 MHz to 225 MHz, the UHF band extends from 420 MHz up to 1.3 GHz. The wavelengths of these bands vary dramatically; for instance, a 54 MHz VHF signal has a wavelength 6 meters long, while a 450 MHz UHF signal has a wavelength of just 70 centimeters. The short wavelengths of UHF signals allow them to pass more easily through obstructions and to bounce of the ionosphere, so they can travel well beyond the horizon of the transmitting tower.
Antenna Design Principles
The purpose of an antenna is to capture the electromagnetic energy of the radio waves, inducing a signal current into the antenna which is passed to the lead wires. Most antennas are designed to capture the same signal at multiple points, called "elements," combining the signal currents and thus amplifying the strength of the signal. The number and length of the elements, the spacing of the elements, and the thickness of the elements are all functions of the wavelength of the frequency the antenna is designed to capture.
Because VHF wavelengths are relatively long, VHF antennas require longer elements. However, the greater efficiency of VHF frequencies in inducing current means that fewer elements are needed, and the rod (or "boom") to which they are attached can be shorter, that is the case for a UHF antenna. For instance, an optimal antenna for 144 MHz would require 12 elements, ranging from 41 1/8 to 34 3/4 inches in length, arrayed on a 17-foot boom.
UHF wavelengths are much shorter than VHF, so their elements are correspondingly shorter. In addition, UHF antennas also require many more elements in order to provide an adequate amount of signal amplification. An antenna optimized for 432 MHz would need 33 elements on a 24 1/2-foot boom. Those elements would, however, measure just 13 11/16 to 10 11/16 inches long.