Common Types of UHF and VHF Antennas
An Introduction to VHF Antennas
VHF antennas or “very high frequency” antennas are designed to receive signals between 30 MHz and 300 MHz. VHF signals don’t degrade as much as UHF or Ultra High Frequency signals, so they are used for longer range broadcasting. What are some of the more common VHF antenna designs?
When you have a basic dipole antenna and add a reflector and director element to it, you have created an array antenna. The antenna reflector is longer than the driven element, while directors are usually shorter than the driven element. An array antenna can have as few as two elements, while some have as dozens. An array antenna can even have an array of folded dipole antennas.
A stacked array antenna is made by stacking two or more array antennas in the same vertical plane. Horizontal stacking is almost always narrows the beam width relative to vertically stacked antennas, which is why most commercial FM and TV stations have vertically stacked antennas.
The array antennas are ideally half a wavelength apart so they aren’t parasitic, though wider spacing improves their gain. Ideal stacking distance depends on the gain of the individual antennas. The stacked antennas need to be connected to a common transmission line.
Multi-Element Yagi Antenna
A VHF Yagi antenna contains a director and a reflector. A Yagi antenna can have a gain of up to 20 dB, making them popular for weak signal communication modes. Yagi antennas can be combined into arrays. The dipoles in a multi-element Yagi can serve double duty, acting as a director or reflector depending on the frequency band. Spacing the Yagi antennas 5/8 of a wavelength apart yields the maximum gain.
The major weakness of Yagi antennas is that at higher frequencies, they are difficult to feed and match signals.
Log Periodic Antennas
The log periodic antenna is a high gain antenna that works on a broad frequency band. Log periodic antennas incorporate a number of active dipoles connected together in logarithmic mathematical formula modeled off an infinite spiral. This is the basis of the name “log periodic”.
VHF log periodic antennas have broadband characteristics similar to a dipole array. Log periodic antennas also have directional characteristics like a dipole array.
Conical Array Antennas
Conical array antennas have reflector elements that extend straight out from the antenna’s supporting beam. The driven elements are bent forward to create a conical shape. The conical array effectively increases the diameter of the dipole and its bandwidth without having to have a very large physical structure. Conical array antennas can receive both high and low band VHF signals.
An Introduction to UHF Antennas
Ultra High Frequency or UHF antennas send or receive signals between 300 MHz and 3,000 MHz or 3 GHz. UHF signals are usually weaker than comparable VHF signals. This means they typically need to be installed in a way to capture as much signal as possible. They tend to be much longer than VHF antennas, since the antenna length determines the frequencies it can receive and UHF wavelengths are longer than VHF ones. What are some of the more common UHF antenna designs?
Fan Dipole Antenna
A fan dipole antenna has two triangular sheets of metal or rods that resemble a bow tie. They capture a wide bandwidth. Fan dipoles tend to be longer than rod dipoles. A multiband fan dipole antenna consists of two to five half wave dipole antennas mounted on a common parallel feed point.
UHF reflector antennas resemble a fan dipole antenna but have a signal reflector behind them. The reflector could be a mesh screen or solid reflector. Solid reflectors may be a flat sheet or a parabolic reflector. The parabolic reflector is curved and concentrates the signal on the antenna. This can improve the gain of a dipole antenna by up to nine dB, increasing their power eight fold.
Corner reflectors don’t have a curve but instead use a simply bent sheet of metal. The corner angle of the deflector can be 90°, 60° or 45°. The smaller the corner angle is, the lower the antenna’s impedance. As the angle degreases, the side length of the deflector has to increase. One of the benefits of the corner reflector antenna is that it improves the gain of the antenna over the entire UHF band.
The driven element of the antenna should be located at the center of the reflector. The dipole can’t be too close to the corner of the reflector or the beam width will become broader and you risk a multi-lobe pattern from the antenna.
Whether a solid or mesh reflector, the reflector should extend beyond the edge of the dipoles. Mesh screens are almost as good as a solid metal reflector as long as the mesh spacing is less than a fifth of the wavelength.
The only weakness of a single reflector antenna over an array is that the array has a wider field of view. Conversely, calibration and testing is easier when there is only one antenna to work with but you can’t do calibrations or tests on a non-interference basis.
The dimensions of corner reflectors are practical at 220 MHz and above. For frequencies of 900 MHz and higher, the reflector yields much higher gain and a sharper pattern.