An off-air antenna is a technology to gather over-the-air signals from broadcast TV stations. We use them to collect and transmit SCADA signals too. So, they have several key uses. At one time, we used them commonly to aggregate local TV signals to be rebroadcast from cable TV headends. These days, optical fibre or satellite connections have replaced the over-the-air signals. While MICAN is focused on commercial applications for Yagi, Log Periodic, and dipole antennas, it is noted that there is a major resurgence of this technology as many consumers are cutting their use of cable TV and acquiring a combination of an internet streaming service with a regional collection of local off-air signals.
There are several variants of commercial off-air antennas. These include, but are not limited to:
- Log Periodic
- and variations and combinations of the above, such as
- Vertical Pair Array
- Horizontal Pair Array
- Vertical Quad Inline Array
- Horizontal Quad Inline Array
- Quad Box Array
- Diamond Box Array
- VHF / UHF All-band
Antenna selection is based upon a variety of factors. Installation height, line of sight, multipath, distance, frequency, signal type, and more.
The number of antenna elements will dictate the gain of the antenna and the gain is related to the beamwidth of the antenna. Different antennas with different beamwidths will incur different multipath impacts. In Canada, Log Periodic antennas are preferred. While Yagi style antennas are popular and normally lower cost, they tend to ice up in the winter months and when this happens the pattern of the antenna inverts often causing a loss of signal.
How and where you install the antenna has a significant affect on performance. Normally tower structures are used. Mounting hardware to attach the antenna to these structures can vary widely based upon loading, wind, and other environmental factors.
A receiver is used to demodulate the over-the-air signals to analog or digital baseband. In rebroadcast situations, we capture the signals and then convert them at the IF (intermediate frequency) as it is better to avoid multiple modulation / demodulation processes in an ecosystem. Below is a classic design by MICAN for a signal collection point for off-air. At the top of the tower, we typically install an all-band antenna on a rotor so it can be used as a live replacement if any of the dedicated systems fail.