To find its way to its destination and to communicate with air traffic control, an airplane uses a variety of radio frequencies with a communications antenna. The onboard radio equipment uses several sizes and types of antennas, each one created for a specific frequency band, to accomplish this successfully. Each of these antennas has unique a frequency and application properties, as well as positioning on the aircraft. There are requirements for even the connection between the antenna and the avionics. The VHF band between 118 MHz and 137 MHz is used for aircraft short-range communication with air traffic control.
Additionally, the signal must be sent in all directions (omni) with a vertical polarization because the location of the receiving station is not always the same or known (upright, vertical antenna). There is a line of sight capability on these VHF frequencies. VHF range is defined as the distance between where you are standing or flying and the horizon. The curvature of the Earth is typically not followed by these signals. Okay, let’s say just a little—say, 20%.
Communications #
The real range is also influenced by the height of the communications antenna. Consequently, a plane flying at FL400 has a greater VHF range than one flying at 1500 feet. Additionally, it depends on the strength of your transmitter (most have 7, some 10 or even 16 watts). The effect of atmospheric conditions can sometimes be seen on VHF, but is more pronounced on HF (3 MHz–30 MHz) and LF. Atmospheric circumstances can either aid enhance range or make some frequencies unusable (300 kHz – 3 MHz).
VHF Radio Range #
You might want to use the following formula to determine you communications antenna and such VHF range: VHF Range = 1.33 VHF Range = 1.33 × (√H-aircraft + √H-gs). Place: VHF Range in nautical miles, H-aircraft is the aircraft’s height in feet, and H-gs is the antenna’s height on the ground in feet. The actual range is smaller than the theoretical maximum due to transmitter power, receiver sensitivity, antenna cable losses, and antenna efficiency. Use a multiplication factor of 1.2 for a more accurate range to account for that effect.
COM Antenna #
A vertical, slightly curved backwards quarter wave rod positioned on the aircraft’s bottom or top. Usually stainless steel or white. Blade antennas are occasionally used; they have a larger bandwidth (are less selective), but a disadvantage is a somewhat worse SWR (standing wave ratio, a factor indicating how good the antenna performs). The SWR should ideally be 1:1, meaning that there should be no signal reflection from the antenna back to the radio, but it should be functional up to 1:2.0. In other words, “The Standing Wave Ratio is a measure of how much power goes out of the antenna and how much is “reflected” back towards the transmitter,” as someone once put it. Remember that a solid “ground” is required for this antenna. This is not a concern for metal airplanes because there is an abundance of good metal that serves as a “conductive ground.”
Composite Aircraft #
Installing an aluminum sheet that is at least 20″ by 30″ in size to serve as the ground can address this issue for composite aircraft (anything less and the installation will not radiate its full power). Install the communications antenna such that it can “see” its own electrical reflection about a third of the way from the border of the sheet. A reasonably large oval sheet should work fine for bent whip antenna. A dipole, which is a symmetrical antenna and completely overcomes the ground problem, is sold by some antenna manufacturers. However, this type must be positioned vertically in order to be omnidirectional in its emission pattern.
Conductive (carbon fiber) composites are used to make a variety of composite airplanes. This serves as a shield (Cage of Faraday) and will shorten the range of radio transmission and reception. As carbon fiber is not an ideal ground conductor in this situation, you will need external antennas and a sheet of metal.
Long Range Communications #
VHF, as was previously established, is only effective for line-of-sight communications and is not well suited for direct communications across very great distances (although sometimes ionospheric conditions can open up the band and reflect even VHF beyond the curvature of the Earth). The HF band (3–30 MHz) is typically utilized for good, dependable long-distance communications.
Most modern airplanes, especially commercial airliners, are outfitted with satellite communications systems to stay in touch with their home bases and to offer wireless Internet access to passengers while they are in the air.
Vertical Fin #
The aircraft has specialized HF band radios for long-distance communications, as well as a vertical antenna in the vertical fin or a cable running from the wingtip to the tail. For instance, when transatlantic flights travel outside of VHF range, they use HF to speak with either Gander, Canada, or Shanwick, Ireland. When onboard circumstances need consulting with the home base, it can also be employed. These HF radios are now utilized as a backup, and some pilots who are also radio amateurs have used them to speak with other hams during the flight’s cruise phase.
