Friday, May 23, 2008

Aircraft Antenna patterns

Aircraft antennas are divided by frequency range, speed/type of aircraft, visible/covert, active or resonant, shape - blade/dish or other, and ESD resistance.

An aircraft can have an antenna array that includes a radar dish in the nose, HF wire, VHF and UHF blades, steered microwave data dish in the tail or on top of fuselage for satellite. Some aircraft have huge arrays of VHF and UHF antennas under the belly and wings, and some have SAR antennas on the belly or in "chipmunk cheeks" near the nose.

What they all have in common is the requirement to receive clean RF signals uncluttered by the aircraft's shape. velocity, or attitude. Active antennas of course cannot transmit, but for all resonant antennas the principle of antenna reciprocity means that they will transmit and receive with exactly the same power lobes in the antenna pattern.

The skin of an aircraft is imperfect. It is made of riveted plates of aluminum which corrode over time. The gap (negative space) between the rivets create spaces that are antennas just the same as if they were wires or positive space structures. The act of semi-conducting, meaning in this case "partial conduction through active regions", makes the skin of the aircraft a component of the radio system. I have actually measured the 2nd and 3rd order Inter-modulation Products of aluminum ground planes built to simulate the skin of aircraft. While the IMs are low power, they pose a problem for reconnaissance aircraft by creating phantom signals.

Since an aircraft is a conductive body, it is an antenna with it's own resonance frequencies. Since all antennas have to (sub-ohm) bonded to the skin (Ground plane) of the aircraft, the signals getting from the antenna to the radio are also modulated by the fluctuating current on the skin of the aircraft. This source of extraneous RF energy that can create IMs in the radio.

Do to the counter modulation effects of the aircraft skin, which vary with the attitude of the aircraft to the signal source, the pure sinusoidal waves that would be detected by a ground station are misshaped when detected by the same equipment mounted on an aircraft. The degree of misshape varies in a fixed manner relative to source position so the actual signals can be offset by having the offset stored in memory and multiplying the received signal by the offset (after digitisation).

Velocity affects antenna design to a huge extent. Antennas for low speed aircraft need not be extremely streamlined, rugged, or ESD resistant. Antennas for jets must be ruggedised and tested to loads from all angles as well as extreme vibration, as a loose antenna could damage a plane. The faster an aircraft moves, the more static electricity is generated. Military antennas and radomes have a coat of slightly conductive paint over them which carries the static charges to the body of the aircraft.

Some antennas are embedded in aircraft. Antennas on composite wing aircraft are subject to destruction by lightening. Some antennas are covert and extend electrically after a plane is air born.

Active antennas are used for receive only applications. They are non-resonant and can have 150-1 band widths. A typical high quality resonant antenna will get 6% to 10% bandwidth at 3db points. An active antenna may have power carried to it by the center conductor of the coax, in which case the RF and Power are separated at each end of the cable by an appropriate network.