The field of design is antennae for aircraft. In particular the field of design is shunt antennae. Shunt antennae have been used many places over the years. Basically the term refers to antennae, which are grounded at one end and fed low voltage and high amperage radio frequencies to cause radio frequency (RF) propagation.
Slot antennae fall into this category and they have been used on aircraft vertical and horizontal tail surfaces for many years. Their use on tail surfaces causes the whole tail to become a radiator and results in an almost equal 360-degree propagation of the RF signal. The entire tail surface becomes a radiator of the RF signals from the antenna, which increases the surface area of the antenna and increases the propagation of the RF signal in all directions.
Prior to the advent of slot antennae, commercial jet transport aircraft were equipped with “long wire” antennae whose high-speed drag was unacceptable, though used, on the early jets. These antennae were required to communicate on high frequency (“HF”), a band of frequencies in the range of 2 mhz–20 mhz, designated by international treaty and used for contacts over 200 miles. 2182 kilohertz is the international distress signal. Other types of aircraft currently use the long wire antenna to attain this lower range.
A vertical stabilizer HF slot antenna, which covered most of the band, was developed by Eastern Air Lines (EAL) for the B727. The design was apparently never patented and is a version that is found on every Boeing today. Its failure to cover the lower frequencies is due to its shorter length, which is limited by stabilizer space considerations. To feed it efficiently its tuners are mounted in the vertical stabilizer, which is a very harsh environment of temperature and pressure extremes.
Most jets now flying internationally use tail mounted HF slot antennae, some with more success than others. No one had produced an HF slot antenna mounted anywhere other than the tail and it was believed by the airplane industry that any antenna placed elsewhere would be ineffective. Industry wisdom has for years held that for slot antenna to be effective they had to be part of the empennage to get it to properly radiate the signal. The empennage is the tail fin area with the vertical stabilizer and the horizontal stabilizer.
The Lockheed Hercules C-130 models A thru H and its civilian version L-382 were all produced with two antenna masts and two long wire antennae terminating high on the vertical stabilizer. On the later version, Lockheed Hercules C-130 “J” model, Lockheed incurred significant development costs to modify the empennage structure at the base of the vertical stabilizer to make room (physically and electronically) for a “notch”(shunt) antenna because it was believed there was nowhere else for the antenna to go. Due to the particular constraints of the existing Hercules stabilizer anti-icing system it was believed by the industry that the base of the vertical stabilizer was the only location for a notch antenna. All models of the Hercules have this stabilizer anti-icing system, therefore it is a constraint to the location of mounting an antenna that must be considered on all models. This notch antenna installation is available from Lockheed on older “A”–“H” models for retrofit at a price of approximately $250,000 with a long down time. The latest model Hercules C-130“J” are manufactured with this style notch antenna at a price buried in the purchase price.
Avionics equipment on aircraft is mounted so that it is constantly surrounded by conditioned air in its ideal environment. Tuners placed high in the tail are operating under difficult conditions, which occurs with the previous design for the Hercules C-130“J”. Significant efficiency benefits result from the closeness of the tuners to the antenna element. Shunt antennas work at a very low voltage of approximately 1.5 Volts and a high amperage of 75 Amps plus. This makes long feed lines from tuner to antenna counterproductive due to voltage drop and the consequent power loss. Vertical stabilizer mounted shunt antennae mandated mounting the tuners high in the stabilizer in non-pressurized, unheated, and inaccessible areas. It would be advantageous to mount the tuner units where they should be, inside and very close to the feed end of the antenna.
Therefore there is a need for a slot antenna that can be mounted away from the empennage area. Furthermore, the tuners should be mounted in an interior environment with a short feed line from the tuners to the antenna element.