Aeronatical Mobile Installation
On with the story…
Above is a BeechJet 400A that just had an High Frequency (HF) radio installed. The techniques and equipment used in the installation are very similar to those used by amateurs when they set up their stations. As you can see, this radio station is more beautiful than the average ham shack.
Not all aircraft have HF radios on board, in fact few small jets or private airplanes have an HF radio. HF is used primarily on trips where the plane is flying across an ocean or when it will be operating in an island environment or sometimes in less-developed countries. Aircraft use a broad spectrum of bands allocated to them from 2MHz up to about 20MHz to fit their needs, very similar to the way that we amateurs pick the best band for the given conditions and the part of the world we want to reach.
At the top of this page is the jet which we made our installation. This is a brand-new airplane, and a very nice one at that. The HF antenna is the faint black wire that runs from about the middle of the cabin toward the top of the tail and then back down nearly parallel to the vertical stabilizer.
To the left, you can more clearly see the antenna (I’ve emphasized it with a white line). We can see the pilot-side (left) engine nacelle. Just below the rear of the engine, there is a small access hatch used for luggage and equipment access. Most of the HF radio equipment is housed up inside this compartment. The equipment is divided into three main components: the controller, the exciter/receiver, and the power amp/coupler.
The controller, shown at the right, is a relatively small box that contains the few controls. Unfortunately I didn’t realize that the picture was fuzzy until it was too late however, you can get the idea. Basically, there is an LED frequency readout, a tuning knob, volume, squelch, a clarifier, and a few memories. Pilots don’t need the sophisticated selectable filters, DSP’s, and fine tuning that we amateurs expect.
The physical dimensions are about 3″ tall and 8″ wide. It is mounted on the pedestal in-between the two pilots seats. It is a simple little box, yet a cable of approximately 40 wires has to be strung from the tail where the actual radio is installed in the front of the airplane to connect this controller.
I must apologize for my poor picture-taking ability. The picture at the left is the HF power-amp/coupler. The exciter/receiver is mounted next to it on the right. The 40 wire cable (which we make ourselves) comes from the controller back to the tail and into the exciter/receiver. The exciter drives the power amp with a couple of watts. The power amp delivers 80-100 watts of RF into the coupler.
Near the top of the coupler picture, you can see what looks like a small handle. That is actually two right angle BNC connectors with a coupling connector in-between them. The top connector plugs into the power amp and delivers the power directly into the bottom connector which feeds the coupler. The coupler is nothing more than an automatic antenna tuner (industrial grade of course!).
This coupler uses all fixed-value components inside to find a good match. Inside, it contains several boards full of various sized inductors and capacitors which can be arranged in various configurations to match impedance. Small vacuum relays are used to add or remove the capacitors and inductors from the matching circuit. When the pilot changes frequencies and keys the microphone, the tuner goes to work with about 10 watts of power. From the outside, you can hear the relays chattering as it searches for the best match.
Again, we will look at the picture of the rear of the airplane and the antenna. From the coupler, a rather short piece of 9913 coax, is used to go from the coupler up to a feed-through on the top of the fuselage. This can be seen just above the silver thrust-reversers on the engine. The wire goes up to a spring loaded pulley mounted on the vertical fin where it turns and heads forward on the fuselage. Near the middle of the cabin, it is mounted to the top of the cabin and grounded to the frame. The spring loaded pulley on the tail maintains pressure on the antenna incase of stretching or giving in any part of the antenna.
To the left is a picture of the cockpit of this BeechJet. All primary instruments on this aircraft are computer controlled and displayed on color vacuum tube displays (before you get too paranoid to fly, be assured that all basic flight instruments have the traditional mechanical backups). Down between the two pilots seats is the pedestal which extends back nearly as far as the back of the seats. The HF controller on this airplane was located on the very rear of this pedestal.
The HF transceiver is tied into the normal audio panel which can’t be seen in the picture very well. It is located on the extreme outboard of each pilot’s side. This audio panel allows the pilots to select, at a flip of a switch, which radio(s) he wants to listen to and with a turn of a knob, he can select which radio he wants his microphone connected to. This is crucial when there may easily be seven or eight receivers and three or four transmitters on any given airplane.
All of these systems work together flawlessly even under the harsh conditions that they face. They must be built to tolerate turbulence, several G’s (the increased force of the airplane in a climb or steep decent), extreme temperatures, rough landings, etc.
Eventually, this system may be replaced by a satellite link in which each airplane would be equipped with a satellite transceiver with an antenna. However, because of the complexity (and therefore cost) of building satellite antennas that can be aimed at a satellite on top of an airplane, it will be many years before the under-developed countries of the world give up such a simple, cheap, and rugged method.