So, how does a modern airplane use shortwave radio successfully? You may wonder what kind of high frequency aircraft antenna are used.
In the old days, aircraft used wire antennas. At first, you saw a trailing wire. Then, you saw wires attached to fuselage, wings and tail. But these techniques don’t work well at high speed and altitude. Something new was needed.
So, by the 1970’s, airplanes switched to shunt-fed antennas in the vertical stabilizer, or static vertical tail. You may have heard of hams using shunt-fed towers for 80 and 160 meters, shown above left. This is the model for modern high frequency aircraft antenna. Shunt feeding basically means “in parallel”.
For comparison, you can think of the shunt-feed wire as the aircraft antenna feed, the vertical stabilizer as the tower, and the airframe as the ground. That’s your modern shortwave antenna on an airplane, as shown above center and right.
A 400 watt transceiver is connected to the antenna feed and airframe (in parallel) using a sophisticated antenna coupler At its top end, the antenna feed connects to the vertical stabilizer (“the tower”). At the bottom end, the vertical stabilizer is connected to the airframe (“the ground”).
High Frequency Aircraft Antenna – About Resonance
With the shunt-feed, antenna couplers use motorized vacuum capacitors and long coils to find a match between transmitter and airframe. You will find the result is a non-resonant antenna that works quite efficiently.
Don’t you need resonance for a good antenna? No. Resonance is simply a magic point where an antenna impedance is purely resistive. If an antenna is too short or too long, that simply means you have capacitive or inductive reactance (respectively) that needs to be cancelled by an antenna coupler.
If you read the theory, you will find non-resonant antennas work just fine when properly coupled (matched) to the transmitter output.