The front end of a transmitter requires to have the antenna matched to it's designed impedance (for example 50 Ohms) otherwise components in the transmitter front would be damaged. This is achieved by matching electrical wavelength of the antenna with the transmitted frequency wavelength.
Every frequency has an associated electrical wavelength. A radio frequency of approx' 7Mhz would have a wavelength of approximately 40 Meters. An Antenna Coupler is required using a Variable Inductor (a tune-able coil) with variable capacitors, to achieve 50 Ohms, approximately matching an antenna to an HF frequency.
Aircraft today tend to have solid state antenna couplers which match the antenna electrical wavelength in around a second or two.
Clearly a length of 40 Metres for an antenna is too long for the average aircraft airframe. Antenna's are therefore made in lengths that are fractions of the average wavelength of the frequencies most used in a transmitter. A mathematical 32nd wavelength, 64th wavelength for example or whatever the airframe would allow to be installed.
Using a Boeing 75/76 family there are two antenna’s in the leading edge of the vertical stabilizer. These antenna's are folded in a zag zag form with an approx' physical length of 18 inches each (a Boeing Techie may eventually post here more accurate details). The antenna's electrical length is longer due to the folding design. There are two matching electronic antenna couplers used to match an antenna to its equivalent electrical length of the frequency selected by the pilot to transmit.
The tone heard through the headset is a low powered oscillator transmitted carrier for the antenna coupler to match the transmitter and antenna. Transmitting on HF SSB there is no carrier transmitted unlike AM.
Some one with a more detailed and better explanation may post here later, hopefully wihout any flaming!.
Last edited by p1fel; 20th August 2011 at 16:55.