And the Beast Lives

The saga of high frequency communication

Tuning the antenna will allow greater efficiency through the operating frequency spectrum. The wavelength of a radiated signal is determined by the frequency and speed of transmission. The speed is a constant at 300,000,000 meters (186,000 miles) per second, which is also the speed of light. Dividing the speed by the frequency will result in the wavelength. As an example, a 10 MHz transmission will have a wavelength of 30 meters.

In most modern day installations, the antenna is a fixed length. The only way to compensate it for frequency variations is through tuning. The variables here are inductance (some antennas are enhanced by adding coils of wire) or capacitance. Some HF system manufacturers install a variable capacitor in line with the antenna and will alter this value allowing the antenna to operate at acceptable levels throughout the intended frequency spectrum. Other installations will accomplish antenna tuning in the antenna coupler.

HF radio has been the means of communication of choice for mariners. The onset of affordable satellite communication has impacted the use of HF and many aeronautical HF radio service providers have switched off their systems. However, the capability of HF to transmit data far exceeds the current capability of the Iridium satellite telephone network. Equipment pricing of HF radio is far below that required to utilize the Inmarsat Satcom constellation.

In recent years, this data capability has breathed new life into an age-old technology. With flight deck available data being the main focus of enhanced air traffic management and communications, it appears HF radio systems will be with us for many years to come.

The moral of this story is obvious: it is better to transmit than to receive (sometimes).

Jim Sparks has been in aviation for 30 years and is a licensed A&P. Currently when not writing for AMT, he is the manager of aviation maintenance for a private company with a fleet including light single engine aircraft, helicopters, and several types of business jets.

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