The Frequency of Optimum Transmission (FOT) is nominally 85 percent of the MUF. Generally, the FOT is lower at night and higher during the day.
In addition to frequency, the route the radio signal travels must also be considered in optimizing communications. A received signal may be comprised of components arriving via several routes, including one or more sky wave paths and a ground wave path. The arrival times of these components differ because of differences in path length; the range of time differences is the multi-path spread. The effects of this can be minimized by selecting a frequency as close as possible to the MUF.
Since many of the variables affecting propagation follow repetitive cycles and can be predicted, techniques for effectively determining FOT have been developed.
A number of propagation prediction computer programs are available. One such widely used and effective program is Ionospheric Communications Analysis and Prediction (IONCAP), which predicts system performance at given times of day as a function of frequency for a given HF path and a specified complement of equipment.
A closer look at the components
The components in an HF radio system fall into three groups: transmitters, receivers, and antennas. In many modern radio sets, the transmitter and receiver are contained in a single unit called a transceiver.
Most aviation applications will require a flight deck located control panel. This is where the system can be switched on and tuned. Other features include squelch that allows noise reduction and a clarifier that modifies the frequency oscillator within the receiver to be fine-tuned to improve signal quality. Many units include switches to allow storing frequencies and a mode select for side band operation. This feature gives an increase in range but distorts the voice quality. Newer controllers may also include a selector for messages.
A receiver/exciter is the device that will demodulate the signal captured by the antenna and supply the audio portion to the flight crew. One common option is selective calling (or SELCAL). This feature will bring the crew’s attention to calls to their specific aircraft and is accomplished with a recognition code implanted on the transmitted signal. Once a SELCAL-equipped signal reaches the system, it is decoded and either an aural or visual alert is provided to the flight crew.
Transmitters vary in construction, though they all contain a means of producing a carrier wave as well as a method of imposing the information to be transmitted on the carrier. Power amplifiers, either internal or external, will then adjust the final output to achieve an appropriate broadcast range. The exciter synthesizes a carrier, which is combined with a lower frequency signal derived from a source of information such as a microphone. The resulting signal is converted to the frequency that is to be transmitted.
Tuning or frequency selection is achieved in modern transceivers by using electronic micro-circuits. Some equipment uses crystal control for achieving desired frequency. The power amplifier boosts the output power of the signal to the desired wattage for transmission before sending it through a cable to the antenna. In addition, the transmitter may contain filters that are used to clean the output.
Once the signal reaches the antenna it is propagated into the atmosphere. HF communications systems on aircraft employ several types of antenna including “Short Wire, Long Wire, V and Shunt.”
The wing type “V” is probably the most effective for slow to moderate speed aircraft. With a length usually between 9.2 to 13.7 meters, the radiation pattern of the transmitted signal is mostly omni-directional and exhibits efficiency at all HF band radio frequencies. Disadvantages include being a physical obstacle and high drag, especially on low wing aircraft.
A shunt antenna built into the leading edge of the vertical stabilizer is most common on high performance aircraft. There is no additional drag and the radiation pattern is practically omni-directional. Efficiency is generally good, but can deteriorate as a result of poor electrical bonding.
Automatic Direction Finders The grandfather of all radio navigation aids By Frank Labue July 1999 Ok, I know. Why are we talking about ADFs when there are VOR/ILS and GPS...
From star-gazing to satellite navigation systems
A look at analog and digital communications.