Don’t worry it’s just the bits you need to know to make it work and pass your exam.
This is a VHF radio signal waveform. The highest parts are called crests, the bottom parts troughs. One wave has both one crest and one trough, it starts and ends half way between the crest and trough. . The frequency is a measure of how many complete waves go past a given pit in one second. Called a Hertz. The higher the frequency, the closer the wave crests and troughs are together. This is know as wave length. As we go up in frequency the wavelength gets shorter.
The height of a wave is known as its amplitude.
Is when a radio wave is modified by super imposing another signal onto it. We can modulate the amplitude with AM, of the frequency with FM.
Radio waves lose strength as they travel, known as attenuation. The higher the frequency the more the attenuation.
Objects And Radio Waves – In exams
**Radio waves only really work inline of sight or when bounced off an object.
**If a radio wave has an object such as the earth or a mountain between the transmitter and receiver it will not work.
High above the earth radio waves come into contact with ions (electrically charged particles). This dramatically affects radio waves and changes at times of day. At night we get better reception when the layers combine.
**Main Factors Affecting Reception
- The frequency – above 30 MHz range decreases as frequency increases.
- Power output of transmitter
- Time of day – however waves above 30 MHz are less effected
Line Of Sight
For all intention purposes pilots should regard VHF radio as line of site only. That doesn’t mean they need to be visible, what it means is if you could see far enough are there any objects obstructing your view. In other words if something like a mountain is between two aircraft radio either will not work or will be severely impacted.
When two radio stations with an object between them need to communicate, they can use a third station that cansee both. So lets assume an aircraft on the wrong side of a mountain has done an emergency landing in a paddock . They could communicate with an overflying aircraft and ask that aircraft to relay the messages to ATC. As the aircraft over head has the ability to communicate with both.
This is a regular occurrence when you start flying a lot of cross country you will hear others relaying a message.
VLF – Very Low Frequency 3 kHz to 30 kHz
LF – Low Frequency 30 kHz to 300 kHz
MF – Medium Frequency 300 kHz to 3000 kHz (3MHz)
**HF – High Frequency 3 MHz to 30 MHz – sometimes in exams
**VHF – Very High Frequency 30 MHz to 300 MHz – nearly always in exams
UHF – Ultra High Frequency 300 MHz to 3000 MHz (3 Ghz)
*Aircraft Use VHF
*Aircraft use VHF in the range 118.0 to 136.0 MHz
*HF is used for long range uses in aviation, including handheld HF radios when you fly in remote areas
VOR navigation aids used by instrument aircraft use 112 to 118 Mhz
NDB navigation beacons use 200 to 500 kHz
Increase Altitude For Better Reception
A better VHF signal is often achieved simply by climbing altitude.
Normal Ranges For VHF
You generally won’t get asked this in an exam , but it is very handy to know
At 500 feet – 24 nm
At 1000 feet – 34 nm
At 2000 feet – 48 nm
At 3000 feet – 58 nm
At 5000 feet – 75 nm
At 7500 feet – 92 nm
At 10000 feet – 106 nm