Long haul radio usage

[FullyFlapped]

Guys,

Something which I think you'll find very trivial, but I'm a bit confused ...

On reading trip reports of GA aircraft which have made exotic trips (including "earth-rounding"), frequent reference is made to the lack of VHF comms at many places worldwide - not least half way across an ocean, etc. In these cases, it seems to be standard practice to use HF radios to contact ATC over long distances, and also to talk to you guys in the "heavies".

My query is this : why HF and not UHF ? Sorry if it's a dumb question !

Cheers,

FF :ok:

[barit1]

Anything (VHF, UHF etc.) above HF will usually be limited to line-of-sight range. HF can carry much farther, more reliably. It was the radio amateurs who first discovered HF's capabilities over 75 years ago.

[FullyFlapped]

Ahhhh ... I see - thanks !

FF :ok:

[pax britanica]

And line of sight at FL350 is about 200 miles altho some atmospheric conditions can improve this to over 300 miles if using powerful transmitters and tall towers. But once beyond around 400 miles from shore its HF or satcoms and HF is a very noisy (static)poor qulaity and difficult medium at the best of times and is very wearing to use for any length of time
PB

[boofhead]

And for even more boring detail, HF does its thing by skipping the radio wave (signal) off the ionosphere (electrical/magnetic layer above the stratosphere) back down to the earth. Thus "Skip Distance". If the frequency is too high (MF/VHF/UHF etc) the signal will just go right through the ionosphere and not bounce back ("skip").
If the signal is powerful enough it can bounce off the earth and back up to the ionosphere for another skip, and if really powerful can bounce right round the world, letting you talk to yourself.
The ionospheric layer varies in height by day and night (breaking up at dawn and dusk, hence HF comms are not so good at those times). By choosing the right frequency you maximise the range (higher frequencies by day and lower at night).
You still have line-of-sight reception, like VHF, for the first hundred miles or so, but can have a dead band between the end of the line-of-sight signal and the first skip distance, and between the several skips. Once again changing the frequency can fill this gap (maybe).
HF antennas have to be variable in length or have adjustable gains to make them the right (electrical) length for the frequency chosen. See also "losing the fish".
Most ATC comms are done using Single Sideband (SSB) HF, which cuts out the energy needed to send both sidebands (Double Sideband as VHF/UHF uses) and put all the transmission energy into just one sideband, which has a logarithmic increase in transmitted energy and thus range. The upper and lower sidebands are identical in frequency and exactly opposite in amplitude, so one can be deleted without affecting the signal, provided both transmitter and receiver are using the same sideband. ATC has standardised on USB (Upper SideBand).
If frequencies became cluttered, messages could be sent at the same time over the same frequency on USB, LSB and DSB (upper, lower and double sideband) with no interference to the receiver. DSB is also called HF/AM transmission but is not much used any more, even by radio stations. Some Hams still use it and it can give surprisingly good results, especially at night when the ionosphere is low, giving a shallow angle to the skip signal, like skipping stones off water.
If you are trying to pick up a transmission on HF and it sounds garbled, try changing the selector to AM or to the other sideband if you have that choice, and it might fix the problem by matching the transmitter and receiver. If that does not work, tune off by a KHz or two.
And if you ever get a chance, listen to Speedbird 12 near Tiger intersection trying to contact "Bombay Bombay" (Mumbai Mumbai now). Some things never change.

[derekl]

If I may nitpick boofhead's otherwise excellent, lucid (and entertaining) description, VHF airband comms are AM (Amplitude Modulated) transmissions which means -- as he correctly implies -- much of the transmitter energy goes into the carrier wave while the speech content goes into the two sidebands which waggle around either side of the carrier frequency.

With SSB (Single Sideband, suppressed carrier, usually used on HF 3-30 MHz), one sideband is selected for final (power) amplification and transmission. That way, all the transmitter's energy goes into sending information, not the base frequency signal. At the receiver end, the carrier frequency is re-inserted to give the reference point (or base frequency) for the speech information in the single sideband signal that has been transmitted.

Detection (more correctly, demodulation) involves measuring the variation in frequency between the sideband and the base frequency which then supplies the original voice frequencies.

DSB signals may also be "suppressed carrier" yet not be AM, where the carrier is always transmitted.

UHF speech (above 300MHz) is usually NBFM (Narrow Band Frequency Modulated) as opposed to your FM stereo which is WB FM (Wide Band Freqency Modulated).

[Old Smokey]

Good information here, but I think that it strayed a bit from questions in the original post -
to talk to you guys in the heavies.....why HF and not UHF ?

Most Civil aircraft available for relay don't carry UHF, only HF and VHF. Many Military aircraft carry UHF.

Regards,

Old Smokey

[BEagle]

Military UHF is not NBFM - in the 225 to 400 MHz band it is plain old AM!

[FullyFlapped]

Cheers guys, very interesting and useful !

FF :ok: