Hf R/t

[Squawk7777]

I am looking for references (web, CAA/FAA publications etc.) about HF R/T. I have used HF in a past a few times, but nobody has ever really explained to me. Is there anything to it? Are the R/T procedures different than on VHF?

Cheers,

7 7 7 7

[turn right heading 365]

The procedures for HF r/t is really no different than for VHF, ie you still talk into the mic and press transmit, and the phraseology is the same.

However HF is prone to a lot more interference than VHF because it makes use of the ionosphere and the surface of the Earth to bounce the radio waves over long distances, utilising what is known as the sky wave. To complicate matters there is also a ground wave which travels in pretty much the same way as line of sight VHF but because it is a longer wavelength than VHF it is prone to a lot more attenuation as it follows the surface of the earth and they type of surface it travels over affects the quality, such as sea, desert, etc.

Types of interference are the sky wave and ground wave arriving at the RX out of sync, causing fade. The ionosphere is made up of a number of layers of ionised particles but there are 4 layers which most affect HF propagation. Can't remember their names but they compress at night into 2 layers, since they get their energy from the sun. Therefore HF is affected by the time of day as well. This is partly overcome by using higher freqs during the day, ie 4 - 8 mHz depending on the distance of the RX from the TX, and lower at night , 2 - 3 mHz. HF tends to settle down during the day and night, it is dawn and dusk when a lot of problems occur.

Temperature inversions can cause a funnel effect, as with VHF and you find that waves travel a lot further than you would normally expect

Sunspot activity and subsequent upsets in the Earth's magnetic field cause problems and you might find using a higher or lower freq might help. An Ionospheric Prediction Service monitors these sorts of variables and over a given route would advise on the most appropriate frequencies. HF networks exist whereby various stations over vast distances are monitoring the same freqs, so that if the unit you are calling, say Djakarta can't hear you, Delhi might, and would hopefully relay the message.

As a pilot you might be lucky enough to have SELCAL short for selective calling. Individual aircraft are allocated a unique selcal code similar to a transponder code but in letters rather than numbers say EJDL. The first 2 letters are 1 part of the code and the last 2 the second. Then as you fly along you don't have to monitor the HF because, when a ground station wants you they just send a tone on HF with your code and alerts you to an incoming message. Unfortunately the ground operator has to listen to all the freq's with their attendant noise, chatter and atmospheric warbles and make the most of it!

There are various suppression devices to limit the effects of interference, one of the most common being Single Sideband (SSB) where, if you imagine a sine wave being chopped off about the middle, you just get the top part, with improved quality. If you listen to SSB tx's without the appropriate filter you get the classic duck talk sound. There are other devices to limit noise also, I think a generic one is called CODAN, carrier operated device anti noise which does exactly what it says on the tin!

Although it can be a pig at times, at others the quality of reception can be almost as good as VHF particularly in the 6 - 8 mHz range but this is the exception rather than the norm, so good mic technique is essential, as is a good pair of ears.

This is probably enough waffle and hope it goes some way to answering your question.

[GlueBall]

During the initial call, a good procedure would be to state the frequency you're on and in what part of the World you're at. That way, the ground stations can answer your call more quickly, as operators monitor many frequencies. They can also direct the antenna towards you for better reception. Lower freqs work better at night, higher freqs during the day. Sometimes it takes many calls to establish contact. Wait at least one minute for a reply between calls.

[411A]

You can receive/transmit for very long distances as well. Was able to work an amateur in Texas while over the Indian Ocean one night on twenty metres, yet Bombay would not answer on any frequency...and that is nothing new either.

[redsnail]

RT technique is more or less the same for VHF. Just announce what freq you are using.

A rule of thumb for selecting what freq to use. For the region you are operating in you should have about 3 to 4 to choose from.
High sun = high freq eg 8 or 10Mhz
Low sun (or night) = low freq. 3Mhz

Having said that, it's as often as not that 8Mhz will work like a dream at 6am and all you'll get is hash on 3Mhz.

[A Very Civil Pilot]

I'm in the middle of a 733 course. it says not to use the Hf R/T whilst refuelling. Injury to personnel or fire may result!

[fantum farter]

Think that is because hf uses the aircraft skin as an aerial-so lots of electromagnetic radiation going on all over the a/c something else to make us all infertile!!!!!

[411A]

Amazing how junior guys can develop strange ideas....:rolleyes:

[redsnail]

HF does need an antenna. On smaller aeroplanes it's the long wire that we are all familiar with. Larger aircraft often house the antenna in the leading edge of the fin.
I was always advised to never test HF inside a hangar. The hangar starts to arc and spark. The engineers told me it is quite impressive. I was also advised not to hang onto the antenna if someone is transmitting.

[Fujiflyer]

With reference to refuelling one should never operate a radio transmitter at the same time. Arcing between conductive parts can occur at any frequency if the field strength is high enough (which is dependent on the transmitter power and the distance from the antenna). Mobile phones are normally banned near fuel pumps for this reason - the output frequency of these being an order of magnitude higher than VHF.

Out of interest what is the typical power output of aviation HF and VHF equipment?

Fujiflyer :)