novice
Posts: 90
Joined: Tue Aug 07, 2012 2:55 pm

HF Transmitting

Sat Jan 19, 2013 1:04 pm

"The HF band used in aviation ranges from 2 to 22 MHZ. Different frequencies have different range capabilities; i.e., the higher the frequency the greater is its range. Therefore, frequency is chosen by the pilot to meet the range between the transmitting and reception points, and the ground station will monitor a range of frequencies because personnel are unsure of the exact distance to the aircraft. This is also the reason why you can be received by a station 2000 nautical miles away but not by a station only 500 nautical miles away."

Why do the higher frequencies in the HF range travel further then the low frequencies in the HF range?
Why can you receive a station 2000 nautical miles away but not 500 miles away, would it be because the station at 500 nautical miles away is using too low A HF? and 2000 using the correct high frequency i.e. high?

Thanks
 
rfields5421
Posts: 5543
Joined: Thu Jul 19, 2007 12:45 am

RE: HF Transmitting

Sat Jan 19, 2013 2:44 pm

It is the way the wave of a HF transmission works based upon the frequency.

On any frequency there is a what is known as a skip pattern. With HF Ham Radio, I've been able to contact other users at 2,000, 4,000, and 6,000 distances, but not at 1,000, 3,000 or 5,000 distances. I would have to use a different frequency for those distances. This isn't set in stone - because atmospheric conditions, sunspots and other things can make the frequency I used on day, unusable the next day.

If you want to get into more details, you need to get into very detailed radio wave propagation theory. There are plenty of web resources.

As far as aviation is concerned, today's usage is significantly better than it was when I worked HF on a USN EC-121 back 40 years ago. Then I had a frequency overlay for the broad area chart, and tuned the frequency manually to contact the various oceanic centers, and USN centers. We went long periods of time with no available contact on some missions.

Today SECAL works a lot like a telephone. The aircraft is assigned a SECAL code, and the frequency choice is pretty much controlled by the equipment. The crew doesn't have to look up frequencies and tune radios.

However, it is not unusual to be out of contact for long periods of time. A crew can try to contact a new oceanic center, and be unable to do so for several minutes to hours.

Part of the issue with realizing AF447 had crashed was because the Cape Verde Center never received the calls from the aircraft.
 
novice
Posts: 90
Joined: Tue Aug 07, 2012 2:55 pm

RE: HF Transmitting

Sat Jan 19, 2013 3:03 pm

Thanks for that detailed explanation rfields5421, thats cleared that up   
 
tdscanuck
Posts: 8572
Joined: Wed Jan 11, 2006 7:25 am

RE: HF Transmitting

Mon Jan 21, 2013 6:14 am

Quoting rfields5421 (Reply 1):
Today SECAL works a lot like a telephone. The aircraft is assigned a SECAL code, and the frequency choice is pretty much controlled by the equipment. The crew doesn't have to look up frequencies and tune radios.

You still need to tune the radios...SELCAL is just listening to whatever frequency the HF is tuned to. Normally, when you are handed off from your last VHF center to oceanic control, you will be given two HF frequencies. You contact oceanic control on either of them and do a SELCAL check on that frequency...if you pass the check you can stop listening to HF but you have to leave the radio on that frequency so the SELCAL system can monitor it and let you know if they call you.

If you're running CPDLC that happens over dedicated datalink frequencies, but you would normally still tune HF to the oceanic center's frequencies of the day.

Tom.

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