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HF radio question

Old 6th Jul 2013, 15:32
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HF radio question

Hi all,
When you transmit on HF frequency is it the same principle as VHF ie (two aircraft cannot transmit at the same time)??

I hear different HF station locations on the same HF freq, but I have always wondered what the correct etiquette is?

Please note I am new to HF radio ops?
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Old 6th Jul 2013, 15:51
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I don't know the answer, but I must admit, that is a very good question. I can only say that having used HF for all of my career ( over 30 years), I would say more than one aircraft can transmit at a time. For example, lets say Jakarta and Brisbane are both using 13887(I made that freq up) ...in my experience you can have aircraft talking to Brisbane and Jakarta at the same time. But I could be very wrong.
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Old 6th Jul 2013, 17:43
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The one with the loudest shout wins!
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Old 6th Jul 2013, 20:41
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The etiquette is not to "talk over" someone else, but HF is very different to VHF in dispersion characteristics, in that it is not limited to "line of sight".
HF signals can refract and bounce many times, sometimes all the way round the earth.
This used to be the basis for Ham radio communications, and multi-skip worldwide comms was common during sunspot maxima.

Sometimes you can't hear someone else on the freq , but someone thousands of miles away can hear you and others simultaneously. It is not always predictable, so "listen before transmit" is a good policy.
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Old 6th Jul 2013, 20:49
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HF is still very popular with Radio Amateurs.

It is possible, although undesirable, to listen to two people talking on the same frequency at once when SSB is in use. You don't get the earsplitting howls that occur with AM or FM comms.
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Old 6th Jul 2013, 21:07
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Those squeals are known as heterodynes or beats, and can still occur on SSB, because they happen when the carrier frequencies are close but not exactly the same. Agreed, they are less than on AM, but FM should reduce the squealing considerably.
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Old 6th Jul 2013, 23:34
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Several times now I've heard Gander loud and clear over 9700nm away....

It's highly unlikely that my calling my "local" HF ATC would interfere with Gander's, but I tend to wait anyway.
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Old 7th Jul 2013, 02:18
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It is possible, although undesirable, to listen to two people talking on the same frequency at once when SSB is in use. You don't get the earsplitting howls that occur with AM or FM comms.
The earsplitting howls occur only with AM.
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Old 7th Jul 2013, 03:02
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I just did my first CPDLC crossing...what a change from the days you would go almost 10 degrees trying to get the position report received, that is, go from 40W to almost 30W trying to give the 40W report. Now, silence.....admittedly, I started in the AF which lacked SELCAL.

Last edited by galaxy flyer; 7th Jul 2013 at 03:04.
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Old 7th Jul 2013, 08:38
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A quite lengthy but thorough treatment of the complexity of HF propagation can be downloaded here.

There's more information there than you are likely to need, so skip over the complex stuff and search for the basics - page 10 might even be a good place to start.

It's a fascinating area which is why we radio amateurs keep tuning up and down the bands. You never know who you might find - and where.

Last edited by FullOppositeRudder; 7th Jul 2013 at 08:39.
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Old 7th Jul 2013, 20:34
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Listen to HF from home:

http://websdr.ewi.utwente.nl901/

8879 (USB) is good for both Shanwick and Mumbai but, be patient...
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Old 8th Jul 2013, 03:54
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Without getting too verbose...

If you hear someone else talking, don't. If the airwaves sound clear then go for it. Use your judgement*.

*Clarification for CMLs: This means you use your brain, not search the book for an answer.
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Old 8th Jul 2013, 11:15
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To answer your question downwind, many people can and regularly do talk on an HF freq at once. The freqs used in the Asian area are a case in point. Listen to 5658, 6556, 8842, 10066 for an idea what happens. It is possible to contact, say, MNL while SIN is working another station and both can pass their respective messages. It is good etiquette to wait till the freq is clear but on many that never happens. Thank Heavens for CPDLC!!
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Old 8th Jul 2013, 13:19
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Pssssst.....it's 8942

Along with 11396 or 5655

The ones we generally "try" and use anyway.

Last edited by nitpicker330; 8th Jul 2013 at 13:20.
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Old 8th Jul 2013, 13:52
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SSB

QUOTE
Those squeals are known as heterodynes or beats, and can still occur on SSB, because they happen when the carrier frequencies are close but not exactly the same. Agreed, they are less than on AM, but FM should reduce the squealing considerably.
UNQUOTE

As Ham Radio Op ,and aviation enthusiast let me inform that SSB has NO carrier (it is indeed suppressed) and aircrafts use the USB (Upper Side Band).
At a certain location you may hear different stations on the same freq.,and if the signals are similar in level you'll hear both of them clearly, but rarely two stations will try to contact the same ground station at the same time , there are procedures and Selcals in place to avoid that,at least for commercial aviation.

Last edited by Lberto; 8th Jul 2013 at 13:57. Reason: typo
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