Voice communications
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The whole business about dopplershift vis-a-vis FM or AM transmissions is flawed anyway.
Doppler shift - if it was a concern ,would have exactly the same effect on AM or FM transmissions ( or USB/LSB or any other mode).
The FM or AM mode refers not to the frequency of the actual transmission but to the modulation applied to the carrier.
Doppler shift - if it was a concern ,would have exactly the same effect on AM or FM transmissions ( or USB/LSB or any other mode).
The FM or AM mode refers not to the frequency of the actual transmission but to the modulation applied to the carrier.
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No it doesn't ...only the perceived frequency from the POV of an observer ( receiver)
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Dave, You beat me to it!!!
Lets say I transmit on 145.000
The FM bandwith is wider so Doppler shift is less noticable to the receivers filters but there is still doppler shift going on.
The AM bandwith is narrower so the Doppler shift is slightly more noticable.
I switch to SSB and wow, the Doppler shift is very noticable due to a narrow bandwith.
The same amount of Doppler shift is experienced by all three modes.
Doppler shift is connected to frequency NOT mode (AM/FM/SSB)
M5DND
Lets say I transmit on 145.000
The FM bandwith is wider so Doppler shift is less noticable to the receivers filters but there is still doppler shift going on.
The AM bandwith is narrower so the Doppler shift is slightly more noticable.
I switch to SSB and wow, the Doppler shift is very noticable due to a narrow bandwith.
The same amount of Doppler shift is experienced by all three modes.
Doppler shift is connected to frequency NOT mode (AM/FM/SSB)
M5DND
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Also FM capture effect
If there are two transmissions a weak AM transmission can be heard under another stronger one. This is not the case with FM. Capture effect - Wikipedia, the free encyclopedia
Definition: capture effect
This is the main reason why AM is used rather than FM
Definition: capture effect
This is the main reason why AM is used rather than FM
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am v fm and other points
capture effect maybe one reason.I dont know for sure but I think its a legacy issue more than any other.
AM (ancient modulation
) has been around forever and suspect they chose it because the tech was there , it was simple and it worked.
Bandwidth is an issue especially as you get lower in the bands with less hertz between channels
As receivers became more selective and transmissions became cleaner and narrower (in the radio sense) more channels were possible where adjacent channel rejection became less of an issue.
as for doppler shift I think this is specious. why? maybe because the military have used it without an issue.ARC 27 ARC 41 etc
as for the transmitter or the receiver moving it matters not.the wave form does not know which antenna is moving and the doppler effect is the same.Its relative motion.How could it not be?
I have heard doppler shift affecting transmissions from satellites where the relative speed is quite high.
as for it affecting cop cars frankly I think someone is either misguided having a loan or using a known fact to support the reasons for a type of equipment being fitted.
Back of the envelope here... at 60 mph a car covers 88 feet per second.In that second the signal has travelled >982 million feet.The doppler effect has to be there but it is miniscule. It would be like a snail moving away from or toward a tone generator and trying to hear the change in note.
As an amateur operator for 30 plus years I have noted females using SSB tend to come through better in marginal conditions.Not scientific just anecdotal.
all imho.interesting topic regardless.
AM (ancient modulation
) has been around forever and suspect they chose it because the tech was there , it was simple and it worked.Bandwidth is an issue especially as you get lower in the bands with less hertz between channels
As receivers became more selective and transmissions became cleaner and narrower (in the radio sense) more channels were possible where adjacent channel rejection became less of an issue.
as for doppler shift I think this is specious. why? maybe because the military have used it without an issue.ARC 27 ARC 41 etc
as for the transmitter or the receiver moving it matters not.the wave form does not know which antenna is moving and the doppler effect is the same.Its relative motion.How could it not be?
I have heard doppler shift affecting transmissions from satellites where the relative speed is quite high.
as for it affecting cop cars frankly I think someone is either misguided having a loan or using a known fact to support the reasons for a type of equipment being fitted.
Back of the envelope here... at 60 mph a car covers 88 feet per second.In that second the signal has travelled >982 million feet.The doppler effect has to be there but it is miniscule. It would be like a snail moving away from or toward a tone generator and trying to hear the change in note.
As an amateur operator for 30 plus years I have noted females using SSB tend to come through better in marginal conditions.Not scientific just anecdotal.
all imho.interesting topic regardless.
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I think canard68 is right about the reason being historic. Interesting that marine radio comms are on FM..
Yes - just waiting for someone to pop up and tell us that is OK because ships move slower than cars and aircraft!
Where do these offbeat ideas come from?
As an amateur operator for 30 plus years I have noted females using SSB tend to come through better in marginal conditions.Not scientific just anecdotal.
Thankyou.
BTW - also had a Shuttle QSO on NBFM back in the 80s.
Yes - just waiting for someone to pop up and tell us that is OK because ships move slower than cars and aircraft!
Where do these offbeat ideas come from?
As an amateur operator for 30 plus years I have noted females using SSB tend to come through better in marginal conditions.Not scientific just anecdotal.
Thankyou.
BTW - also had a Shuttle QSO on NBFM back in the 80s.
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I wholly agree that the female voice comes across clearer but having worked just about every major DXpedition in the last 30 years I queried Dave's suggestion that DXpeditions tended to take YL ops, which I beg to disagree with. Try getting a YL "DXCC" - it's far from easy! Bit off topic, sorry.
So what is the the reason for having to re-tune constantly if one is following an amateur satellite? I've never done it but read many times that the frequency changes significantly as the satellite passes by. Presumably that is Doppler shift? I became involved in this AM v FM discussion donkey's years ago and capture effect was then put forward as a reason for using AM.
So what is the the reason for having to re-tune constantly if one is following an amateur satellite? I've never done it but read many times that the frequency changes significantly as the satellite passes by. Presumably that is Doppler shift? I became involved in this AM v FM discussion donkey's years ago and capture effect was then put forward as a reason for using AM.
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So what is the the reason for having to re-tune constantly if one is following an amateur satellite
Getting into supposition now ...but most comms satelites are in relatively high orbits .
A geostationary satelite - no problem - but one that is not will actually be moving at a fair lick....umpteen thousands of mph relative to the earthbound receiver ....maybe fast enough for there to be some very very slight doppler effect....
Still can't agree that doppler effect would have any noticeable effect on A/G comms for normal aircraft speeds whatever the mode.... even at Mach 4 or 5.
And finally - the transmitted frequency does not change even when a doppler effect is present - only the perceived frequency at the receiver
Getting into supposition now ...but most comms satelites are in relatively high orbits .
A geostationary satelite - no problem - but one that is not will actually be moving at a fair lick....umpteen thousands of mph relative to the earthbound receiver ....maybe fast enough for there to be some very very slight doppler effect....
Still can't agree that doppler effect would have any noticeable effect on A/G comms for normal aircraft speeds whatever the mode.... even at Mach 4 or 5.
And finally - the transmitted frequency does not change even when a doppler effect is present - only the perceived frequency at the receiver
There's a bit of a write up here
A Closeup of Doppler Shift
Suffice it to say that the velocity of aircraft is so low compared with "c" that when you run the numbers the Doppler effect has no practical effect on Air to Gnd comms (i.e. we don't need to retune because we are flying towards or away from the Ground station).
Now as for the "Perceived frequencies" argument, at a simplistic level whose clock are you using to measure the perceived frequency
? , the one at the transmitter or the one on the receiver
Relativity, anybody
A Closeup of Doppler Shift
Suffice it to say that the velocity of aircraft is so low compared with "c" that when you run the numbers the Doppler effect has no practical effect on Air to Gnd comms (i.e. we don't need to retune because we are flying towards or away from the Ground station).
Now as for the "Perceived frequencies" argument, at a simplistic level whose clock are you using to measure the perceived frequency
? , the one at the transmitter or the one on the receiver Relativity, anybody
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To reply to the original question.
AM is used in aviation because it does not suffer from the capture effect. With an FM signal the receiver will only pass the strongest signal unlike AM.
With AM, multiple transmitting stations can be heard not just the strongest which allows the operator to ask for a repeat message should a strong signal overpower a weaker one.
The Doppler effect is used in aviation, as in DF (Direction Finder) on both VHF and UHF where the Receiver Ae array is switched to cause the effect and therefore determine the bearing of the aircraft transmitting relative to the ground station.
Also with NavAids as in a DVOR which switches the transmitted signal to cause the Doppler effect on the aircraft receiving the signal - allowing them to navigate using a network of ground beacons.
Hope this helps.
AM is used in aviation because it does not suffer from the capture effect. With an FM signal the receiver will only pass the strongest signal unlike AM.
With AM, multiple transmitting stations can be heard not just the strongest which allows the operator to ask for a repeat message should a strong signal overpower a weaker one.
The Doppler effect is used in aviation, as in DF (Direction Finder) on both VHF and UHF where the Receiver Ae array is switched to cause the effect and therefore determine the bearing of the aircraft transmitting relative to the ground station.
Also with NavAids as in a DVOR which switches the transmitted signal to cause the Doppler effect on the aircraft receiving the signal - allowing them to navigate using a network of ground beacons.
Hope this helps.
Last edited by Track Jitter; 15th Feb 2009 at 09:50.
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Doppler shift tuning rate is linked to the frequency.
At 28Mhz the Doppler shift rate is less then at 400Mhz
On the low orbiting satellites if we use 28Mhz, this gives us a shift of 5-10khz Starting 2.5-5khz high in frequency at aquisition of signal. Drooping in frequency,It will be on the "Nominal" frequency when closest. and then drop down 2.5-5khz when the satellite reaches the loss of signal.
At 144Mhz the doppler rate is far higher, can be 10-20khz (from memory at moment!)
At 430Mhz is is even greater (Cant quote on this one, to early in day!!!)
Hope this make sense!!! Need more coffee!!
M5DND
At 28Mhz the Doppler shift rate is less then at 400Mhz
On the low orbiting satellites if we use 28Mhz, this gives us a shift of 5-10khz Starting 2.5-5khz high in frequency at aquisition of signal. Drooping in frequency,It will be on the "Nominal" frequency when closest. and then drop down 2.5-5khz when the satellite reaches the loss of signal.
At 144Mhz the doppler rate is far higher, can be 10-20khz (from memory at moment!)
At 430Mhz is is even greater (Cant quote on this one, to early in day!!!)
Hope this make sense!!! Need more coffee!!
M5DND
Last edited by m5dnd; 15th Feb 2009 at 08:21. Reason: Should have read Wiggys link first!! But...
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whose clock are you using to measure the perceived frequency
I would have though that from the context it was pretty obvious I was talking about the receiver end not the TX.
At the TX end the percieved and actual Frq will be the same no matter what.
I would have though that from the context it was pretty obvious I was talking about the receiver end not the TX.
At the TX end the percieved and actual Frq will be the same no matter what.
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Thanks everyone... I really love this place 'cuz you end up learning a lot more new things than you initially thought you would! I think I have my answer although I am tempted to ask why they use FM for maritime communications.... I wonder.
Back to reading my book on superstring theory...!
Back to reading my book on superstring theory...!
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heathrow director
nice to see some (amateur / ham ) radio people here.
just looking at the satellite orbit speeds.many of them are of course are geo stationary but those that do orbit are travelling (from what I can read ) 7 to 10 km per second.Rather quick.
2 collided a few days ago at around 8 km per second.An old russian one and an Iridium comm's satellite I hear.Hope they had airbags on that model.nasty prang
Having listened to sat comm's a longlomg time ago the doppler effect was without any doubt present.
I accept that capture effect is one of the reasons am is used but perhaps it became a reason with increased traffic?
Having tried to get a word in sideways to the tower at peak times I would think using fm and blotting out all the doubled up transmissions would be a boon.
again its an interesting discussion thanks to all.
just looking at the satellite orbit speeds.many of them are of course are geo stationary but those that do orbit are travelling (from what I can read ) 7 to 10 km per second.Rather quick.
2 collided a few days ago at around 8 km per second.An old russian one and an Iridium comm's satellite I hear.Hope they had airbags on that model.nasty prang
Having listened to sat comm's a longlomg time ago the doppler effect was without any doubt present.
I accept that capture effect is one of the reasons am is used but perhaps it became a reason with increased traffic?
Having tried to get a word in sideways to the tower at peak times I would think using fm and blotting out all the doubled up transmissions would be a boon.
again its an interesting discussion thanks to all.
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I accept that capture effect is one of the reasons am is used but perhaps it became a reason with increased traffic?
I think (but stand to be corrected) that the main reasons AM is used for Air/ground are firstly that as a viable mode it predates FM, secondly that the technology for AM RT is simpler and thirdly FM takes more spectrum space per channel.
I think (but stand to be corrected) that the main reasons AM is used for Air/ground are firstly that as a viable mode it predates FM, secondly that the technology for AM RT is simpler and thirdly FM takes more spectrum space per channel.
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dave
agree 100% as per my previous long winded post.Legacy and simplicity.
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Just a thought for those who think that Doppler has some bearing in the AM vs FM choice. Have you ever noticed a Doppler shift when listening to either AM (medium/long wave) or FM (VHF) radio in your car whilst driving along the motorway? No. The effect is there (and affecting both the AM and FM transmissions), but is absolutely negligable given the relative speeds of a car or aircraft and the speed of light.
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As stated, the primary reason is that of the FM capture effect. For a typical FM receiver in a 25 kHz channel only 4 to 8dB is needed to completely swamp out the other signal.
With an AM receiver you will hear (or be aware of) the other signal with +20dB of difference.
With AM you will be aware in almost 100% of cases that a transmission collision has occured. With FM it will be a lot less than 30%.
Secondly is the necessary bandwidth. An FM signal in a 25 kHz channel is 16 kHz wide (16k0F3E for marine). An airband AM signal in a 25 kHz channel is 6 kHz wide. This makes it possible for ATC to implement the offset carrier Climax system within the same 25 kHz channel. It couldnt be done with FM.
Now that AM is down to 8,33 kHz and FM is using 4FSK in 6,25 kHz channels technology has moved on but 20 years ago the above was state of the art.
AFAIK doppler was never an issue. Even for Concorde to use the Climax system meant that any shift was minimal.
With an AM receiver you will hear (or be aware of) the other signal with +20dB of difference.
With AM you will be aware in almost 100% of cases that a transmission collision has occured. With FM it will be a lot less than 30%.
Secondly is the necessary bandwidth. An FM signal in a 25 kHz channel is 16 kHz wide (16k0F3E for marine). An airband AM signal in a 25 kHz channel is 6 kHz wide. This makes it possible for ATC to implement the offset carrier Climax system within the same 25 kHz channel. It couldnt be done with FM.
Now that AM is down to 8,33 kHz and FM is using 4FSK in 6,25 kHz channels technology has moved on but 20 years ago the above was state of the art.
AFAIK doppler was never an issue. Even for Concorde to use the Climax system meant that any shift was minimal.
