AM v FM
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AM v FM
Ok a bit of a technical anorak question ...
The Vhf and Uhf Airbands have historically used Ampletude Modulation as opposed to the technically superior Frequency Modulation method. Quite apart from continuing compatibility with legacy kit ... I seem to remember that one of the prime reasons for sticking with AM is that it doesn't suffer from the Capture Effect as does FM. So what is the Capture Effect and why is this a bad ?
I also thought that with AM the more power (ERP) used ... a thumping great carrier wave is produced but modulation sufferes.
Could someone indulge me and bring me back up to speed ...
Coff.
The Vhf and Uhf Airbands have historically used Ampletude Modulation as opposed to the technically superior Frequency Modulation method. Quite apart from continuing compatibility with legacy kit ... I seem to remember that one of the prime reasons for sticking with AM is that it doesn't suffer from the Capture Effect as does FM. So what is the Capture Effect and why is this a bad ?
I also thought that with AM the more power (ERP) used ... a thumping great carrier wave is produced but modulation sufferes.
Could someone indulge me and bring me back up to speed ...
Coff.
Last edited by CoffmanStarter; 15th Mar 2013 at 18:43.
Ecce Homo! Loquitur...
FM Capture Effect With the same frequency being reused at airfields within the LOS of high flying aircraft I can understand the need to stay with AM.
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Cheers ORAC that has jogged the old grey cells
PPRuNe the best immediate aviation tech resource around
I wonder how many people have learn't something new
PPRuNe the best immediate aviation tech resource around
I wonder how many people have learn't something new
Last edited by CoffmanStarter; 15th Mar 2013 at 18:54.
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There's probably also a historical element to this. The complexity of an FM tranceiver when "R/T" was introduced (i.e. 30s/40s) would be far greater than an AM tranceiver; this at a time when economy of production was a primary concern.
The amount of deviation required (at the time) to provide reasonable voice quality would also mean a smaller number of voice channels could be used in the band vs the number of AM channels that the same bandwidth could accommodate.
In FM, a constant amplitude is radiated (more or less) with the frequency of the carrier being varied in proportion to the modulating signal. In theory, therefore, the antenna cannot be perfectly matched to the transmitter in FM, however the amount of deviation required is not significant in proportion to the carrier frequency. Thus ERP is not practically affected by FM.
However, in AM (DSB), the same information is transmitted twice, as both the upper and lower sidebands are modulated with the signal. Single Side Band (or SSB) is therefore preferred as it allows greater energy to be applied to the intelligence in a smaller bandwidth. Even better is SSB-SC where the carrier itself is suppressed, and the entire power output is applied to the intelligence-carrying signal.
The amount of deviation required (at the time) to provide reasonable voice quality would also mean a smaller number of voice channels could be used in the band vs the number of AM channels that the same bandwidth could accommodate.
In FM, a constant amplitude is radiated (more or less) with the frequency of the carrier being varied in proportion to the modulating signal. In theory, therefore, the antenna cannot be perfectly matched to the transmitter in FM, however the amount of deviation required is not significant in proportion to the carrier frequency. Thus ERP is not practically affected by FM.
However, in AM (DSB), the same information is transmitted twice, as both the upper and lower sidebands are modulated with the signal. Single Side Band (or SSB) is therefore preferred as it allows greater energy to be applied to the intelligence in a smaller bandwidth. Even better is SSB-SC where the carrier itself is suppressed, and the entire power output is applied to the intelligence-carrying signal.
In FM, a constant amplitude is radiated (more or less) with the frequency of the carrier being varied in proportion to the modulating signal. In theory, therefore, the antenna cannot be perfectly matched to the transmitter in FM, however the amount of deviation required is not significant in proportion to the carrier frequency. Thus ERP is not practically affected by FM.
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From a military point of view , I was taught during my trade training that the reasons we use AM is that FM provides a better Signal to noise ratio which is great for listening to music but its complete overkill for the normal vocal range, their fore it would be a waste of money to implement FM for military use.
I think the reason for AM was that the original technology at the time was AM only, from the wartime 4 channel VHF to the ubiquitous 1985/1986 10 channel VHF used on Canberras, V Force, Transports etc etc in the RAF up to the 1970s.
I recall the VHF frequency was achieved in the equipment by using a plug in crystal and then by a set of frequency 'doublers' and 'treblers' - which was why all airborne frequencies were divisible by 9 eg 117.9/140.58/121.5
I recall the VHF frequency was achieved in the equipment by using a plug in crystal and then by a set of frequency 'doublers' and 'treblers' - which was why all airborne frequencies were divisible by 9 eg 117.9/140.58/121.5
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Eulogy - while that argument is true, for secure voice, the improved SNR afforded by FM was required for military use. Hence the first widespread use of secure (radio) voice in the UK Armed Forces used FM for the Clansman RT353 set. The Clansman ground-air RT344 was an AM set, but despite the higher frequency range (and therefore "cleaner" signal), did not offer secure voice.
Seems a bit nonsensical to me. The ERP is irrelevant to the mode of transmission, but is a function of power into the antenna and the gain of the antenna. ..ERP is not practically affected by FM..what does this mean?? So I have to agree with Whopity.
Surely the biggest advantage in using AM for airborne comms is the ability to detect 'doubling' which will not be apparent in FM due to the capture effect.
Surely the biggest advantage in using AM for airborne comms is the ability to detect 'doubling' which will not be apparent in FM due to the capture effect.
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Seems a bit nonsensical to me. The ERP is irrelevant to the mode of transmission, but is a function of power into the antenna and the gain of the antenna. ..ERP is not practically affected by FM..what does this mean??
So say a 100W set, with a 3dB feeder loss and an antenna with +6dBi gain would have an ERP of 100x 0.5 x 4=200W, or +23dBW.
Now, the feeder loss and antenna gain are dependent on f0. In FM, f0 is varied by the deviation, thus the feeder loss and antenna gain are actually not constant. However, my point was that the amount of variation is SO small in comparison with the nominal f0 that it may be considered insignificant. THUS, ERP is not affected in any practical way by use of FM (even though in theory the two factors described ARE frequency dependent).
So I don't understand either Whopity's call of "crap" or your agreement with him.
Please explain...
Last edited by Roadster280; 15th Mar 2013 at 22:41.
I cannot fault your argument of course, but what I'm saying is that that your comment is so obvious that it should not need to be said. In the non theoretical world that I live why would I even think that the ERP of a 5khz deviated signal at say, 130Mhz would be affected or not, compared to say an AM transmission at the same carrier frequency. I could equally say that ERP is not affected in any practical way by use of AM (given same power into antenna, gain and losses). The words or concept just do not make sense to me.
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I think we are arguing over points of agreement. The only reason I said what I did was that in the OP, Coffman had mentioned ERP. I was simply pointing out that ERP was not affected. Obviously 
Also noted a booboo in my maths. Duly edited.
Also noted a booboo in my maths. Duly edited.