VHF Line of sight maths explanation
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in France, luncheon often renders radio completely useless,
Or some other French FIS frequencies...Actually, Spain is pretty good for this too, if you try "conversational" English. They don't want their lack of English recorded on tape, so they pretend they didn't hear. Only isolated examples of course...
It seems obvious to me, from flying, that one runs out of signal strength long before it gets blanked by the earth's curvature.
Beacon Outbound
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That's a curious thing to be certain of. I can go to 20,000ft or so. How many watts are required to guarantee the statement? It has to be a finite value.
Let's assume a distance to be covered of 500 km (well over 250 nm, enough for a visual horizon from 40000' +) at the highest available frequency to us in the VHF comms band, 136 MHz
The free space loss is calculated with the formula
L = 32.5 + 20log F + 20log d = 130 dB
L is Loss in dB
d is distance in km
F is operating frequency in MHz
Let's also assume a less than perfect antenna/cable combination in the aircraft, giving 6dB overall loss, and a slightly better ground installation giving only 3dB loss.L is Loss in dB
d is distance in km
F is operating frequency in MHz
A rather non-sensitive radio could have a sensitivity of 2 uV over 50 Ohms, which equals around -100 dBm.
So in this case your required transmitter power is in the region of:
-100dBm + 130dB + 6dB + 3dB = +39dBm
+39dBm is about 8W
A typical GA VHF radio has 8W carrier power
In the case of a good receiver (0.5uV over 50 Ohm sensitivity , -113dBm) and a better antenna installation on both sides (3dB loss in the A/C, 2dB loss on the ground) this TX power requirement could drop to:
-113dBm + 130dB + 3dB + 2 dB = +22dBm
+22 dBm is about 0.160 W or 160 milli-Watts
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You also need to allow a 10db-30db loss from crap cabling, corroded connectors, etc
Then knock a good 30db off the dynamic range due to interference from aircraft strobe inverters, alternator voltage regulators and ignition harnesses...
If you don't believe me, go for a flight and listen to some others' radios...
Then knock a good 30db off the dynamic range due to interference from aircraft strobe inverters, alternator voltage regulators and ignition harnesses...
If you don't believe me, go for a flight and listen to some others' radios...
Beacon Outbound
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IO540
Just because you have a bigger engine than others doesn't mean you can't (on occasion) just accept the information given to you.
You asked for numbers, I gave you numbers. You must be of the 'don't try to change my mind with facts' school of thought.
If I look at your fudge factors, best case it adds up to 40dB, worse case it would by 60 dB. That would mean that TX power would need to be in the 10s of kW up to 1MW range to get any kind of distance. We don't use this kind of power and still successfully manage to communicate on most occasions. Therefore your assumption of all these extra losses is wrong. Not slightly out, not in the right order of magnitude, but WRONG. By a factor 10000 to 1000000 or so. Or 40dB to 60 dB if you do in fact understand what a dB is.
Regards,
CFM56-7B26
Just because you have a bigger engine than others doesn't mean you can't (on occasion) just accept the information given to you.
You asked for numbers, I gave you numbers. You must be of the 'don't try to change my mind with facts' school of thought.
If I look at your fudge factors, best case it adds up to 40dB, worse case it would by 60 dB. That would mean that TX power would need to be in the 10s of kW up to 1MW range to get any kind of distance. We don't use this kind of power and still successfully manage to communicate on most occasions. Therefore your assumption of all these extra losses is wrong. Not slightly out, not in the right order of magnitude, but WRONG. By a factor 10000 to 1000000 or so. Or 40dB to 60 dB if you do in fact understand what a dB is.
Regards,
CFM56-7B26
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I can believe a 10dB loss of signal strength due to corroded connections - you only had to listen to the FM radio on my old Vauxhall Cavalier when you wiggled the antenna wire.
Regarding the remaining 30dB, What IO540 actually said was a loss of dynamic range of 30dB, not an attenuation of the RF signal by this amount.
By this, I understand him to mean that the noise "floor" at the receiver comes up by that amount due to the interfering effects he mentioned and you get a signal to noise ratio of say 25dB rather than 55dB (or whatever). What you hear in your headset, for the same RF strength, is still intelligible, but has more background hiss.
Regarding the remaining 30dB, What IO540 actually said was a loss of dynamic range of 30dB, not an attenuation of the RF signal by this amount.
By this, I understand him to mean that the noise "floor" at the receiver comes up by that amount due to the interfering effects he mentioned and you get a signal to noise ratio of say 25dB rather than 55dB (or whatever). What you hear in your headset, for the same RF strength, is still intelligible, but has more background hiss.
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Perhaps I should apologise for being a little flippant with my observations on this.
However, I do recall, during my time working for the Queen, stationed at RNAS Brawdy South Wales in a ground based crash waggon, clearly hearing the transmission " Lossie Tower, Rescue one do you read? over." Etc. From a Landrover. I cannot remember whether these were VHF or UHF sets that we used. (1964)??
So if the ionospheric influence is also a variable how goes the calculation?
However, I do recall, during my time working for the Queen, stationed at RNAS Brawdy South Wales in a ground based crash waggon, clearly hearing the transmission " Lossie Tower, Rescue one do you read? over." Etc. From a Landrover. I cannot remember whether these were VHF or UHF sets that we used. (1964)??
So if the ionospheric influence is also a variable how goes the calculation?
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Hugely variable...
I was a young radio ham in the late 1960s (OK1OFA) and we used to talk to England on 2m (150MHz) VHF.
I think that is somewhat behind the horizon... mind you, the antenna had about 100 elements so it was somewhat directional
Of course, nothing political was discussed, if you wanted your parents to keep their jobs...
I was a young radio ham in the late 1960s (OK1OFA) and we used to talk to England on 2m (150MHz) VHF.
I think that is somewhat behind the horizon... mind you, the antenna had about 100 elements so it was somewhat directional
Of course, nothing political was discussed, if you wanted your parents to keep their jobs...
