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VHF transmitter power

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VHF transmitter power

Old 19th Sep 2021, 10:15
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VHF transmitter power

Most airliners have 3 VHF radios, 1+2 are usually identical, and VHF 3 is a lower power unit chiefly used for ACARS data comms.

I had a browse of my old FCOMs but can't find those details, perhaps unsurprisingly. Does anyone with m/t connections know what the actual transmitter powers are for these units?

Thanks!
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Old 19th Sep 2021, 10:27
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A (very) quick Google suggests a currently-available drop-in replacement for airliners is the VHF-2100 from Collins Googling that shows a minimum of 25w output power in this document (though it seems to be more VDL than comms oriented). This FCC Document shows the VHF-900 (replaceable by the VHF-2100) also has a 25W carrier so that might be the common standard.

With VHF, if I remember my basic RT, you don't need a massively powerful transmitter as it's line-of-sight limited, a relatively weak transmitter still has quite a long range in the atmosphere, though happy to be corrected, been many a year...

Hope that helps.
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Old 19th Sep 2021, 11:36
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I find it difficult to imagine that airlines would burden themselves with another box in the spares inventory.
I am almost certain that they would have only one type of VHF Comm box the aircraft.
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Old 19th Sep 2021, 13:08
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VHF Antenna 1 and 2 are usually on the roof. Antenna 3 is on the belly. So occasionally VHF 3 may not work so well on the ground, depending on the aircraft's orientation.
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Old 19th Sep 2021, 14:49
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A320 family #1 and #3 on the roof, #2 on the belly.

B767 VHF L+C on the roof, VHF R on the belly.
Same for B787 .
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Old 19th Sep 2021, 14:49
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VHF will go a very long way (200 miles + reliably) line of sight. Aircraft antennas though are optimised more for wind resistance not performance which is one reason for upping the transmitter power a bit as is the large number of frequencies - meaning that interference with another airport or controller by operating high power on a given channel is minimal.

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Old 20th Sep 2021, 07:52
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Thanks guys. I found the specs in ARINC 716 and ARINC 750. Max permissible transmit power is 40W, and 25-40W is what appears to be built into most bizjet and larger aircraft. VHF3 is in fact a lower power transmitter, in some configurations it appears to be on the battery backup bus. On the A380, VHF1 antenna is fwd top, VHF2 is fwd bottom, and VHF3 is on the top midship. Antenna placement should really only affect range on the ground.
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Old 20th Sep 2021, 13:57
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I don't have access to the manuals, but it is entirely possible that the same VHF transceiver unit can be fitted in either VHF 1, 2, or 3 locations of an aircraft avionics bay, and a 'jumper wire' connection within the control plug for the VHF 3 position switches the transceiver to use lower transmit power in that role, (and therefore draw less current when transmitting).

25-40W is plenty of power for an airborne Comms transceiver, (clear transmission path). In my previous life; a 1W 'walkie-talkie' on 78Mhz worked on the clear path from a hill near Newbury to Crystal Palace TV transmitter station; about 60 statute miles.

The commonly used aerial configuration for fitment to a vehicle or aircraft is a 1/4 wave or 5/8 wave resonant single element 'ground-plane', (the metal body of the vehicle forms the other half of the dipole), and I imagine that is what is inside the shark's fin antennas fitted to aircraft.
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