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TOPIC: Radio range testing for part 25 airplanes

Radio range testing for part 25 airplanes 1 month 1 week ago #907

  • fawcett_j
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When I became a lead FTE working on VHF radio systems (amongst others…), my instructor told me just to remember “100-80-60 1.23√h”. In other words, the line-of-sight range of a VHF radio in NM is 100% of 1.23 times the square root of the altitude in feet; cert requirement for range is 80% of that value and masking (antenna coverage) checks have to be performed at 60% of that value. Was he right?
FAA Advisory Circular AC25-7C contains some numbers in table 170-1. Let’s take an example of FL350; line of sight distance is given as 230.1NM (which is indeed 1.23√h). But simple geometry would suggest that the correct formula for line of sight distance is 1.06√h (see attached picture if you don’t believe me). So where does the 1.23√h come from? Well, “line of sight” is not quite the right term to use because radio waves (or indeed any electromagnetic waves) experience diffraction in the atmosphere such that the route travelled by a radio signal between an aircraft and a ground station is actually a curve in the same direction as the curve of the Earth’s surface. Here the formulae are more complex but it can indeed be shown that 1.23√h is the right answer.
What about the 80% and 60%? Well, here the AC does not give values as a function of altitude but simply distinguishes between above and below 18000’. Above 18000’ the required minimum range is 160NM and antenna coverage checks must be completed at at least that range. 160NM is pretty much exactly 70% of 230NM so it looks like my instructor was optimistic on range and nervous about masking! What do you think?
Of course, matters get more complicated when the ground station is not at sea level on the surface of a perfect sphere but that could be the subject for a
whole other discussion!
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Radio range testing for part 25 airplanes 1 month 1 week ago #908

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Is there any MIL equivalent of the FAA quoted? As regards masking, it would be further more complex if different VHF antennae are above or under the fuselage. Masking due to wing while banking @ different angles would affect the ranges. Can someone guide on the Civil or Military certification requirements?

Radio range testing for part 25 airplanes 1 month 1 week ago #909

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Civil aircraft have antennae on top and bottom of fuselage too (eg on Airbus, VHF1+3 on top, VHF2 on bottom)! For masking, flying one complete 360º turn LH and another one RH lets us see the masking effect of all aircraft structure (principally wings, engines, fin and fuselage) for both upper and lower antennae. So it isn't more complex for military aircraft (except that they may have more frequency bands to be checked, but that just means repeating the same type of checks again and again).
I'm not familiar with a MIL equivalent of the AC. Hopefully someone else can help.
The AC includes some explanation of acceptable dropouts during masking checks, see attached.

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