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Why does the glideslope use UHF, while the localiser uses the VHF band? I mean, the glideslope could have used the VHF band too, or vice versa, so why is it so?

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The higher the frequency the smaller the transmitter, so fitting the G/S on a small tower near the touchdown zone becomes practical. Compared to the large LOC array at the other end of the runway.

[Of] the advantages of using UHF frequencies for glideslope operation is only a small antenna and ground plane is required. As a rule of thumb, the higher the frequency, the smaller the antenna can to be (avionicslist.com).

Here is a table for the localizer and glide slope frequency pairing (fcc.gov).

As for why not UHF for the LOC, my understanding is the range. A pilot can receive the LOC farther out than the G/S (8 miles more).

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  • $\begingroup$ It also has to do with establishing the width of the the proportional beam. LOC beams vary with runway length but are typically in the range of 4 to 6 deg wide (full scale to full scale). The G/S is 1.4 deg wide. When you say "small antenna", you're really talking about an antenna array. The size and relative placement of the antenna elements are driven by both the frequency and the desired beam width. $\endgroup$ – Gerry May 14 '18 at 12:51

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