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This is a glide slope antenna from an aircraft. They are installed in or near the nose cone of an aircraft for the reception of the glide slope signal beams.

The antenna is tuned for 329 to 336 MHz and has an impedance of 50Ω. glide slope antenna I have no idea what the center tube is for, the connector on the other side is behind one of the legs with the rubber glands. Sometimes the antenna is missing the center post, perhaps it's only for rigidity.

You can somewhat see the way it mounts to the fuselage in this picture: bottom plate You can see the N connector, and it seems the center post is directly screwed into the bottom plate.

I'm wondering what type of antenna this is. To me it looks like a folded monopole antenna or something, but I'm not sure. I'd like to know the beam characteristics of it, how to calculate the dimensions, etc. I have no idea how the internal structure of the antenna is, since the connector is one of the legs, so I can only assume the rubber glands isolate the element from the bottom plate. Perhaps it's like an end-driven λ/2 dipole that is folded back or something, I'm really confused and I don't know. But I'm very interested.

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    \$\begingroup\$ Antennas generally cannot be understood intuitively - so complex is the wave behaviour. Experiments with careful measurements or solving numerically 3D field differential equations are the way to find what an antenna does and antennas can be developed step by step by making mods and checking did it improve the operation. A dipole and a whip are exceptions. I bet the aircraft body is an essential part of this antenna. Aircraft antennas should work well when rotated to different positions due the movements, so this probably works with something more complex than a linearly polarized wave. \$\endgroup\$
    – user287001
    Apr 23 at 16:04
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    \$\begingroup\$ Aircraft navigation signals are horizontally polarized, with communication signals being vertically polarized. There are a number of different designs. A starting point might be FAA-RD-79-75 but it also references another interesting one I've not yet found, FAA-E-2429. Your antenna is quite standardized now. So there should be design documents somewhere. Interesting question. \$\endgroup\$
    – jonk
    Apr 23 at 17:49
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    \$\begingroup\$ Also these may help out: FAA: Siting Criteria for ILS and DIY info on aircraft antennas. \$\endgroup\$
    – jonk
    Apr 23 at 17:58
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It is better understood as a pair of symmetrical, single-turn loop antennas. Each loop consists of one curved portion, plus the center post. It's mounted so the loops are horizontal, so it's horizontally polarized, and mounted on the nose of an aircraft (sometimes covered by a radome) so it will have a fairly broad, forward-looking radiation pattern.

Grounding the center post helps make the antenna sturdy and invulnerable to lightning or static discharges.

The picture you've shown has a single feed and only uses one loop. The other loop's feedpoint is simply grounded. But the same design can be built in a dual-feed version. I believe that is used to provide redundancy -- i.e., two cables, two receiver front ends, etc -- to reduce the chances of failure at a critical moment in the approach.

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  • \$\begingroup\$ So in the design I've posted, both the center and the end without the connector are shorted to ground? Wouldn't this basically mean the non-driven loop is simply inert? \$\endgroup\$
    – polemon
    Apr 23 at 18:29
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    \$\begingroup\$ Yes, the non-driven loop is mostly inert in the single-feed version. It probably has some effect on the pattern but not enough to matter. \$\endgroup\$ Apr 23 at 18:35
  • \$\begingroup\$ Oooh! Now this makes sense! Also, seeing it as a loop-antenna that is "laying down" makes sense now that I look at from that perspective. But wouldn't it make more sense having it in a vertically polarized orientation for a glide-path application? Or is this more of a regulatory constraint? \$\endgroup\$
    – polemon
    Apr 23 at 18:40
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    \$\begingroup\$ The glide-slope transmitter has to radiate two narrowly controlled beams at slightly different vertical angles. Horizontal polarization was probably chosen to make the transmitting antennas easier to implement. The receiving antenna is the "dumb" part. \$\endgroup\$ Apr 23 at 18:59
  • \$\begingroup\$ @MarkLeavitt While I'm a pilot, I'm no expert on these details. However, I do recall reading that navigation signals have been assigned to be horizontally polarized as a matter of policy. (And that communications are also by policy to be vertical.) That doesn't mean the policy is ignorant. For example, the navigation signals need the "Earth surface" as a reflector. So it just makes scientific sense. But I think the science has been codified now into policy. Even if it is "difficult" at times. But I may be wrong about how I've interpreted what I've read, too. \$\endgroup\$
    – jonk
    Apr 24 at 2:01

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