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I think I understand the basic principles of a collinear antenna. How the Coaxial Collinear Antenna Works? seems to describe it well.

However, I've just repaired a 1090MHz (ADS-B receive only) antenna I built some time ago. Or at least I thought I'd repaired it, turns out I've left the final 1/4 wave still blu-tacked to the window! What purpose does this half-length section serve?

I'm not sure exactly what design I used when I first built it, but they do seem to vary a little:

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    $\begingroup$ Welcome to ham.stackexchange.com! $\endgroup$ – rclocher3 Apr 20 at 20:53
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    $\begingroup$ I think we should have a rule: "antenna simulation or pattern and efficiency measurements, or it didn't happen." I kind of trust N1HFX's theoretical design (I can't fully understand it at the moment, nor can I be bothered to – it's 11pm), but I'm sure they aren't accounting for a lot of the parasitic effects. $\endgroup$ – Marcus Müller Apr 20 at 20:54
  • $\begingroup$ Your question, however, Mat, is excellent! $\endgroup$ – Marcus Müller Apr 20 at 20:55
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    $\begingroup$ So, we know that a quarter wavelength piece of waveguide inherently (because that's the definition of "a quarter of a wavelength") shifts the phase of the signal by 90°. An open end reflects back – 180° shift. Add another 90° for the way back – it's a full 360°! That means the open end looks like a short to ground for RF. $\endgroup$ – Marcus Müller Apr 20 at 20:58
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The quarter wave element compensates for the ground plane (as in a J-Pole). In many designs this same function is served through a counterpoise;

On transmitters the antenna of an HT, a wire, the vehicle ground. No matter the form, the effect is the same, bleed off, either through radiation or termination to ground, to disperse the "excess power".

On receive, the quarter wave element or section acts as the ground eliminating(reducing in practical terms) the need for a ground plane to function optimally.

If you are not placing your antenna on a ground plane, with radials for optimal function, then the quarter wave portion is needed for optimal reception.

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