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I made a 1090Mhz antenna for ADS-B following the instructions from sprut.de. The antenna looks like below and I made it from the copper centre of some stripped coax:

Antenna diagram

The instructions say:

Now we need a "dummy-ground", that has to be connected to the shield of the coaxial cable.

Why does this virtual ground / counterpoise need to be connected to the shield? Will it be slightly less effective or completely useless if left unconnected?

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It will be mostly useless.

Every electrical circuit has current flowing in a loop. When a circuit contains an antenna, there are always, in some form, two halves to the antenna, which have opposite voltages and currents (and the current loop is completed by "empty space", much like in a capacitor). When you see an antenna that looks like one wire sticking out, there is actually something else forming the other half, usually being whatever the circuit ground is (possibly a metal casing or a power supply ground).

What you have there is an unusual form of ground plane antenna, where a plane of metal (or approximation by wires) is used to form that other half. If you leave it disconnected, then you're removing it from the circuit (except for some very small amount of capacitive coupling) and the coax shield (and, in part, whatever it's attached to) will be pressed into service as the other half of the antenna.

This is undesirable because it means the antenna does not have stable characteristics (depends on metal near the coax), is not overall as far away from obstructions, and because noise from your receiver or other equipment that is conducted along the coax shield will be added to the received signal. (This last effect is not truly eliminated unless you also have a choke/unun at the feed point.)

(By the way, the instructions describe as a series of dipole antennas. Note that an ordinary dipole, with a feed point at the midpoint of the wire, does not require any sort of ground or counterpoise. This antenna has its feed point at one end of the wire.)

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  • $\begingroup$ Thank you. I knew that the capacitive effect was important but this answer spells out clearly why. If you have a link or another sentence about how one would add a choke / unun that would be excellent. $\endgroup$ – David Feb 1 '16 at 19:55
  • $\begingroup$ @David Lots can be said about that, and it's not really the same question. A really simple choke: Take some slack in your coax and wind it into a neat coil of several turns (around a handy piece of plastic pipe or bottle) just before it enters the antenna. Try it and see whether it helps your SNR. If so, read up on building better ones. If not, don't worry about it. $\endgroup$ – Kevin Reid AG6YO Feb 1 '16 at 20:17
  • $\begingroup$ @KevinReidAG6YO [. . .] the instructions claim that this is a series of dipole antennas. It isn't really, because the feed point is not at the center. I mention this because a true dipole, being symmetric, does not require any sort of ground or counterpoise. The IEEE definition of a dipole antenna only suggests a current node at each end with no regard to feed placement. Given the length of the 130 mm elements in the example antenna, they are very close to being dipole antennas and behave as such. $\endgroup$ – JSH Feb 2 '16 at 17:06
  • $\begingroup$ @JSH I've edited to remove that claim. $\endgroup$ – Kevin Reid AG6YO Feb 2 '16 at 17:46

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