What Kind of Antenna Is This?

Sorry, I don’t have a picture, but this antenna is on the roof of a hospital. It looks like an inverted, open umbrella (if you stripped off the fabric). A verticle pole, with surrounding poles coming out of the base at a 45 degree angle.
Strange looking thing…what advantages does such an antenna design offer?

Depends on the length, but it apparently is a commercial quarter wave VHF or UHF ground plane antenna.

The sloped surrounding poles at a 45 degree angle provide the “ground plane” that, on a vehicle, would be provided by the metal surface of the car body. Without it, the antenna would be “half an antenna” and the signal would simply go “SPLUT” right into the ground, and not go anywhere. It needs the ground plane to help push the signal off to the outside world.

(NOTE TO RADIO PURISTS: I know that last part is not “technically accurate,” however, it’s the way I describe the effect to my customers - they seem to get it that way.)

And the reason the radials (the surrounding sloped poles) are sloped at a 45 degree angle? It pulls the impedance resistance closer to 50 ohms, the impedance used by most commercial two-way radio gear.

Here’s one you can build yourself.

This is a long shot, but it could be an inverted discone, if the central pole is just a mechanical support and there are guy ropes radiating down from it to hold the radials. Then there’d have to be a separate ground plane, but it might not be obvious - it could be part of the roof, or fine radial wires anchored practically horizontally. This antenna would have good VSR for frequencies over a wide range. It’d be vertically polarized and have a perfectly circular radiation pattern.

At least, in principle, it might be. Anybody see any reason it couldn’t?

If its not the 1/4 wave ground plane, I was thinking inverted discone, too.

These are good for airband signals. Inverting it would (somewhat) limit the coverage to signals coming from above the antenna. Is there a helicopter landing pad on the roof?