Reg Edwards wrote:
An approximate 1/2 wave dipole bent into the form of a circle is not an
omnidirectional antenna. But it is good enough not to make a lot of
difference.

It behaves as a magloop with the tuning capacitor set to zero and is
therefore not easily tuneable.

It has a fairly high Q and therefore a narrow bandwidth appreciable
smaller than a 1/2-wave dipole, and the feedpoint impedance is
appreciably lower than the normal 70-ohms.

The narrow bandwidth make a mess of the required wide bandwith of 87.5-108
MHz. This is the main disadvantage compared with a straight 1/2-wave
dipole.

The impedance mismatch is high at the band edges of 87.5 and 108 MHz. But
this applies whatever type of antenna(s) is involved.

The antenna I was talking about is he

http://www.towerhillaerials.com/view...&orderNumber=0

As far as I know this design is the only commercially available
omnidirectional FM antenna for your general public.

Unless you want to go to 90-degree phased crossed dipoles your simple
circular loop will be as good as anything.

I think turnstile might be better than the "halo".

Or, ideally, you could switch between two well-spaced antennas, orientated
at 90-degrees, each with its own feedline to the receiver.

I don't think that one can buy just a folded dipole for FM. Sure for
broadcasaters (big bucks) but not aimed at your general public. Would like
to see me proved wrong though. :c)

The other omni-directional antennas you mention are vertically polarised
and are not suitable for reception of FM broadcasting stations.

I'm not sure what you say is true. I've been led to beleive that most UK FM
stations are vertical or slant. That ubiquitous "halo" you see, is always
mounted horizontally though. Perhaps wrongly.