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.

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

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

The other omni-directional antennas you mention are vertically polarised and
are not suitable for reception of FM broadcasting stations.
----
Reg, G4FGQ

=================================

In the UK, it seems that apart from the "halo" type omnidirectional
antenna
(ie folded dipole bent into a circle) there are no other commercial
omnidirectional offerings. I mean marketed for the listener. So, it
looks
like one has to homebrew an FM antenna.

Does anyone know of any online plans for an already mechanically
engineered
FM/VHF omnidiectional antenna? Could be turnstile, slim jim, j-pole,
ground
plane, co-linear, or whatever. Or, any designes published in magazines.

TIA.