In message , Jim Lesurf
writes
In article , Ian Jackson
wrote:
In message , UnsteadyKen
writes

tony sayer wrote...

Think how much better it would be with a decent Vertical dipole;-)..

Who knows why they use these halos?,

1. Halos are compact and very easy to mount on (around) a vertical mast.
They don't clutter up the space required for other aerials too much.

2. Halos have a well-defined, pretty well omnidirectional gain of around
minus 3dBd. The fact that that there a minus sign there doesn't
necessarily indicate that it doesn't perform adequately.

Depends on how you define 'adequately'. ;-

'Adequately' could be that even if you put up a 'better' aerial,
reception is not noticeably - and in many cases, not measurably -
improved.

Yes, the user may still get enough signal to keep them happy. Although for
VHF - as people have said - a vertical dipole will probably provide more
gain.

'In the open' (I hesitate to say 'in free space'), a vertical halfwave
will have 3dB more gain than a horizontal halo. If you mount it 1/8 to
1/4 wave away from the mast (which you more-than-likely will) it will
also be broadly directional (sort-of cardioidal) with up to 3dB gain,
which may - or may not - be what you want. Of course, you also have to
take into account the relative field strengths of the horizontal and the
vertical signals, and if some of the required signals are predominantly
vertical, a halo is definitely not a good choice.

Another way to look at the halo is to wonder where the power goes if you
try to use one as a TX antenna. This may give a clue to the inefficiency
when used for the task for which they are fitted.

A horizontal halo used to be popular as a radio amateur aerial for 144
and 432MHz (especially for mobile work). This is because horizontal was
the norm. However, in the early 80s, the influx and immediate popularity
of ready-made FM transceivers brought a move to vertical polarization -
especially for relatively short-distance communication. These days, the
sight of a halo (whether on a vehicle or on a building) is quite a
curiosity.

I'd have on my list of reasons for why they are used:

N) Because they look to the punter like they are getting something more
impressive than a 'bit of wire' (i.e. a dipole). So the installer can
charge more for them.

A properly installed vertical dipole hardly looks like a 'bit of wire'.

and also sometimes:

N+1) So the installer doesn't have to spend any time aligning them toward a
local TX. Fit, down the ladder, present the invoice.

I doubt if installers have much difficulty in pointing a directional FM
aerial (of whatever type) roughly in the right direction (certainly less
exacting then pointing a TV aerial). Even a vertical dipole on the side
of a mast should be on the correct side. However, a halo is indeed a
100% no-brainer.
--
Ian