In message , Tarmo Tammaru
writes

"Reg Edwards" wrote in message
...
Many, if not all, FM broadcast stations in the US transmit both a
vertically
and horizontally polarized signal. So, rotating the antenna 45 degrees
is
not giving up anything.

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

Simultaneous transmission of vertical and and horizontal polarised signals
from a single antenna system is impossible without upsetting the desired
radiation coverage pattern.

What you mean is your clever US broadcasting engineers have designed
antennas which radiate "Circularly Polarised" signals.


If you go to a broadcaster's web site, they will say X KW horizontal, and X
KW vertical, with no reference to right or left hand polarization. I take
that to mean cross polarized, rather than circular. Obviously, the reason
for the vertical component is car radios. As for messing up the pattern, I
would think that in the majority of cases they want equal propagation in all
directions

Tam/WB2TT

Transmission of vertical and horizontal, with no phase shift, produces
slant polarization. The degree of slant depends on the ratio of the
powers (ie equal powers give 45 degrees).

Transmission of vertical and horizontal, with 90 degrees phase shift,
produces elliptical polarization. Equal powers produces circular.

In the UK, most transmitters use slant or elliptical (to improve
reception for cars etc). Generally, I think that the only
horizontal-only transmissions are from a few low power 'fill-in'
transmitters. Most UK transmissions give more power to the horizontal
(so you don't see vertical domestic antennas).

Circular/elliptical is better than slant as the relationship of the
'slant' of the transmitting and receiving antennas don't matter. The
first UK transmitter to use it gave noticeably consistent signals in
difficult places (eg built-up areas).

Where slant polarization is used, I suspect that it is because it is
easier to transmit (just by slanting the antenna).

I think I'm right in saying the Irish Republic generally uses vertical
for FM (or at least used to). This obviously helps a lot for reception
in vehicles.



As Cecil says, nobody gains anything power-wise. For the same transmitter
radiated power everybody's signals are 3 dB down (half-power) relative to
simple linear polarision when both transmitting and receiving antennas
have
the same polarisation.

The advantage of circular polarisation is that it doesn't matter which
polarisation your antenna is orientated because, in practice, when
erecting
it, the polarisation received by your antenna is usually a matter of
guesswork anyway.

Only with relatively-rare, direct line-of-sight broadcasting propagation
is
there any certainty in the polarisation of received signals.
----
Reg.






Ian.
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