wrote in message
...
I have been looking for a decent CP design to try out on my
repeaters .
For many years I have been looking at the reasoning for FM
broadcasters having used CP pol with great success.


In the UK, Band II VHF FM sound radio broadcasting began to fixed receivers
using horizontal polarisation (HP) for reasons including a belief that
interference from car ignition systems was predominantly vertically
polarised, and because it was found easier to achieve a good
omni-directional pattern in the horizontal plane from a transmitting antenna
based on a vertical slot (Babinet's principle) - several such slot antennas
were stacked vertically to obtain some gain and to avoid illuminating the
sky.

Later, as transistors became available and vehicular VHF receivers of
sensible size became practical, a new market emerged but it was poorly
served by the HP transmissions*. When local radio was launched in the UK,
in Band II, the new transmitters were equipped with antennas that radiated a
VP component as well as HP, and in time all Band II transmissions were
converted to mixed polarisation. Circular polarisation is one example of
mixed polarisation, but its ability to provide cross-polar discrimination is
not used in FM broadcasting.

Take a look at: http://downloads.bbc.co.uk/rd/pubs/reports/1970-35.pdf

The experience in other countries has probably been similar - same physics.



I also have a small indication that linear antennas (eg:linear
collinears)and non-linear antennas(eg:folded loop dipoles) by design
may have slightly different characteristics in the far field that
tend towards a greater degree of cross/CP from the mechanical design .


Huh?

* HP is less effective than VP for VHF communication with mobiles because
the ever-present ground reflection has reversed polarity (i.e. it's in
antiphase with the signal propagating over a direct path).



Anyone had good success with installing a CP repeater antenna(2m or
70cm) to assist with the deap fade nulls in mobile uplink to the
repeater.
I want to try rhp for TX and lhp for RX - anyone tried this
combination before?

If you use the same antenna for transmitting and receiving, and it is
fundamentally circularly polarised, then it will provide and respond to the
two different senses of CP automatically because the definition of the sense
of circular polarisation depends on the direction of propagation. But do
you think the horizontally-polarised component will help with deep nulls?

Chris