Hi All
I have been corresponding with Bob NM7M and Karl K9LA on this subject over
the last few years as it has interested me since I was exposed to the
phenomena of Polarisation Coupling Loss in the mid 80s. I am the "forgotten"
G to which Karl refers!! If you will excuse me I would like to add to the
thread.
While in P29 in the mid 80s and working with the P&T there we shared a site
with the PNGBC (broadcasting service). We had a Consultant over from the UK
looking as to how the PNGBC could improve the coverage. While on a site
visit I marvelled at the vertical antenna (190ft) to which the Consultant
informed me this was half the problem and that they could improve the service
in the E-W direction if the antenna was horizontally polarised. A
improvement of 20dB in an E-W direction was mentioned. N-S would remain much
the same!! This is in addition to the normal D layer absorbtion and another
reason why the Medium Wave is not very popular for daytime broadcasting and
why the "tropical broadcast bands" are used in this part of that part of the
world.
When returning to G Land I joined the said Consulting company and had the
pleasure of being able to quiz the aforementioned Consultant further !! He
cited another instance of the PCL where the Company was Consultants to a
Broadcaster who had a specific requirement for a target area. There was a
tender and two bidders - One bidder A giving the normal equipment specs,
antenna requirements etc and the other bidder B saying the requirement was
impossible to meet since there would be a 30dB loss in the propagation path
to a specific target area. Needless to say Bidder B won the tender due to
the technical competence of their bid and supplied a number of 2MW AM (8MW
pep) medium wave transmitter and antennas one of which was a 8dbi vertical
array located on the sea on the LOWER end of the MW broadcast band.
Migrating birds were seen to do a right angle when flying over the area - not
surprising when you consider the field strengths involved !!
The gyrofrequency has been mentioned as 1.4-1.6MHz at the poles due to the
high strength of the magnetic field. What has not been mentioned in the
discussion is that the gyro frequency in South America is a mere 700kHz due
to the lower field strengths in the region.
The PCL is a complicated equation based on the difference between the
frequency and polarisation of the transmitted wave and the gyrofrequency
(dependent of strength of the magnetic field), and the direction of the
magnetic field covered by declination - difference to the normal geographic
direction and inclination - the angle of the magnetic field to the
horizontal.
CCIR 435-5, & Proc IEE Vol 112 No.1 Jan 1965 are good starting points for
folk who want to dig further into magneto-ionic theory and its effects on
propagation !!
Food for thought
best 73s
Bob
G3REP
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