Topband: Polarisation Coupling Loss - a broadcasters perspective

[RPARKES197@aol.com]

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