Topband: Magnetic Variation and Gyrofrequency

[Bob G3REP]

There are maps published by the CCIR (now ITU-R) showing the world wide
magentic deviation, declination and strength in the CCIR documentation on
Broadcasting on the MF Bands. Makes interesting reading rather than trying
to work it out.

Such reports as  CCIR Report 894-2 Vol VI, XViith Plenary Assembley
Dusseldorf, 1990.  has examples of this as does earlier CCIR Reports.
Unfortunately I do not have the references to hand (back in G-land).

Depending on the difference in frequency between the transmitted signal and
the gyro frequency and the magnetic lines of force, ie deviation
(difference of horizontal component to north ) and the magnetic declination
(difference of the magnetic field to the horizontal) you can lose up to
20-30 dbs of signal.  This will also affect each hop to different extent.

The Gyro freq is at the lowest in South America (700kHz) and its greatest
between Australia and the Antartic (approx 1.7MHz).

The subject is covered in Ionospheric Radio Propagation NBS Monograph no.80
by Keneth Davies but has been updated in his book "Ionospheric Radio"
published by Peter Pereguins 1989 on behalf of the Institution of
Electrical Engineers.

Another book is Radio System Design for Telecommunications by Roger Freeman
(Publishers Wiley 1997 $120)  but it is more expensive than Davies !!!! 

Incidently an interesting anecdote to this subject was a project for a
Superpower Transmitter with a 2MW transmitter and a 8dBi antenna.  There
there were two bidders.  
Company A said the system would give coverage of a certain area and Company
B said the signal strength would be 30dBs down on that predicted by Company
A because of the effect of polarisation coupling loss caused by the
diffrence between the transmitted frquency and the Gyrofrequency, and the
direction of transmission.  Company B got the job because they knew what
they were talking about !!

Try surfing the Internet to the ITU-R and you could download all the
reports you like for a credit card number !!

best 73s

Bob Parkes
S21YP/G3REP



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