A
propagationist is a person who likes to monitor sections of the radio
spectrum,
to see what signals they can receive; particularily on
frequencies that for most of the time are dormant.
However these same
frequency bands (usually the higher end of HF assignment, and the VHF
& UHF spectrum)
because of some unusual influence become the
provider of a signal from a far off location.
Commonly known as: Good
DX !! which gives great satisfaction to a "propagationist"!
Propagation
effects are well documented and can be as a result of, tropospheric
ducting, ionospheric reflection/refraction, meteor showers, sunspot cycle,
auroral reflection, aircraft enhancement, etc etc.
Much interest is
now shown about frequencies at 162MHz.
The two adjacent marine channels
87 and 88 (161.975 and 162.025 MHz) are allocated (world wide) for AIS
(Automatic Identification System).
All seagoing vessels over 400 g.r.t.
(gross registered tonnage) are required to be fitted with the AIS
reporting equipment.
This is a maritime, VHF radio data transmission,
which operates continuously & automatically;
regardless of whether
the vessel is in the open seas, coastal/inland areas or is moored or
anchored.
Max' RF power is 12 watt, narrow band FM carrier modulation
(NBFM), with a 9.6 Kb/sec GMSK modulation scheme.
GMSK or
Gaussian minimum shift keying is a continuous-phase frequency-shift keying
modulation scheme.
It is similar to standard minimum-shift keying
(MSK); however the digital data stream is first shaped with a Gaussian
filter before being applied to a frequency modulator.
This has the
advantage of reducing sideband power, which in turn reduces out-of-band
interference between signal carriers in adjacent frequency
channels.
However, the Gaussian filter increases the modulation memory
in the system and causes intersymbol interference,
making it more
difficult to discriminate between different transmitted data values and
requiring more complex channel equalization algorithms such as an adaptive
equalizer at the receiver.
GMSK has high spectral efficiency, but it
needs a higher power level than QPSK,
for instance, in order to
reliably transmit the same amount of data.
GMSK is most notably used in
the Global System for Mobile Communications (GSM). Source:
Wikipedia
Some people (propagationists!) now
consider this as a large floating network of mobile VHF
beacons.
The information available by monitoring 162MHz
will be a good indicator for say;
openings on the 144-148MHz amateur
allocation.
To enjoy this "propagation - study" I have contrived a
system as shown below.
An
omni-directional Slim-Jim antenna with a MAR6 masthead amplifier. ('cut' for 162 MHz),
Re-used from the 2011 ARISSAT 1(SK) satellite project
{The name
"Slim Jim" stems from its slender construction and the use of a
J type
Integrated Matching Stub (JIM) that facilitates feeding the aerial at the
base,
thus overcoming any problem of inter-action between the feeder
and aerial.
The feed impedance is 50 ohm.}
The gain
of the amplifier is variable; by adjustment of the collector load - being
a 1KΩ "pot" in the power supply.
In use, I have noticed the MAR6 can
mix strong off-air signals and self-oscillate (i.e. 'taking off').
By
adjustment of the load pot, this condition can be avoided - listening for
clean audio from receiver, and noting the value of current, which gives
nil self-regeneration!
Fortunately some very clever people have
produced the software (downloadable from the WWW)
to decode and display
the information on your computer screen i.e. Shipplotter
Click to enlarge.
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during a particularily good 60 min' monitoring session.
Also notice that some reception was from sea going ships 550,625 & 740 miles distant.
The Hepburn tropospheric forecast is included for reference.
Good "tropo" conditions gave the above result on 28th Oct 2012
I have also constructed a 5 element Yagi for AIS reception; for further info follow this link
I acknowledge my interest in AIS was aroused mainly because of the efforts of :- VK2KRR