Are YOU a PROPAGATIONIST ?

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.
The tabulation shows a total of 2050 decodes from 66 stations (ships/navaids etc)
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