Page 33 - EngineerIT August 2022 Digital

AMATEUR RADIO

The mysteries of VHF radio propagation

ropagation on VHF, UHF and microwaves keep intriguing the least to 1967. In that year Henry Root, W1QNG, wrote a two page
amateur radio fraternity. The point of view put forward in article for the technical correspondence section of QST (magazine
Pmany articles, that frequencies above 30 MHz are only useful of the American Relay League, ARRL), discussing the theory of
for point to point communication, is a very narrow view, as again aircraft scatter, with the title “Using Aircraft Reflections in VHF
shown on 8 July at 16:08 when a radio amateur in Secunda, Cor Communications”.
Rademeyer ZS6CR, picked up a signal on the two metre band from When an aircraft crosses the path of a radio signal, the aircraft
Namibia. This started a major discussion on WhatsApp, which will cause the signal to scatter in all directions. Backscatter is
resulted in other amateurs commenting on their experiences. primarily what is used in radar systems that track aircraft in
There are many propagation modes that could have been at the sky. Here transmitter and receiver are normally co-located.
play. Joining the discussion was Carl Minne ZS6CBQ who looked The signal will also be reflected in the forward direction and be
up flight radar history and noticed that at that time flight DAL200 received by another station in a different location. This is known
from Atlanta was exactly in place over Botswana flying at 12 000 as bi-static radar. It stands to reason that the amount of signal
metres. He also noticed three other flights over this spot, more reflected is very small and there are equations available that can
or less at the same time at different heights. There were three big be used to calculate the path losses and expected level of the
Airbus 380’s, the one from ETH846 to Addis Ababa, SAA 2968 and received signal. There is also some magic that happens when
a smaller 737 to Windhoek. It became obvious that the signals the aircraft flies directly in the path between the two stations.
Cor Rademeyer had heard were truly from Namibia but scattered This magic manifests itself as signal enhancement, sometimes as
off one or more aircraft. much as 20 to 30 dB or more. As a result of this, aircraft scatter
propagation is sometimes referred to as aircraft enhancement.
Aircraft scatter communication was first noticed in 1937
The first radio-based detection off aircraft was accomplished There are several factors that affect the possible contact via
by L. A. Hyland of the U.S. Naval Research Laboratory in June aircraft scatter:
1930 using a 33 MHz CW signal. The detection was accidental, • the speed at which the aircraft is flying;
as Hyland was working on ground-based, direction-finding • whether the aircraft flies in the path between the two stations
equipment at a military airport and astutely noted that the or crosses the path;
received direction-finding signal increased when aircraft passed • the size of the aircraft also affects the amount of signal that is
through the direction-finding beam. scattered; and
The history of amateur use of aircraft scatter goes back at • the number of aircraft in the sky.

Aircraft scatter is more common in areas where the airspace is
busy, as in Europe or the US. Here in southern Africa, there are
considerably fewer aircraft in the sky, although there are times
when the airways are busy, for instance in the early morning or
late afternoon with aircraft landing and taking off from OR Tambo
or Lanseria airports.
This story was first broadcast in Amateur Radio Today, a
one-hour programme broadcast on Sunday mornings on various
amateur radio frequencies. It is also available as a podcast from
www.sarl.org.za.
Brian Jacobs produces the segment “Focus on VHF and
Above. It is dedicated to promoting the application and
propagation research of the VHF, UHF and microwave bands. He
also manages the SARL next generation VHF beacon programme
(http://www.amsatsa.org.za/Beacon.htm) project to place a
number of beacons around South Africa to study tropospheric
and other propagation modes. Brian and his team are currently
working on a beacon in the Karoo. A crowd funding scheme was
recently launched to generate funding for the project.
Aircraft scatter communication, unlike meteor scatter, has
very little commercial application but it is a great amateur radio
project. That is what radio amateurs do - explore the science of
radio communication. n

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