[Milsurplus] 75 MHz Aeronautical Marker Beacons

Thu, 17 Jan 2002 09:05:06 -0800

George:

The "cone of silence" is a natural phenomenon of the Adcock antenna
array used on the old LF enroute navigation beacons (also called LF
radio range stations).  The system was also known as the "4-course
range" beacon.  Each beacon had 4 narrow "beams" that radiated outward
from the site.  The beacon was set up to that the navigator would hear a
continuous tone if the receiver was in the center of the beam.    If the
aircraft strayed off the beam center, it would receive a series of dots
or dashes, depending on which side f the beam the airplane was on.  The
dots and dashes were timed so that they provided a continuous tone when
the airplane was "on the beam."  The Adcock antenna array's radiation
pattern was such that it did not radiate directly upward from the
station site.  This way, the navigator knew when the airplane passed
directly over the station and served to confirm the airplane's exact
location.  The stations operated on several channels in the 200-400 KHz
band.

The time the airplane wa in the cone of silence depended on its altitude
because the point of the cone was at ground level and the cone expanded
upward from there.

Eventually, the 75 MHz marker beacons were installed and served to
provide a more accurate way of noting passage directly over the
station.  The 75 MHz signal (then known as an "airway marker beacon")
was AM modulated with a tone (sorry, I don't recall the audio tone
frequency) that was used by the receiver to turn on a white light on the
75 MHz receiver's control panel.  The tone was a series of beeps so that
the white light flashed when passing over the station.

The invention of the airborne LF radio direction finder obviated the
need for the 75 MHz airway enroute marker beacons because the DF set was
more accurate in identifying station passage.  The 4-course radio range
beacons were still in use into the late 1960's is many remote areas
because the newly invented VHF omnirange (VOR) system was coming into
use but didn't have the long range needed in remote areas, including the
northern reaches of Canada and places south of our border.

Ultimately, the 75 MHz marker beacon and the LF beacon became part of
the VHF(localizer)/UHF(glideslope) instrument landing system (ILS). 
While the ILS provided left-right alignment (VHF localizer) with the
runway and angled descent path (UHF glideslope), there was still a need
to determine exactly how far the aircraft was from the runway threshold
horizontally.  About 5.2 miles from the runway threshold, a 75 MHz
beacon was installed pointing straight up.  ILS approaches were (and
still are) designed so that the aircraft intercepted the glide slope at
the 75 MHz beacon, which is still called the Outer Marker.  A low power
LF transmitter with a vertical omnidirectional antenna is co-located
with the Outer Marker so it can be easily found with an LF automatic
direction finder (ADF).  Officially, the Outer Marker's LF transmitter
is called the Outer Compass Locator.

A second 75 MHz transmitter is installed closer to the runway threshold
and is modulated with a different tone that illuminates a different
light to once again mark the airplane's horizontal distance form the
runway threshold.  It's called the Middle Marker and doesn't have an LF
beacon associated with it.

More recently, with the advent of high precision ILS receivers in
airplane's a third 75 MHz transmitter has been located very near the
runway threshold and is called the Inner Marker.  It is modulated by
beeps of the same tone as the old marker beacon that identified the
center of the original LF 4-course range station.

While the ADF has eliminated the need for the 4-course range stations,
there are still many LF omnidirectional beacons around the world for
long range air navigation.  There are also many low power LF beacons
along our shores for use by boats and ships.  The beacons work over the
horizon and help ships locate their intended destinations.

Eventually, all of these will probably be replaced by precision GPS-like
systems.

Somewhere in my collection of stuff, I still have an old 2-tube 75 MHz
aircraft receiver.  Interestingly enough, it's tech manual describes it
as a "75 UHF Marker Beacon Receiver."  Interesting that 75 MHz was once
considered UHF.

Over my 40-year flying career, I've used everything from LF 4-course
range systems to modern GPS, inertial and Doppler radar navigation
systems.  Well, I reckon that kinda dates me! :)

Hope this bit of history is helpful.

Regards,

Dick W1NMZ

aGEnuine ham wrote:
> 
> Group:
> 
> If my records were any better, I should be sending this to an individual,
> but don't remember with whom I was having this discussion a couple of
> years ago.  So I apologize for the bandwidth, but maybe someone else is
> interested in ancient history.
> 
> 75 MHz marker beacons (initially called Z Markers) were comissioned on
> Jan 1, 1939 in the continental U. S.
> >From the text book: "Principles of Aeronautical Radio Engineering" by
> Sandretto, first edition, 1942, pages 150 to 167.  While this book
> contains numerous factual and typographic errors, I believe this
> statement is accurate from the context and references to CAA test dates
> in 1938, and it is mentioned several times in different discussions.  The
> 75 MHz "Z marker" beacons replaced the so called "cone of silence"
> beacons on 278 kHz which had been installed in association with the
> original LF radio range stations.
> 
> So, it is unlikely that Amelia Earhart had any 75 MHz beacon equipment on
> board for an international flight in 1937.  I have found no further
> references to beacon implementation overseas, but beyond those deployed
> by the military during WW2, I suspect it was well into the late 40s
> before they were common world wide.
> 
> George W5VPQ
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