New Life for LORAN — Part 2
located in St. Anthony, Newfoundland. For the US West
Coast Chain ( 9,940 rate), the control monitor is located
in Petaluma, CA. The control monitor insures the proper
timing and synchronization of all the LORAN stations in
the chain.
The TCS rack mentioned earlier controls the pulses in
the new solid-state transmitter, shown in Figure 2. You will
also notice there are vertical panels consisting of four
sections. Each section is called an HCG (Half Cycle
Generator). Two sections work in parallel to form the
positive half of the LORAN pulse and the other two
sections form the negative half of the pulse. The HCG
consists of beefy inductors, hefty capacitors, large diodes,
and large SCRs (Silicon Controlled Rectifiers), which
“bang” a tuning coil to “build” the LORAN pulse. In a way,
this transmitter can be thought of as a sophisticated,
electronically controlled, spark-gap transmitter. This is a
totally different concept than what was used in the vacuum
tube transmitter that can be described as a super large
class B linear amplifier.
The HCGs are fired in groups at various time intervals
to resonate and amplify the 100 kHz pulse. This is no
simple task because the whole output network must take
into account the LORAN antenna. Once the peak of the
pulse is reached, some means must be made to dampen
the trailing edge of the LORAN pulse.
That dampening of resonance is
accomplished by a network with the distinguished title of “tailbiter.” The TCS
has a lot of responsibility for HCG firing,
tailbiter operation, and output network
tuning. The output network is constantly
being massaged as the antenna sways
(changing impedance) or environmental
conditions change (rain, snow, icing, low
humidity, dust storms).
The Accufix 7500 transmitter
produces 1.3 megawatts — a bit less
than the AN/FPN- 45 vacuum tube
transmitter’s 1.6 megawatts — but the
solid-state unit has some virtues that the vacuum tube rig
lacks. If an HCG should fail, the Accufix will still put out a
signal, albeit at a slightly reduced output power. An HCG
can be swapped out while the transmitter is operational.
Corrective maintenance for the Accufix is simpler, less
time consuming, and a lot cheaper than what was
required for the vacuum tube transmitters. There is only
the one solid-state transmitter running 24/7, as opposed
to the two vacuum tube rigs which were alternated every
two weeks.
If you remember, earlier I mentioned that the vacuum
tube transmitters could require an electrical “demand” of
nearly 1,000 kilowatts and that I calculated that each
transmitter required 100 kilowatts just to light the
filaments. The entire building that contains the new
transmitter, frequency/control equipment, heating/cooling
equipment, and lighting uses about 170 kilowatts. That’s a
dramatic reduction of electrical energy consumption by
nearly six times. Utility bills will go from about $9,000.00
per month for the vacuum tube LORAN equipment to
about $2,200.00 per month for the solid-state LORAN
equipment.
LORAN Antennas
Figure 2. The new Accufix 7500
solid-state transmitter. The right side
is a mirror image of the left side,
prominent in the picture.
Photo courtesy of ETC K. Anderson.
There are three basic types of
LORAN antennas used at the US Coast
Guard stations (see Reference 3). Two
are common monopoles (towers) —
either 625 feet tall or 700 feet with top
loading elements (i.e., “capacitance
hats”) to decrease capacitance to
ground. Under each type of monopole is
a counterpoise of radials buried in the
earth. The monopole sits on a stout
glass insulator which isolates the tower
from ground. At the LORAN frequency
of 100 kHz, we can calculate that one
wavelength is 3,000 meters. Therefore,
the 625 and 700 foot towers are about
Figure 3
SEPTEMBER 2004
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