New Life for LORAN — Part 2
electromagnetic waves. None of the “T” sections
are connected to another. The bottom of the
vertical wire is connected to the bottom of the other
three vertical wires from their respective “T.” If you
use your imagination, you can think of the SLT as
a conical monopole with the upper half sliced off or
you can think of it as a “fat” monopole.
I will relate to you this “sea story” regarding
the SLT, although I have yet to find the source of
the story. More than 30 years ago, when the Coast
Guard was modeling SLT antennas, the computer
predicted an impedance of “about” 5. 5 “real” Ω.
Compared to a 700 foot monopole resistance of
4.0 Ω, that’s a big increase and, hence, more
power radiated (in theory, assuming loss
resistance is a fixed value). However, when the SLT
was erected and impedances were measured on
the actual antenna, it was found to be 3.3-j15 Ω, a
true disappointment. My guess is, though, that the
bandwidth of the SLT is much broader than that of a
monopole. If nothing more, the sight of flashing aircraft
warning lights makes an impressive display from 25
miles away, as seen while driving on Interstate 90 at
Figure 4. Backup generators for the vacuum tube transmitter.
Each unit is rated at 400 k W. Photo courtesy of ET1 K. McKinley.
7% of a wavelength. As a result, the impedances of the
625 and 700 foot tall antennas are 2.5-j25 Ω and 4.0-j23
Ω, respectively. Not much there, huh? You can bet that
great effort is made to produce a good counterpoise to
reduce ground loss as much as possible.
The third type of LORAN antenna — the one used at
George — is the SLT. I’ve seen “SLT” defined several ways,
but the drawings at LORAN Station George call it a
“Sectionalized LORAN Transmitting” antenna. There are
four other LORAN stations using an SLT, so it’s not
unique to George. The SLT is a wire antenna consisting of
four “T” sections slung between four towers. Each tower
is 695 feet tall and arranged in a perfect square — 1,451
feet on a side. One “T” section held up by two towers is
shown in Figure 3. The horizontal sections are suspended
and insulated from the towers; the towers do not radiate
I mentioned earlier that the vacuum tube station
could use as much as 1,000 kilowatts at one time and
that the new solid-state station uses about 170 kilowatts.
What happens when the lights go out? The Coast Guard
doesn’t want that LORAN signal off air for very long.
Despite the very good service and reliability of the local
utility (Grant County Public Utility District), power
interruptions do occur.
NUTS & VOLTS
1 — www.navcen.uscg.gov/loran — then click on “LORAN-C
User Handbook.” Although this publication contains information
regarding LORAN stations long since shut down, it is a good
tutorial for providing insight as to how LORAN works.
2 — Private correspondence 5 March, 2004, with Bill Roland, a
retired engineer from Megapulse, Inc.
3 — www.navcen.uscg.gov/pubs/rnavbull/rnbull38.pdf —
then go to page 13 for details.
4 — www.loran.org/Newsletters/NewsletterIndex.htm —
then click on September 2003. www.loran.org is the homepage of
the ILA (International LORAN Association) previously known as
the Wild Goose Association.
5 — www.loran.org/Newsletters/April,2003.pdf — page 3,
“US Coast Guard reports interference to GPS from TV antennas;”
an incident involving certain powered (active) UHF/VHF marine television antennas creating interference to GPS receiver operation.
6 — http://webhome.idirect.com/~jproc/hyperbolic/
loran_c_future.html — “Excellent though GPS may be, its
problem is that it is so low powered that the signal can easily be
blanked out or disrupted — as demonstrated at a 1997 Moscow air
show, where a jammer destroyed the signal over a radius of 200
km.” The website — maintained by Jerry Proc — is loaded with all
sorts of information regarding LORAN and its history, as well as
information regarding other forms of radio navigation.
7 — A nautical mile is approximately 6,076 feet or about 1.151
statute mile. It is defined as one minute of longitude at the Equator
— 1,852 meters.
8 — www.megapulse.com/how%20used.html — A concise
article with diagrams explaining the extension of LORAN use.