Nuts & Volts - September 2004

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 night!

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

Backup Power

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.

References

F o r

E l e c t r o n i c s

NUTS & VOLTS E v e r y t h i n g

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.