LF stands for Low Frequency, that portion of the RF spectrum extending from 30 through 300 kHz. In Europe, where there are numerous broadcast transmitters between 150 and 250 kHz, it is often called ``Long Wave''. Under ideal conditions in mid-winter the high power European broadcast transmitters can be heard on the U.S. East coast.
In the United States, users of the LF band include the US Navy, WWVB, LowFERs between 160 and 190 kHz (Longwave Home Page and Longwave Club of America Home Page) LowFERs are limited to one watt and a 15 meter antenna under FCC part 15. They have demonstrated some amazing ranges under what would appear to be very restrictive rules. Check out these web pages to see what they are able to do. The Longwave Club of America publishes an interesting and informative newsletter LOWDOWN. A serious LF person should subscribe to be sure not to miss new and important information on the LF scene.
The electric power companies also transmit signals on the power lines at those frequencies. Their signals are called Power Line Carriers (PLC) and use the power lines to conduct the signals. Some unintended radiation occurs and when listening at the noise threshold of LF these can be heard as modulated and unmodulated carriers. When using a mobile LF receiving setup you can hear these PLCs come way up in strength as you pass near or under long distance power transmission lines. The power companies never applied for or received FCC licenses for this operation. Now with the potential for amateur allocations, power companies are voicing some concern on the potential for interference with their systems.
Starting at 200 kHz up to around 420 kHz Non-Directional Beacons (NDB) dot the North American continent. NDBs are located at or near many airports to aid navigation using direction finders on the aircraft. These signals are a good first test of LF receiving systems and can challenge listeners to see how far away they can be heard. Inland NDBs run around 200 watts with a simple Marconi antenna. NDBs on the end of a chain at the coastal edge can run 2.5 kW to reach further out to sea.
Operation on LF presents unusual challenges (read ``problems''). The wavelength at 187 kHz is 1 mile, and a quarter wave is 1,320 feet! It gets worse. At 137 kHz where the wavelength becomes 1.3 miles and a quarter wave vertical would reach up 1800 feet. Worse yet at 76 kHz the wavelength is 2.46 miles.
Hence for those of you interested in building your own equipment, getting the maximum out of necessarily inefficient antennas, using DSP to fight man-made noise, LF is a wonderful place to experiment. If you wonder why LF we can argue the need for a pool of trained LF engineers and listeners for "national" needs such as was needed at HF in WW2. Very Interesting indeed. Who will build LF systems in the future? You, perhaps.
Many people use active antennas, while others prefer ferrite bars, or remotely tuned whips.
Transmitters can be either very efficient switchers or audio amplifiers with bandwidth extending to 200 Khz or beyond. Glenn KA0ESA and Andr� N4ICK are busy building such contraptions. AMRAD purchased and has tested a commercial transmitter made in Holland. See a review here.
LF signals can propagate by ground wave or by sky wave. Most studies on LF propagation have concentrated on the ground wave mode and little data and analysis exist on sky wave propagation. J.S. Belrose et al presented data on LF skywave propagation in the Proceedings of the IEEE in May of 1959. This paper seems to be the best source of data on the subject. AMRAD is looking at acquiring data on European LF broadcast stations to help engineer the system necessary to achieve a transatlantic QSO on LF. Some early data has been collected by Sandy, WB5MMB and is shown here. Note the abrupt drop in signal level around 0700 GMT which corresponds with sunrise in the area of the transmitter. We are curious to know what other phenomena might be exploited to achieve a transatlantic QSO.
All these narrowband modes can make good use of computer signal processing. This can use either a general purpose PC with sound card software or a more special purpose Digital Signal Processor (DSP). The DSP chips are much simpler than a PC while being much more powerful having been optimized from the ground up for signal processing. Several AMRAD members are working on DSP to include Bob WA3WDR and Dave K8MMO. Bob provided some insight in his fine AMRAD article here.
For an excellent example of DSP reception see image of frequency vs. time plot for commercial LF station DBF39. (From Marco Bruno - IK1ODO ) Also check out our trip to Nags Head NC and some of the spectrograms we got there.
Work by AMRAD Members Bill Farmer W3CSW (with
Frank Gentges K0BRA and Andre N4ICK looking over his shoulder) on using the
RX320 along with a DSP program has resulted in a calibrated frequency accuracy
of about 1 hertz on the DSP spectrogram. This work has combined the use of Gerd
Neiphaus' program GNRX320 and the PADEN DSP program Spectran. This has turned out to be
a powerful LF weak signal monitoring set up. Bill was able to discriminate
between two beacons running on almost the same frequencies. A screen capture
can be seen here. The procedure for getting this accuracy is provided here. Argo/Spectran/Jason Authors Alberto
di Bene, I2PHD and Vittorio De Tomasi. IK2CZL have a lot of further
improvements planned so stay tuned to their web site as these changes start
showing up so we have seen nothing yet. See our journey to Nags Head NC
where we used this setup extensively and it was a delight.
References: must reading, from cover to cover, the RSGB's ``The LF Sourcebook'' and Ken Cornell W2IMB's The Low and Medium Frequency Radio Scrapbook. Ken Cornell passed away recently and the availability of his book is limited.
Click on these links for images of early AMRAD LF experiments:
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Americans have been extremely creative to extract the last drop of energy available to them within the harsh limitations of the Part 15 rules. Now with higher power radio amateurs have the potential to open up new applications and to exploit propagation phenomena that Part 15 rules would not allow. How about you?
Me, the reader of this page? Moi? Yes, You !
Why not getting involved in LF? Instead of using your 2-meter hand-held to discuss the road traffic that you encountered on the way home, how about helping on the LF scene? If you can distinguish what is the business end of a soldering iron, or if you are good at writing in Pascal or C, how about sending us a short e-mail telling us about your capabilities? We need help in converters, receivers, transmitters, baluns, DSP, receiving and transmitting antennas and new ideas and concepts in general.
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