AMRAD Low Frequency Web Page


WHAT IS LF?

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.


WHY LF?

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.{1}

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.


LF TRANSMITTING ANTENNAS:

  • At LF frequencies, traditional antennas of acceptable length exhibit efficiencies well below 0.1% (yes, one tenth of 1%); hence the search is on for improving traditional antennas, and finding possibly new configurations that may yield better efficiency. AMRAD is looking for ways to build efficient antennas with common materials on a typical radio amateur suburban/urban lot. The search continues.
  • AMRAD has built a 1500 foot long bipole antenna. This is a wire that goes through the woods and is connected on one end to a 300 foot steel well casing and a ground rod in lake on the other. It is tuned to resonance with two large coils near the ends and a ferrite toroid transformer gets it all to 50 ohms for the transmitter. Initially it was not strong back here in the Washington DC area. But now it looks like the signal is pretty good further out. It may favor sky wave to ground wave. Recent results have been good with reception into London, Ontario Canada by Mitch Powell VE3OT. His zipped .wav file can be downloaded here. Note that it is almost 750k. This reception is about 324 miles or 521 km. Closer in we have reception confirmed from Steve Dove W3EEE/G3YDV at Mt. Gretna PA.
  • AMRAD built a small Marconi antenna with only a single wire in the top hat. It is 30 feet high with a 50 foot long top hat. It does not work real well. We will look at adding more top hat wires. The tuner is built with Walmart/Kmart plastic storage boxes as the coil forms.
  • AMRAD built a vertical loop with mixed results. More refinement to reduce resistance losses are being contemplated. "Zip wire ain't efficient here!"
  • If you need a field strength meter to measure and adjust LF antennas take a look here: PA0SE Field strength meter for the 137 kHz band

LF RECEIVING ANTENNAS:

Many people use active antennas, while others prefer ferrite bars, or remotely tuned whips.

  • Bill, W3CSW has been building some attic loops antennas rotated with syncros and remotely tuned with a capacitor decade box. You can read about it here.
  • The simplest and quite effective LF antenna is the E-Field probe. It is essentially a short whip with a very high impedance amplifier at the base to convert the signal impedance to the coaxial cable impedance. To this day Ralph Burhans wrote the best information. It appeared in the magazine Radio-Electronics over the months of March, April, May and June of 1983. Our local library central library in Fairfax Virginia has these on microfilm. Other large public and university libraries as well as old-timer basements should also have these magazines. The effort to find them is well worth it. These antennas work well today and Ralph Burhans imparts a lot of good wisdom on the whys and wherefores of LF receiving antenna design.
  • AMRAD member André N4ICK has a nifty design that combines an E-Field probe and varactor tuned preamp to limit overload by strong stations. You can see his work here. N4ICK LF preamp
  • In addition, an isolation transformer between the LF receiving antenna and the shack with the receiver is needed to limit the AC powerline currents flowing into the antenna ground system. If powerline noise and trash are made to flow in the antenna ground they will couple into the electric field around the antenna and thusly into the antenna signal. Once corrupted, it is difficult to remove and can reduce receiving sensitivity by may decibels. No good LF receiving station should be without one. AMRAD has built some and a description of how to build one is here.

LF TRANSMITTERS:

Transmitters can be either very efficient switchers or audio amplifiers with bandwidth extending to 200 Khz or beyond. Glenn KA0ESA and André N4ICK are busy building such contraptions. AMRAD purchased and has tested a commercial transmitter made in Holland. See a review here.


LF SIGNAL PROPAGATION:

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

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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.


LF RECEIVERS:


DSP SOFTWARE:

  • Weak signals can be copied using narrower bandwidths than customary on HF.
  • While 500 Hz is considered narrow on HF we could copy really weak signals on LF with bandwidths well below 100 Hz and even below 10 Hz under some conditions.
  • European LF hams have been using slow CW (QRS) with speeds like 3 seconds for a dot which would result in 9 seconds for a dash.
  • LowFERs in the United States have perfected narrowband forms of binary phase shift keying (BPSK) and have demonstrated automated detection sometimes taking all night to recover a weak signal.

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.

New JT9 Mode for MF and LF:

If you are on 472 or 137 kHz, you may be interested in a new mode called JT9, designed especially for making QSOs on these bands. JT9 uses the structured messages introduced in 2003 for the JT65 mode, now widely used for EME and for QRP operations at HF. JT9 can operate at signal levels as low as -27 dB (in a 2500 Hz reference bandwidth), with one-minute timed transmissions. It also offers slower transmissions of 2, 5, 10 and 30 minutes duration, and the slowest mode can decode signals as weak as -40 dB. With one-minute transmissions, submode JT9-1 has a total bandwidth of 15.6 Hz -- less than one-tenth the bandwidth of a JT65A signal. The other submodes are narrower still: a JT9-30 signal occupies about 0.4 Hz total bandwidth.

JT9 is implemented in an experimental version of WSJT called WSJT-X. Some further details can be found at http://www.physics.princeton.edu/pulsar/K1JT/wsjt.html. The author, Joe K1JT, requests feedback with your experience with JT9. Please tell him you saw the link on the AMRAD website.

DSP INTEGRATION:

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.


WHAT IS AMRAD DOING?

Click on these links for images of early AMRAD LF experiments:

More recently:

  • AMRAD members Sandy, WB5MMB and Hal, WB3KDU went to the mountains to see if they could copy the New Years day transmission from SAQ at Grimeton Sweden. SAQ uses the last existing Alexanderson alternator to generate the VLF signal at 17.2 kHz. They copied the signal with a 40 foot mast and 4 40 foot ground radials for an antenna. A hombrew upconverter was used ahead of an Icom R71A with a 500 Hz CW filter. The transmission was logged at 1:00 AM EST on 2 January. The location was N38-59-06 W78-00-00 on ridge about 1000' above the valley and 1/4 or more miles from nearest house or power line. You can hear how they did here with their 700k zipped .wav file.
  • AMRAD has been coordinating with European Radio Amateurs on LF and is co-sponsoring the LF Transatlantic Challenge with the Bobek award.
  • AMRAD applied for and received an experimental FCC Part 5 license to operate at 1 watt EIRP on 136.750 Khz. Details are announced at WA2XTF LF transmissions
  • AMRAD LF transmissions continue. See our recent DSP Integration work above.
  • AMRAD went to Nags Head, NC to see how LF listening might be right on the beach. See Field test in January 1999 for more information.
  • AMRAD returned to Nags Head NC over the weekend of January 16, 2000. See our Nags Head 2000 web page for more information.
  • AMRAD returned still again to Nags Head NC and Burke Lake Park VA over 12-15 January, 2001. See our Nags Head 2001 web page for more information.
  • AMRAD had the opportunity to connect to an unused Navy LF Marconi antenna at Annapolis MD. See our Photos from trip to NSS Annapolis. These Navy antennas were no longer needed so they were demolished with high explosives. Some sad pictures were taken. See Demolition of three antennas November 13, 1999.

WHAT ARE THE EUROPEANS AND AUSTRALIANS DOING?

  • The British, The Radio Society of Great Britain.
  • the Germans, the Swiss and the Australians, to name but a few, are doing great things on LF.
  • The European countries have been authorizing radio amateur allocations on LF. QSOs spanning 1,000 miles have taken place recently in Europe with transmitter powers of between 250 and 700 watts, yielding ERPs around 500 milliwatts!
  • For an example of a European radio amateur QSO on LF see a DSP display of a European amateur contact in progress

JOIN AMRAD'S EFFORTS

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.

Write to us at tacos@amrad.org and we promise we will write back to you.

You can also write to each of us individually by using our individual callsign@amrad.org for example n4ick@amrad.org


OTHER LINKS OF INTEREST

DF3LP Main Page


Frank K0BRA

André N4ICK