ARRL

LF (Low Frequency)

Introduction

Unfortunately, hams in the USA do not have an allocation in the LF (30-300 kHz) region. However, the FCC has licensed a few experimental stations at 136-138 kHz. Canada and several European countries have opened up this small RF space for Amateur CW and digital communications. A half-wave dipole at this frequency is 3,416 feet long! However, most experimenters use short, inefficient antennas with very large loading coils. Propagation in this RF region is by surface wave, using vertically polarized antennas. The radio waves follow the surface of the Earth. To communicate further, you simply increase power, similar to daylight AM broadcasting in the MF (300-3000 kHz) region.

In ITU region 1 and 3, broadcasters use the 150 to 280 kHz region to cover entire countries. European broadcasters in this spectrum can be heard on the East Coast of the US occasionally just after sunset. A good outside antenna is recommended for reception of long wave broadcasters.

An LF band does exists in the US, but it's not an Amateur Radio allocation. A lot of "lowfer" (Low Frequency Experimental Radio) activity occurs in the 160 to 190-kHz region--the so-called 1750-meter band, authorized under Part 15 of the FCC regulations. Right now, you don't need a license to operate on 1750 meters, but there are severe legal restrictions on what you can put on the air there. For starters, you can't run more than 1 W input to the transmitter's final stage, and the entire length of the transmission line and antenna combined cannot exceed 15 meters (approximately 50 feet). That's not much antenna for a band where a half-wavelength antenna would be more than one-half mile long! Hams that operate on 1750 meters sometimes use just their call sign suffix as an ID.

Right now, a few hundred experimenters occupy the band in the US, and several of them have set up CW beacons on 1750 meters (many between 180 and 190 kHz), so you might take a listen if you have a receiver that tunes those nether regions. A lot of equipment for the band is homebrew, but commercial equipment is becoming more available.

Several HF/VHF transceivers have general coverage receivers which tune down to 100 kHz or lower, Recent QST Product Reviews now include sensitivity performance at 137 kHz.

Articles

  • Lowfing on 1750 Meters  Members Only
    QST October 1993, p.67-68
    An introduction to an experimental band below the AM Broadcast band.
  • Mother Nature's Radio  Members Only
    QST January 1994, pp. 49-51
    This is an introduction to the Earth's natural VLF radio emissions.
  • Build Your Own Lowfer Transceiver  Members Only
    QST April 1994, pp. 26-31
    This article includes construction of the antenna.
  • Exploring 136 kHz  Members Only
    QST November 1998
  • The Monster Loop  Members Only
    QST September 2000, pp. 38-40
    Build a high performance VLF receiving antenna
  • The AMRAD Active LF Antenna
    QST September 2001, pp. 31-37
    This small state-of-the-art active receiving antenna works well on the MF and HF bands also.

    [The CP-666 JFET is still available in the TO-37 package from Crystalonics in Long Island, New York (631-981-6140). They are very happy to sell small quantities to hobbyists at $17.50 each (April 2009 prices). A J310 or U310 will work if a sacrifice in dynamic range is acceptable.]
  • AMRAD Low Frequency Upconverter  Members Only
    QST April 2002, pp. 34-39
    This project will allow you to use your HF transceiver to listen to activity below 500 kHz.
    Feedback: QST October 2002, p. 39
  • The Transatlantic on 2200 Meters—this article has a schematic of Joe Craig VO1NA’s 136kHz class E transmitter.QST July 2005 pp 42-46.

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