Over The Horizon Radar

Over The Horizon Radar by Andy Tyler, G1GKN
Why should we care about OTHR? Like it or not we are forced to share our allocated HF bands with it. The transmitters can be very powerful, up to 40MW in some cases. OTHR operators have a total disregard for band plans, using any frequency that suits them. OTHR are often secret projects, which restricts what authorities can do to stop them. Indeed many countries deny that such systems exist.

As current microwave radar is capable of 300km range and ceiling at 20,000ft. OTHR can see much further distances down to ground and sea level, although resolution is not as good as microwave radar.
OTHR-B. Back scatter uses ionospheric propagation. Operational frequencies used are 3MHz to 30 MHz. Typical range is 3000km. It selects the best frequency to achieve the distance required. OTH-B is usually coupled to an ionospheric sounder. It uses large fixed, electrically scanned, antenna arrays. Transmit and RX sites are often 100 miles apart. It gives poor spectral resolution due to long wavelengths, however it provides good Doppler resolution due to integration times. It can look at a small target area so is mostly used for long range early warning of both airborne and ground targets.
High Frequency Surface Wave Radar HFSWR is not so intrusive and relies on ground wave propagation, usually in 2MHz to 4MHz range but can be up to about 26MHz. Typical range is 200km.

Main use is for low flying targets, surface targets, customs enforcement, drug enforcement, and people smuggling. It is also used for Tsunami detection and general environmental observation of sea state forecasting and currents.

Typical transmit and receive length of tetrahedral or dipole antenna arrays varies between 20 to 400m. Monopoles or Yagis may also be used. Peak power varies between 1kW and 10kW. The frequency range is between 4 MHz and 26MHz. Integration time for aircraft is 2 to 10secs and ships 100secs.

Andy briefly explained that in practice sophisticated control equipment alters the phasing between an array’s elements to effectively make the “sum beam” sweep across the horizon.


The Russian “Woodpecker” started on 4th July 1976 and consisted of two huge OTH-B synchronised radars, one in Ukraine and one in Siberia. Their beams were directed over the North Pole so that they crossed over the USA. These very powerful beams had a Pulse Repetition Frequency (pulses/sec) of 10, 16 and 20Hz with a RF bandwidth of 40kHz giving ground resolution of 10 miles.

The Russians called it DUGA 3 and NATO nicknamed it “Steel Yard”. The transmit and receive sites were separate. The antenna arrays were vast at 150m high, 500m long and estimated to weigh 14,000 tons. It operated between 4 and 30Mhz, depending on ionospheric propagation. It caused worldwide interference until 26th April 1986 when the Chernobyl reactor exploded, just 8 miles distant. The second station was closed when the USSR collapsed in 1988.

Cobra Mist.

In turn the RAF and USAF colluded to make the AN/FPS-95 Cobra Mist OTH-B system at Orfordness on the Suffolk coast. This was a huge fan shaped log periodic array that operated between 6 and 40MHz. Average power output was 300kW, peaking at 3.5MW. It had a range of between 500 and 2000 nautical miles (3700km)

Unfortunately Cobra Mist never worked properly due to phase errors that the boffins could not resolve. The large Russian trawler fleet operating in the North Sea was finally suspected of disrupting it but this could not be proven. The project was finally abandoned when the Strategic Arms Limitation Treaty (SALT ll) was instigated.


The Jindalee Operational Radar Network (JORN) is an Australian OTH-B network. It was given this Aboriginal name by the designer of the Log Periodic Antenna as the elements were thought to resemble “Bare Bones” hanging up to dry.

The network consists of 3 transmitting stations, one each at Longreach in Queensland, Leonora in Western Australia and Harts Ridge in Alice Springs. The network has 28 phase able log periodic antennas powered by 28 x 20kW transmitters.

There are 3 receiving stations, one each at Stonehenge in Queensland, Laverton in Western Australia and Mount Everard in Alice Springs. Each site is 3.2km long consisting of hundreds of monopoles, each with an integral receiver and Analogue to Digital converter. Beam forming is controlled digitally.

The JORN uses 12 ionosondes and operates between 5MHz and 30MHz. It beams north and west of Australia and has a range of 2000 to 3000miles (3200 to 4800km)


Russian 29B6 OTH-B Radar has a range 3000km up to 100km altitude. It can use 3MHz to 30MHz and often heard just LF of 7MHz. The transmitter and receiver sites are 300km apart. All airborne objects are it’s target. A second Konteyner system is scheduled to be operational in the Far East by 2018, with a total of six networks working by 2020.

Sepher (Sky)

OTH-B used by Iranian military. It is based on a Russian design. There is one system known to be in operation to date with more expected. It is often heard on 20M band.

Pluto ll

British Military system located at RAF Akrotiri, in southern Cyprus. This is a key site for anti-ISIS operations and airborne surveillance, with little information available but it is reported to have a sizable array sited within the base that has caused concern with local people. It is known to operate between 18MHz and 30.5MHz with a 50 to 60kHz bandwidth. It is often heard on 10M.


A French HFSWR experimental system currently beaming over the Mediterranean sea. It could be using 3 to 30MHz. Range 20km.


This French Military OTHR-B. It can scan 360 degrees in azimuth. Uses 3 to 30MHz. It can often be found on 13.98MHz. It has a range of between 800 and 3000km. The antenna consists of 288 bi-cones in a 3 legged star formation. Tunnels under the arms house the transmitters and receivers. Some antennas are used to transmit but all are used to receive.

China is known to have both OTH-B and OTH-SW systems monitoring the South China Sea. Other countries operating OTHR include Denmark, Israel, Romania, Turkey, Ukraine, USA and more. The June 2016 edition of the International Amateur Radio Union newsletter (Region 1) listed 2 OTHRs on 40m, 27 OTHRs on 20m, 23 OTHRs on 17m, 11 OTHRs on 15m and none on 10m. However a recent email from Datta, VU2DSI, (IARU Region3) reports OTHR noise 5/9++ on 28453, 28473, 28483, 28545 khz when the band was good.


  • Most countries are either operating or developing OTH radar.
  • Recently there has been a big increase in OTH-B systems.
  • More sensitive receivers and SDR should mean lower transmit powers and smaller antennas.
  • OTH radar will remain a threat to Amateur Radio for the foreseeable future.

Further reading
Samples of RADAR systems: http://www.sigidwiki.com/wiki/Category:Radar

International Amateur Radio Union Monitoring System (IARUMS)

Wolf Hadel, DK2OM, the Region 1, IARUMS Co-ordinator produces a monthly newsletter that is packed with detailed information about intruders of the Amateur Bands.

The downloadable newsletters on this site are here:


Wolf’s April 2012 Newsletter is dedicated to “Radar Systems on Shortwave”.


Also check out https://en.wikipedia.org/wiki/Radar

OTH Radar Antennas: 20th Century Behemoths https://www.antennex.com/Stones/st0807/st0807.htm

For even more search the web for “radar systems on shortwave”

Thanks to Andy for a very informative talk.

Norman, M0FZW


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