HF Radars

There are two type of HF radar commonly known as surface wave and back scatter over the horizon radars abbreviated to OTHR. The former makes use of ground wave propagation, at the lower end of the high frequency spectrum, to achieve maximum detection range out to about 300km over the sea. The latter makes use of skywave propagation and skip distances out to about 3000kms. A good example is seen in the JORN system in Australia, keeping an eye on Indonesia, the Philippines and southern China.

In recent months we have seen Russia declaring a range of OTHR systems around its borders. Surface wave systems are being deployed in the Black Sea and probably the arctic regions to monitor the country’s economic zone (200nm). Skywave systems are being strategically placed as an element of Country’s air defence network but it is the former I shall concentrate on.

A few weeks ago, I was trying to resolve a noise problem on an HF receiver. The result was that a friend pointed me to a WebSDR (software defined radio) available in The Netherlands. Having downloaded the SDR it was put to immediate use and I was amazed at the number of HF pulse systems across the 2.5 – 6.0 MHz band.

One emitter in particular is possibly new and currently undergoing trials,, so its parameter set is changing quite regularly in RF and sometimes in pulse duration and pulse interval terms but seems to conform to a fairly rigid transmission format, which includes exotic waveforms, although these were not decoded for lack of sufficient resolution.

However, for any radar anorak, like me, out there with an HF SDR, these are reasonably consistent and interesting emitters, always showing very strong signals after dusk. The Dutch WebSDR is found at: <http://websdr.ewi.utwente.nl:8901>

Mar20