The practice of jamming radio transmissions has been around since radio was first used for military purposes when one side would attempt to inhibit the communications of the other side by transmitting on the same frequency at the same time. Indeed such military jamming is usually termed 'electronic counter-measures' or ECM and a whole industry has developed around it to try and produce radio systems that are resilient to jamming known as 'electronic counter-counter-measures' or ECCM. The picture on the right shows an example of a drone in use today for jamming - in this case one known as a Dragon. The payload of the missile is a high power transmitter which can then be sent to the area of interest and remotely activated. You can see the antenna which unfolds from the front of the device when it is deployed.

However jamming is not strictly a tool of the military. Jamming of broadcast radio stations has been a regular activity for many organisations and - and here's the disappointing bit - continues to take place even today. In the height of the cold war, countries such as Russia used to jam short-wave transmissions from countries such as the UK and USA which carried news or information which the Russians viewed as anti-communist propaganda. To try and compensate, broadcasters such as the BBC World Service, Radio Free Europe and the Voice of America used to transmit their programmes on several short-wave frequencies at the same time (usually 4 or more) in the vain hope that at least one of them would be receiveable in a given area clear of the jammers. The Cuban authorities also jammed Radio Marti, a US Government funded station aimed specifically at the country.

Although they haven't admitted it, China continue to jam Mandarin language short-wave transmissions from the same kinds of stations even today (take a look at this article for example). Tune to the BBC Chinese service (try 9580, 11945, 11980 or 13970 kHz or look at the most recent programme schedule N.B. this page is in Mandarin) and you're likely to hear either the familiar 'wobble' sound over the top of the transmission or 'distorted voices' - both symptoms of a jammed frequency. It's odd that such activities still take place when, with the advent of the internet, the kind of 'propaganda' that the Chinese are trying to guard against is freely available to anyone with a computer. Of course, not all the population of China have access to a computer but with internet cafe's many do (though the Chinese authorities do their best to stop people getting access to such information). There are also case of the Chinese jamming themselves!

Whilst such overt jamming of international stations by such political regimes is virtually taken for granted, there have been deliberate attempts to block transmissions from other stations by apparently respectable regimes. Read the story of Radio Caroline whereby the British Government decided to jam its transmissions to stop them being heard during election times. The British Government pulled another fast trick in 1990 when it licensed a station in London on 558 kHz, the frequency used at the time by Radio Caroline. The choice of frequency was clearly chosen to stop Radio Caroline from being heard in London (though the plan vaguely back-fired as the London station was forced to temporarily simulcast on 990 kHz as the 558 signal was so badly interfered with by Radio Caroline).

More recently there has been a story suggesting that the US Government was deliberately jamming radio communications during the recent disaster in New Orleans. Whilst the US Government clearly could have done more for the residents of this flood-stricken town, it belies belief that they would actively try to scupper rescue attempts.

What is clear is that despite significant developments in radio technologies and in international relations, radio jamming transmitters seem to be as active today as they have always been, mucking up military communications and supressing freedom of speech. Let's hope that one day soon, neither of these uses will continue to be necessary.