The Taliban are thought to be operating weak mobile transmitters. A mobile transmitter that can be carried on a bicycle or motorcycle will have to be very weak. It will need to run off of batteries and also have a low antenna (low antennas limit transmission range since the signal is more likely to be blocked by obstacles such as trees, hills, houses, and buildings). So realistically, the Taliban’s mobile transmitters will be limited to below 1kW almost definitely and most likely below 500W (especially for the bicycle version). It simply cannot compete with a large, stationary transmitter with a dedicated diesel generator acting as a power supply. A small radio station can easily output 100kW of effective power (a term for the combination of the true transmitter power and the gain from the antenna), and its very tall antenna will make sure that signal reaches just about every receiver in the area without being blocked. Army equipment, which is not artificially limited by government regulations, can easily surpass this.
Some common claims about the “impossibility” of jamming the Swati Taliban’s radio station and why they are incorrect
1. The Taliban are using mobile transmitters so they can’t be tracked, let alone jammed effectively.
This is a basic misunderstanding of how jamming works. When you jam a signal, you do not do anything to the transmitter itself. There is no high power beam that you shine on the transmitter that somehow makes it stop. Instead, radio jamming works by sending a competing signal to the receivers you are interested in (in this case, home radios). The stronger signal will win and the other signal will be silenced. FM radio, in particular, is very easy to jam due to something called the capture effect. FM radio receivers used in homes and cars are designed to eliminate noise and interference by locking onto the strongest signal available and suppressing weaker signals. This allows a clear, static free broadcast even when neighboring radio stations accidentally overlap on the same frequency (or nearby frequencies). An AM radio, on the other hand, does not distinguish noise and interference from the true signal, so you end up with static or even several voices talking over each other.
As a consequence, jamming an AM source requires lots of power and the resulting reception will be unintelligible because you have to completely drown out the competing signal with a uniformly loud tone (otherwise you could hear the original signal during the quiet moments). However, jamming an FM signal requires only marginally more power than what your opponent is delivering and you can deliver a coherent signal such as an alternative broadcast or even silence.
2. The radios can’t be jammed because they constantly switch frequencies.
Frequency switching is a defense against jamming only when the receivers are also programmed to switch frequencies. By switching between dozens of frequencies every second, it becomes very difficult to jam because you have to send a strong jamming signal for each of those frequencies. This multiplies your cost (in terms of equipment and power) by as many frequencies as your opponent is using. However, home and car radios are not programmed to switch frequencies in conjunction with other devices. If the Taliban transmitted on 50 different frequencies the broadcast would be unintelligible to a home listener because they would only receive 1/50 of the data. FM works by transmitting on one primary frequency called the carrier and a very small neighborhood of frequencies around it, which contains the actual sound information. When you tune your radio to 88.1, that is actually locking the receiver into the 88.1MHz frequency range plus or minus 100kHz for the sound data. If the Taliban switched from 88.1MHz to 96.3MHz, your radio would go silent until you also switched.
The Taliban may be using advanced frequency hopping for their own strategic communications — there are commercially available walkie-talkies that do this — but they could not possibly be using that technology to broadcast their programs to the public because regular FM radios do not support it.
3. The military can’t jam the FM radio without also jamming their own radio communications, in effect harming more than helping.
This suggestion is just silly. While FM can be broadcast on any frequency, consumer radios are only designed to receive FM transmissions in a certain range of frequencies — typically from around 85MHz to 110MHz (though in most regions the range is smaller). The military uses frequencies outside this band to avoid interference to and from civilian FM transmissions. The entire FM radio range could be jammed with absolutely no effect on military communications — or indeed, cell phones, television reception, wireless networks, or anything else. Frequencies are distinct and broadcasting or jamming one range of frequencies doesn’t impact other ranges.
As you can see, jamming a weak transmitter is almost trivial, whether it’s mobile or not, and no matter where it’s located in your target region. You will never be able to jam it 100% since it will always dominate a small area around it, but you can quite limit that area of dominance to a tiny area that makes it insignificant and prevents your opponent from achieving their information goals.
Another technique that could be used by the army which would be more direct is simply using a direction finder and then physically finding and shutting down the transmitters. An FM broadcast is a notoriously easy signal to track because it makes absolutely no effort to hide itself (such as by employing frequency hopping as noted above), unlike military communications. Free plans for building an FM signal locator can be found with a quick google search. The army even has a special type of missile called an anti-radiation missile that has such direction finding hardware built in. From Wikipedia http://en.wikipedia.org/wiki/Anti-radiation_missile:
“An anti-radiation missile (ARM) is a missile which is designed to detect and home in on an enemy radio emission source. Typically these are designed for use against an enemy radar, although jammers and even radios used for communication can also be targeted in this manner.”
ARM missiles are expensive so this would not be an effective use of resources. However, in principle, the army could use basic direction finding equipment to track the signal from a helicopter, fly near the location, visually spot the transmitter, and disable it with a machine gun.
A brief overview of radio broadcasting Continue reading