Myths about radio jamming

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

“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
A transmitter converts an electrical signal into a radio signal using an antenna and a power source. A large power source can broadcast farther than a small power source. A directional antenna can focus that power to broadcast farther than an omnidirectional antenna, but the signal can only be received where the antenna is aimed. This is useful for a point-to-point link but for a radio station, you want more than one person to be able to hear you so you typically broadcast equally in all directions (except up towards the sky and down towards the ground). The farther away from the antenna you are, the weaker the signal, until at some point the signal will not register at all and your radio will be silent (or play static).

Like many physical phenomena, the strength of the signal varies with the square of the receiver’s distance from the source. If you want your signal to reach 10 times as far, you need to supply 100 times the power. For this reason, even a weak transmitter can dominate a small area around it as long as it is relatively isolated from more powerful transmitters. A 1 watt signal will overpower a 100,000 watt signal if the weak transmitter is 1 meter away and the powerful one is 1km away. In fact, the distant transmitter would need to be 1,000,000 watts to equal the power of the nearer transmitter!

However, this is a double edged sword. As you move away from the weak transmitter, its signal falls off more quickly than the signal from the distant transmitter. For instance, moving from 1 meter to 10 meters, the power falls by a factor of 100. However, the power from the distant transmitter is almost unchanged. This is because 1.99km or 1.01km is hardly any different from 1km. In fact, moving from 1km to 1.01km (10 meters difference) reduces the power by less than 2%, whereas moving 10 meters away from the weak transmitter reduces power by 99%. You can see that the sphere of dominance of the weak transmitter is very small indeed.

A larger transmitter of, say, 500 watts, sounds a lot more powerful than a 1 watt transmitter. However, being 500 times more powerful only increases your range by a factor of 22.3 (the square root of 500) — instead of dominating an area a few meters in radius, you can dominate an area a few dozen meters in radius. Hardly enough to control a population in a city or a rural area.


12 responses to “Myths about radio jamming

  1. Awesome insight. I am sadly technologically inept, (ironic considering that I run a blog), so thank you for breaking the issue down and explaining why the claims are incorrect.

    I wonder, then, what the real reason is why the military hasn’t jammed the Swati Taliban’s radio station?

  2. hey Rabia,

    great post. I have forwarded it to just about everyone.


    I am just as technologically inept so that makes the two of us.

  3. excellent post, Rabia. Why don’t the “good” ISI hire you? : )

  4. Publius


    Shouldn’t you be beyond the point of ‘wondering’ at this juncture ?

    The idea that Pak army has deliberately allowed the takeover of Swat for strategic purposes( strategic depth in Afghanistan, money from US, eventual use of same proxies against India etc etc) has often been presented and discussed here and elswhere.

    Do you not find that to be a plausible explanation ?

  5. thanks guys, this was actually a guest post by Jon who was kind enough to spend a week reading everything on the internet about FM radio transmission!

  6. Well, great work Jon. I’m linking to this post right now 🙂

  7. Pingback: GEO News Reporter Killed in Swat as Sharia Deal is Reached « CHUP! - Changing Up Pakistan

  8. Publius, it was a rhetorical question – i.e. one that didn’t require an answer. I obviously know that the Pakistani Army is just making excuses.

  9. Publius

    OK Kalsoom, my bad.

    Rabia, sounds like your are bringing in some international help.:-).

    (and thanks to your guest blogger)

  10. i was going to forward this to all my friends.

    then i read that an infidel has written it. clearly this is the work of satan. the white man always claims we are wrong. i am going to go bury my head in the sand now.

  11. damn I knew he should have picked a fake name

  12. stuka

    Rabia genius, can you now score a military strategist who can contrast Indian and Pakistani counter insurgency tactics in Kashmir and Swat. I am still trying to figure out why your guys don’t do infantry based cordon and search like our guys. I asked MNI coz his bro is in Pak Amry..turns out the dude is a doctor who specializes in plastic surgery and is at present attending a course in the UK on genital reconstruction. 😉

Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out /  Change )

Google+ photo

You are commenting using your Google+ account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s

%d bloggers like this: