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I just want to prefix by saying I asked where to post this question on meta and no one was 100% sure but this seemed to be the most likely fit so please tell me if this is off topic and where I should post instead. Also, I know the site guidelines advise against question about illegal activities, but the scenario below is just an analogy to clearly explain my question, what I'm really interested in is how radio jammers and receivers work?

I understand that radio signal jammers, such as that used security teams to block the signal from an explosives detonator, work by emitting a broad spectrum of powerful radiowaves to drown out those from potential threats. I've also heard that sensitive or poorly built radio triggers are known to have been activated by signals emmited by devices other than that of the intended trigger, such as the signals from police radios. What I am wondering is, with that in mind, what stops the signals from the jammer activating the device?

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  • $\begingroup$ Note that jamming in general is illegal, and jamming in specific is only relevant to amateur radio in the context of direction finding the jammer to get it shut down. $\endgroup$
    – user10489
    Jan 14 at 0:19

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Nothing.

You are correct that jammers generate powerful signals over either a narrow or broad swath of the radio spectrum, depending on the goal. This can even be inadvertent. If you are an amateur radio operator, a great example would be the over-the-horizon radar systems (read more here).

Whether a device will be triggered by a powerful radio signal at some frequency really depends on the specific device, which you allude to in your question regarding the quality of the device. If the receiver is simply looking for any powerful signal on a frequency and it does not have great discrimination, a sufficiently powerful signal that is nearby or potentially at a harmonic can absolutely trigger that circuit. On the other hand, if the device is more like a cell phone or similar, it is very unlikely that this could be inadvertently activated since it requires radio communication with a handshake to the device before it would do anything at all.

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From a practical point of view with your final question of "What stops the signals from the jammer activating the device?" an important thing to keep in mind is RF receivers will tend to try to "lock on" to any signal and often have an automatic gain control (AGC) section that tries to amplify the signal until something is picked up.

What happens in the usual case of no signal is being transmitted? They end up picking up random noise from the environment so you end up with a random data stream. It's the same as when a cheap transistor radio receives noise when tuned between radio stations.

To negate this any practical RF device will normally transmit a preamble of fixed data to give the receiver time to lock and as a sort of preliminary check, followed by some data and finally a checksum or repeat of the data so the receiver can verify it's a valid packet of data. For anything remotely important there's a couple of problems with this:

  1. The same data packet is going to activate any device within range, so they might want to include a relatively long unique ID for each device.

  2. If someone uses a receiver to record what a device, say your garage remote control, sends they can simply replay the data later to open your door.

Most modern remote controls will use a rolling code to stop that sort of replay attack. I'm not sure specifically what your example of explosives detonators use but I'd sort of hope commercial ones have a fair level of cryptographic security to link the transmitter to the receiver. For "home grown" devices of course anything is possible.

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What I am wondering is, with that in mind, what stops the signals from the jammer activating the device?

This has been a concern with the use of cellular activated bombs for some time. Once it was discovered that bombs have been activated by cellular phones the tactic of removing cellular service (which can be by jamming but could be by other means) has proven futile since the bombs could be triggered by entering a space where cellular service was lost.

This same concept has been used in anti-aircraft missiles. A missile guided by radar can lose its target by enough radio noise in the vicinity. The counter to this tactic is "home to jam" where if the missile loses its ability to navigate by radar because of a strong jamming signal then it switches to a mode of seeking out the strongest radio emitter, which is often the aircraft it is targeting. If by chance the jamming is not the original target then it is still something that would be a valid target to destroy, so long as it is a target inside the range of the missile.

Radio jammers work by putting in enough energy in the RF band of interest that any receivers can no longer pick out the weaker signal of interest. For something like GPS the signals are quite weak as they are transmitted by satellites that run on solar power. The satellites are far away, have limited power, and in order to give accurate position data would need to be of a relatively high frequency. It is because of jamming of GPS that there's renewed interest in ground based radio-navigation, systems that can operate with much higher power. Because of the limits of propagation on high frequencies any ground based radio-navigation system would have to operate at relatively low frequencies, frequencies low enough that they can "bend" over the horizon. With the lower frequencies the accuracy would suffer but it is better than nothing, or at least can provide a "sanity check" on navigation by inertial systems, GPS, or other navigation that could provide greater precision. If the two systems deviate too much then that indicates the higher precision system is being "spoofed" (a false signal than a jammed signal) and therefore should be ignored in favor of the less precise navigation.

A radio jammer activating bombs isn't always a bad thing. Consider a military convoy where the lead vehicle contains a radio jammer. If the device is activated by a loss of signal then presumably it will explode before the convoy is in a range were the weapon poses any danger. If the bomb needs some precise code to activate then the jammer will prevent reception until the convoy is clear of the radio activated bombs. An obvious counter to this is some kind of "home on jam" weapon that seeks out any attempts to jam the activation signals, but such weapons would be quite complex and out of the reach of poorly funded armies.

In the example given of a radio activated by a police radio then that is some kind of unique threat. The people planting the bomb would need some kind of idea on the frequency and power of the target radio so as to prevent a trigger by something other than the target, and prevent a trigger by a radio outside the effective range of the weapon. I could go on but I fear this is already getting to be a long answer and I'd be treading into the territory of a lesson on how to build weapons against the police.

The summary of this is that radio jamming works by putting so much noise in the RF band of interest that the receiver can no longer pick out the signal of interest. In the case of a weapon triggered by radio signal there's the potential to be triggered by the jamming attempt, by a remote radio activation, or potentially either. Which kind of trigger to use depends on how the target is expected to act when this is a known threat.

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