Basically, not actively transmitting on your radio for a certain period of time or in a certain area unless you have a very good reason. There are three basic reasons for this:
- The enemy might be listening.
- The enemy might not know what you're saying, but they can locate where you are from Radio Direction Finding (RDF) kit.
- The enemy might know that you're there and can make a broad guess from the frequencies, lengths and volume of traffic as to what is going on.
This also applies to the broader aspects of radar transmissions and also applies to similar fantasy devices.
See also Loose Lips.
- Fail Safe, where the Vindicator bombers are ordered not to transmit once they receive the Go-Code.
- In Star Wars: The Phantom Menace, it was vital that the Queen's shuttle not respond to the distress signals from Naboo to prevent giving away their location and exposing Queen Amidala to Trade Federation forces. Darth Maul was able to track them down but he was a Sith apprentice and his master was the Naboo delegate to the Senate.
- In Battle: Los Angeles, the Marines have to observe radio silence once the realize that the aliens are able to home in on their radio transmissions. They use this fact to lure an alien drone into investigating a radio planted in a petrol station, and then chuck a grenade into the station. Kaboom.
- Spoofed in Get Smart, when Siegfried orders radio silence to shut Shtarker up.
- Capt. Ramius' forged orders in The Hunt for Red October specify that the eponymous submarine is to observe strict radio silence. It quickly becomes clear that his real goal is to go AWOL and defect to the United States with the submarine's technology, triggering the hunt of the title.
- In Dr. Strangelove, General Ripper issues radio silence on his airbase and confiscates all radios, making his men think that they're under attack. After Captain Mandrake finds a radio and discovers that civilian stations are still broadcasting music, he gradually begins to realize that there is no emergency at all, and Ripper is deliberately trying to fool the airbase into starting World War III.
- In Harry Potter and the Deathly Hallows The Death Eaters are able to find the Trio by putting a spell that alerts them when someone says "Voldemort", which Harry, Ron and Hermione generally do
- They're able to do this using the same system the Ministry of Magic used to detect anyone using one of the forbidden curses.
- The Roanoke colonists are forced to do this in The Last Colony by foregoing the use of their technology. Note that this requires far more than simply not making radio transmissions—much of their technology relies on wireless communications, even between different components of the same item.
- In the Shannara series, any use of magic tends to reveal the user's location to his or her enemies. The more powerful the magic, the more likely it is to be noticed; using the Elfstones is like setting off a flare. In the later novels, the Druids' headquarters at Paranor has a scrye pool that can detect magic use throughout the Four Lands not only in the present, but in the recent past as well.
- In the Star Trek: The Original Series episode "Balance of Terror", the Romulan ship heads home under cover of a cloaking device and comm silence. Unfortunately for them, one of the officers violates orders to call home base to report the success of their mission, and the transmission is detected.
- An episode of The West Wing revolves around this: Washington loses track of a submarine off the coast of North Korea and is uncertain as to whether it's been destroyed or is just keeping silent.
- Harpoon has a rule that means a search radar will be detected by anyone in range, giving them bearing and type of the radar instantly, narrowing down your location and ship type quite well.
- The Star Trek: The Next Generation Interactive VCR Board Game uses this as a gameplay mechanic involving a set of pin-backed communicator badges. If you didn't want the Klingon holding the Enterprise hostage to know where you were, you had to take your communicator off. On the other hand, when he calls you have to put the badge back on or risk losing time.
- Company of Heroes features this as a doctrinal ability for one of the Oberkommando West commanders in the second game. At the cost of some munitions, your units disappear from the enemy's minimap... And then you hear absolutely no radio traffic from any of your units or rearguard spotters.
- You can order this in Wing Commander. Whether your wingman obeys depends on the individual. If it's Maniac? Forget it, outside of the final mission series in WC3 where he obeys every order.
- A few missions across the Ace Combat series impose this for storyline reasons, sometimes combined with altitude restrictions or avoiding enemy radar. At most, this is just a break from your wingmen's chatter.
- It is a good idea to refrain from issuing excessive orders in Flashpoint Campaigns, as it will generate large amounts of radio traffic for enemy direction finders. The intercepted traffic may have enough info for the enemy to start shelling your field headquarters in a matter of minutes.
- Done in Red vs. Blue, but for different reasons than normal—they were trying to get rid of the AI O'Malley, who is able to Body Surf through radios. The plan was to maintain radio silence so once they forced him out of his host body, he'd have nowhere else to go. This being Red vs. Blue, of course it doesn't work.
- Averted in the Battle of the Atlantic, where Doenitz micro-managed his U-Boats and made them report their positions on a daily basis, all their contacts and general status. This proved a boon to Allied RDF and their codebreakers, and the combat ships using High Frequency Direction Finding or "Huff Duff" technology to detect the transmissions, trace them to their origin and attack the U-Boats.
- This is also how the German battleship Bismarck was found, after the British cruisers shadowing it lost contact. The captain of the Bismarck and Hitler were talking over the radio in regards to the captain's birthday. Without those transmissions, it's most likely the Bismarck would have survived to later slip into the south Atlantic, and wreak havoc on Allied shipping.
- The tradition continues today; Western warships at least have A LOT of passive sensors that will enable them to determine where their enemies are (or at least in what direction) and sometimes WHAT they are (depending on the specific nature of the emissions). Radiate too long and you will tell that NATO frigate in what direction you lie and how many missiles it will likely take to sink you.
- Ballistic missile submarines go silent when on patrol.
- This is the reason for something called "microburst transmissions", where you record your message, compress it and send it in a fraction of a second.
- Radio is just part of EMCON (EMissions CONtrol) protocols in most navies.
- There are numerous models of what are confusingly (for laypersons) called "anti-radiation missiles" - in fact, what they are is passive-homing weapons designed to locate and follow radar and radio transmissions to their source. Early models had to be pre-chosen according to the frequencies of the enemy radars expected to be encountered and frequently lost their way if the radar shut down. The newer ones have 'memory' of where the target was and broadband seekers that can be optimised in flight, plus numerous other tricks.
- Of course, in the case of the plane trying to blow up enemy radars, commonly referred to as a Wild Weasel, the goal was typically to clear the way for more vulnerable bombers to get through unmolested. If the enemy shut off their radar, they couldn't shoot at the bombers. It wasn't unheard of for bombers that lacked support from Wild Weasels to pretend to be Wild Weasels, and trick the enemy Anti-Air personnel to shut off their own radars.
- Common in pirate radio, as broadcasting has to be limited so regulatory bodies like the FCC don't have time to triangulate the signal. The station is normally built into a van or trailer so the location can be changed periodically.
- This is how Cold War Soviet tank units operated, combined with formation drills, arm and flag signals when appropriate. One tactic used was for signalers to play tape recordings of their normal radio traffic, while the rest of the unit moved to an attack position under radio silence.
- In both civilian and military IED investigation and disposal work, radio silence is sometimes ordered in case the IED's trigger is operating on the same frequency as one of the radios.
- For the same reason, you'll find a lot of no-radio warnings (including cellphones and other communication) at construction sites where blasting is taking place. Electromagnetic waves and errant wires are a great way to generate a wee bit of electricity, and when those wires are connected to things like detonators...yeah, you do the math.