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You press a button, speak, let go, and your friend's handset crackles with your voice. A walkie-talkie is a radio transmitter and receiver in one box, and three ideas explain it: one shared frequency, push to talk, and a range set by what the wave can reach in a straight line.
How do walkie-talkies work?
A walkie-talkie is a half-duplex communication device: several handsets share a single radio channel, and only one of them can transmit at a time (Wikipedia: Walkie-talkie).
Choosing a channel means every set agrees on one carrier frequency. When you speak, the microphone turns your voice into an electrical signal, that signal varies (modulates) the carrier wave, and the antenna radiates it. Every other set tuned to the same channel picks up the wave, strips off the carrier and sends what is left to its speaker.
Pressing the push-to-talk (PTT) button turns off the receiver and turns on the transmitter, according to the same article. Let go and the set flips back to listening. That single switch is why you cannot hear a reply while your thumb is still on the button.
What does half duplex mean?
A half-duplex system carries messages in both directions, but only one direction at a time (Wikipedia: Duplex). A full-duplex system lets both people talk at once, as on a phone call. The same article uses the walkie-talkie as its example of a half-duplex system, and notes that users say "over" to mark the end of a transmission.
Taking turns keeps a handset simple: to send and receive at once, it would need to stop its own transmitter drowning out the far fainter signal it is trying to hear. If two people press together, neither hears the other, because both receivers are off.
Why is walkie-talkie range limited by line of sight?
Above 30 MHz (VHF and higher), radio waves near the ground travel in more or less straight lines, so an obstruction between the two antennas blocks the signal (Wikipedia: Line-of-sight propagation). India's licence-free walkie-talkies use the 446 MHz band (Licence-exemption Rules, 2018, Table V), well inside that range.
Even with nothing in the way, the Earth curves away beneath the signal. For an antenna h metres high, the radio horizon is about 4.12 times the square root of h, in kilometres, a figure that includes the usual allowance for the atmosphere bending radio waves slightly (Wikipedia: Line-of-sight propagation). Without that bending, pure geometry gives d = √(2Rh), which works out to about 3.57√h km. Two radios can reach each other while their horizons meet, so you add the two distances.
Here is that 4.12√h formula at work, from the street to your terrace:
Two friends on flat, open ground, each holding a set about 1.5 m up: d = 4.12 × (√1.5 + √1.5) ≈ 4.12 × 2.45 ≈ 10 km.
One friend on a terrace with the antenna 30 m up, the other still at 1.5 m: d = 4.12 × (√30 + √1.5) ≈ 4.12 × 6.70 ≈ 28 km.
Raising one antenna twenty times higher less than triples the ceiling, because range grows with the square root of height, not with height itself.
These are ceilings set by geometry, not promises. A low-power handset usually falls well short: walls, trees, hills and even your body absorb or reflect the signal. A friend in a basement can lose contact with someone on the street; our post on why mobile signal drops inside a lift shows how a metal box blocks radio. For how other short-range radios trade frequency against power and range, see Bluetooth vs Wi-Fi: frequency, power and range explained.
If the square-root shape still feels abstract, Fermi's AI tutor can draw the figure of the curved Earth and the tangent line with you, and check your understanding as it goes.
Are walkie-talkies legal in India?
India's 2018 licence-exemption rules allow Personal Mobile Radio handsets in the 446.0-446.2 MHz band at up to 500 mW effective radiated power (Licence-exemption Rules, 2018, Table V).
The same table sets conditions: hand-held sets with integral antennas only, no base station or repeater, and short-range peer-to-peer use. Rule 3 grants the exemption on a non-interference, non-protection and shared basis, so strangers may turn up on your channel and nobody is owed a clear one.
Before buying, check the box: a set rated at several watts, or with a removable long antenna, is outside these rules.
How can you test this yourself?
A pair of legal handsets makes a good weekend physics experiment.
Set both to the same channel and stand side by side. Both press PTT together and confirm that neither hears the other.
Walk apart along an open road or ground and note where speech first breaks up.
Repeat with one person at the top of a building or stairwell.
Repeat with a building between you.
Work out the horizon distance for each case with the formula above and compare. The gap between prediction and measurement is the effect of obstacles and low power.
When a result surprises you, explain it in your own words before you look it up.
