Ham Radio Simplex vs. Duplex — Explained Simply

Simplex — Direct Radio-to-Radio

Simplex operation means both stations transmit and receive on exactly the same frequency. No repeater is involved — it is a direct path between two radios. The national 2-meter FM simplex calling frequency is 146.520 MHz. The 70cm simplex calling frequency is 446.000 MHz. These are the channels most hams monitor when operating simplex.

Simplex range is limited to line-of-sight plus some terrain bending — typically 2–15 miles with handheld radios depending on antenna height and terrain. Good for: nearby stations, events where participants are in the same area, or field operating where a repeater is not needed.

Half-Duplex — How Repeaters Work

Repeater operation uses half-duplex: you transmit on one frequency (the input), and the repeater retransmits on another (the output). You receive on the output. Your radio handles this automatically when you set the offset correctly — you do not manually switch frequencies while transmitting. This is called half-duplex because you cannot transmit and receive simultaneously (only one direction at a time).

Full Duplex

Full duplex means transmitting and receiving simultaneously on different frequencies — like a phone call. Some specialized amateur radio applications use full duplex (certain satellite contacts and repeater linking systems), but most amateur radio HTs and mobile radios are half-duplex. Full duplex requires hardware that can simultaneously handle two RF paths without the transmit signal overwhelming the receiver.

Simplex Calling Frequency Protocol

146.520 MHz is a calling channel, not a conversation channel. The protocol: call CQ or announce yourself on 146.520, make contact with another station, then both move to another simplex frequency (like 146.460 or 146.550) to have the actual conversation. This keeps the calling frequency clear for others to initiate contacts. On busy repeaters, a similar protocol applies — but most local repeaters are conversational and do not enforce this as strictly.

• Can I talk simplex through a repeater?
  No — that is a contradiction. A repeater by definition uses duplex (separate input/output frequencies). Simplex means no repeater. If you are using a repeater, you are operating duplex. If you are communicating directly between two radios on one frequency, you are operating simplex.
• When should I use simplex instead of a repeater?
  Use simplex when: you are communicating with a station that is close enough for direct contact (same event site, nearby neighborhood), you want to conserve the repeater for others, or the repeater is busy with other traffic. Simplex also works when no repeater covers your area — hiking in remote terrain, for example, where you and a companion are within line-of-sight distance.