On-the-air-indicator

Description: The circuit is a simple RF-actuated switch that responds to any strong electromagnetic field in the vicinity of the pickup wire. The length of the wire will depend on the required coupling, but a 250 mm length wrapped around the outside of the coaxial cable feeding the antenna should be sufficient for most power levels. If only one band is utilized, the wire can be made a resonant length—495 mm for operation in the 144 MHz band, for example. When RF energy is detected by the device, diode D1 will conduct during the negative half-cycles but will be cut off during the positive half-cycles. This results in a net positive voltage at the base of transistor TR1, forward biasing it into conduction. In single-sideband (SSB) and continuous wave (CW) transmissions, where the transmission is not continuous, this bias would constantly vary, causing relay RLA to chatter. However, capacitor C2 stabilizes the bias voltage until a long gap in transmissions occurs.

The RF-actuated switch circuit operates by utilizing a pickup wire that detects electromagnetic fields generated by RF signals. The design allows for flexibility in wire length based on the desired coupling efficiency, with a standard recommendation of 250 mm for general applications. For specific frequency bands, such as the 144 MHz band, the wire length can be adjusted to a resonant length of 495 mm to enhance performance.

The circuit employs a diode (D1) that plays a crucial role in converting the alternating RF signal into a usable DC signal. During the negative half-cycles of the RF waveform, D1 becomes forward-biased and conducts, allowing current to flow. Conversely, during the positive half-cycles, the diode is reverse-biased and does not conduct, which results in a net positive voltage being present at the base of the transistor (TR1). This configuration effectively enables the transistor to be turned on when RF energy is detected.

In scenarios involving SSB and CW transmissions, where the signal is not continuous, the resulting fluctuations in bias voltage could lead to undesired operation of the relay (RLA), causing it to chatter. To mitigate this issue, capacitor C2 is incorporated into the circuit. This capacitor serves as a smoothing element, maintaining a steady bias voltage at the base of TR1 during brief interruptions in the RF signal, thus preventing rapid on-off cycling of the relay and ensuring stable operation.

Overall, this RF-actuated switch circuit is designed to be responsive to RF energy while minimizing issues related to signal variability, making it suitable for various RF applications. Proper selection of component values and wire lengths is essential for optimizing performance in specific use cases.The circuit is a simple rf-actuated switch which will respond to any strong field in the region of the pickup wire. The length of the wire will depend on how much coupling is needed, but a 250-mm length wrapped around the outside of the coaxial cable feeding the antenna should suffice for most power levels.

If only one band is used, the wire can be made a resonant length-495 mm for 144 MHz band operation for example. When rf energy is picked up by the device, diode Dl will conduct on the negative half-cycles, but will be cut off on the positive half-cycles. The result will be a net positive voltage at the base of transistor Trl, forward biasing it into conduction.

On ssb and cw transmissions, where the transmission is not continuous, that bias would be constantly varying and the relay RLA would chatter. However, capacitor C2 holds the bias voltage steady until a long gap in transmissions occurs.