1B24A — Tunable Gas TR Switch Tube
The 1B24A (also designated as TR 1 B 24 A) is an integral-cavity, tunable narrow-band gas Transmit-Receive (TR) switch tube designed for operation in the X-band microwave frequency range. Its primary function is to provide receiver protection in pulsed radar systems during high-power transmitter pulses, while allowing low-attenuation transmission of weak echo signals during reception.
Principles of Operation and Applications
In pulsed microwave radar systems, a common antenna is typically shared between the transmitter and the receiver. The 1B24A is designed for use in duplexer circuits where a fixed electrical position for the short circuit is not required.
During high-power magnetron transmission, the gas within the tube ionizes, causing the device to act as a parallel short-circuit across the receiver's transmission line and in series with the antenna line. This effectively blocks high-power RF energy from reaching and damaging the sensitive receiver components. Once the transmission pulse ends, the gas deionizes rapidly. During the subsequent reception period, the tube presents low attenuation, allowing the weak incoming echo signals from the antenna to pass through to the receiver with minimal loss.
Electrical and Operating Characteristics
The 1B24A operates across the X-band frequency spectrum, covering a range from 8,490 to 9,600 MHz, with a nominal operating frequency of 9,375 MHz. The tube features an integral cavity with a maximum loaded Q of 350, measured at the nominal 9,375 MHz frequency.
To facilitate rapid ionization when the transmitter fires, the tube incorporates an ignitor (or igniter) electrode. The electrical parameters of this ignitor circuit are defined as follows:
- Ignitor Supply Voltage (Open-Circuit): -750 V to -1000 V (with a maximum starting voltage of -1000 V).
- Ignitor Current: 100 µA minimum to 200 µA maximum.
- Ignitor Voltage Drop: 325 V minimum to 450 V maximum, measured at an ignitor current of 100 µA.
- Ignitor Interaction: 0.2 dB maximum at 100 µA ignitor current.
Performance Under Test Conditions
The operating performance of the 1B24A is characterized under specific pulsed RF conditions: a peak power of 10 kW, a pulse width of 0.5 µs, a pulse repetition frequency of 1,000 Hz (or pulses per second), and an operating frequency of 9,375 MHz, with the ignitor current set to 100 µA. Under these conditions, the tube exhibits the following characteristics:
- Maximum Leakage Power: 30 mW.
- Maximum Deionization (Recovery) Time: 4 µs (measured for 3 dB attenuation / 3 dB down).
- Insertion Loss: 0.85 dB minimum to 1.50 dB maximum at 9,375 MHz. At zero electrode current, the maximum insertion loss is specified up to 2 dB.
Mechanical, Tuning, and Environmental Specifications
The 1B24A is constructed with an integral cavity and is designed to allow physical tuning across its operating band.
Tuning Mechanism
Frequency tuning is accomplished via an onboard tuning screw. The tuning mechanism provides a range of 5 turns, which is sufficient to adjust the resonant frequency of the cavity across the entire 8,490 to 9,600 MHz operating band.
Physical Dimensions and Mounting
The tube is designed for flexible installation and can be mounted in any position (indifférente), both in terms of its physical orientation and its position within its support structure. Its physical outline is characterized by the following dimensions:
- Maximum Height: 3-1/4 inches (approximately 82.6 mm).
- Maximum Body Diameter: 1.760 inches (approximately 44.7 mm).
- Top Cap Diameter: 0.250 inches ± 0.005 inches.
- Reservoir Projection: Designed to fit within a 1-1/4 inch diameter cylinder co-linear with the tube's main axis.
- Iris Assembly: Does not extend beyond the outer surface of the body.
- Exhaust Tabulation: Does not extend beyond the outer periphery of the device.
- Weight: Approximately 150 grams (about 1/4 lb).
Environmental Limits
For storage and non-operating conditions, the 1B24A is rated to withstand ambient temperatures ranging from -40 °C to +100 °C.
