1248HM Radar Tube

The 1248HM (also designated as the 12/48HM) is a 12-inch diameter, flat-faced cathode ray tube designed specifically for radar applications requiring fine symbol presentation and high resolution. It utilizes magnetic focus and magnetic deflection systems to achieve precise beam control. The tube is characterized by its specialized orange-fluorescing phosphor screen and its robust physical construction, making it suitable for demanding display systems.

Screen and Phosphor Characteristics

The 1248HM features a metal-backed screen utilizing a Type 'H' phosphor. This configuration is optimized for radar display applications with specific visual persistence requirements:

• Fluorescence: Orange
• Afterglow: Orange
• Persistence: Very long

Because of the high-persistence characteristics of the Type 'H' phosphor, the screen is susceptible to damage under certain operating conditions. The phosphor is liable to burn even at low values of beam current if operated with a stationary or slow-moving spot.

The 1248HM is closely related to the 12/48L3M, which differs primarily in its screen characteristics. The 12/48L3M uses a Type 'L3' phosphor with orange fluorescence, orange afterglow, and long persistence. This alternative phosphor is designed to enable flicker-free images at repetition frequencies down to 10 cycles per second, while curtailing the persistence of moving targets compared to the Type 'H' phosphor.

Physical Dimensions and Mechanical Construction

The mechanical design of the 1248HM incorporates a flat faceplate and a precise glass envelope. The tube uses a B12A (Duodecal) base and a CT8 cavity-type recessed anode cap for the high-voltage second anode connection.

The physical dimensions of the tube are highly specified to ensure proper alignment within radar display consoles:

The nominal neck diameter is 35 mm, with a tolerance of +1.5 mm and -1.0 mm (1.378 +0.020 / -0.039 inches).

Electrical Connections and Pinout

The B12A Duodecal base provides electrical connections to the heater, cathode, control grid, and first anode. The second anode is connected via the side-mounted CT8 cavity cap. The pin configuration, viewed from the underside of the base, is as follows:

• Pin 1: Heater
• Pin 2: Grid
• Pin 3: No Pin
• Pin 4: No Pin
• Pin 5: No Pin
• Pin 6: No Connection
• Pin 7: No Connection
• Pin 8: No Pin
• Pin 9: No Pin
• Pin 10: 1st Anode
• Pin 11: Cathode
• Pin 12: Heater
• Side Cap: 2nd Anode

Heater and Maximum Ratings

The 1248HM operates with an indirectly heated cathode. The electrical limits and ratings represent absolute thresholds that must not be exceeded during operation.

Heater Specifications

• Heater Voltage: 6.3 V
• Heater Current: 0.3 A

Absolute Maximum and Minimum Ratings

• Maximum First Anode Voltage: 600 V
• Minimum First Anode Voltage: 250 V
• Maximum Second Anode Voltage: 15.5 kV
• Minimum Second Anode Voltage: 9.0 kV
• Maximum Cathode Current: 150 µA
• Maximum Heater-to-Cathode Voltage ($V_{h-k}$): 200 V
• Maximum Heater-to-Cathode Resistance ($R_{h-k}$): 1.0 MΩ
• Maximum Grid-to-Cathode Resistance ($R_{g-k}$): 1.5 MΩ

Typical Operating Conditions and Focusing

Under typical operating conditions, the 1248HM is configured to balance brightness, focus, and resolution. The typical operating parameters are defined as follows:

• First Anode Voltage: 300 V
• Second Anode Voltage: 15 kV
• Grid Voltage ($V_g$) for Visual Cut-off: -90 V
• Average Grid Drive (for $I_b = 50$ µA): 30 V
• Line Width (at $I_b = 50$ µA): 0.25 mm

Focusing System Alignment

To achieve the specified high resolution and a line width of 0.25 mm, the external magnetic focus coil must be precisely positioned. The focus coil should be placed so that the center of its gap is approximately 220 mm from the reference line, which is established at the 36 mm Ring Gauge Position.

Interelectrode Capacitances

The internal capacitances of the tube electrodes relative to all other connected elements are kept low to maintain signal integrity:

• Cathode to all other electrodes ($C_{k-all}$): < 8.0 pF
• Grid to all other electrodes ($C_{g-all}$): < 8.0 pF

Technical Summary

The 1248HM is a specialized 12-inch flat-faced radar cathode ray tube featuring magnetic focus, magnetic deflection, and a very long persistence Type 'H' orange phosphor screen. Operating with a 6.3 V, 0.3 A heater, the tube is designed for high-resolution symbol and target presentation. It requires a first anode voltage of 300 V and a second anode voltage of 15 kV under typical operating conditions, delivering a narrow 0.25 mm line width. Proper operation requires careful positioning of the focus coil and precautions against phosphor burning during stationary or slow-moving spot conditions.