Hammarlund Vintage Air Variable Capacitor 2.14-6.5 pF Bench Tested RF Tuning
$39.00
Vintage Hammarlund CA-291 heavy-duty air-variable RF capacitor with rugged metal plate construction, insulated mounting structure, screw terminals, and precision mechanical adjustment.
The CA-291 is documented in U.S. Signal Corps technical literature as a variable-air capacitor with a specified capacitance range of 6-100 pF. Signal Corps documentation identifies it as stock number 3D291 and specifies 27 plates, 0.008-inch minimum spacing between rotor and stator plates, isolated construction, approximately 1-7/8-inch overall length, and special contact spider plates and wiper contacts originally specified as silver or nickel plated.
This capacitor was used as an RF and oscillator trimmer in military communications receivers including the BC-312 and related BC-342 family. Documented applications include Band B and Band C first RF, second RF, detector, and RF oscillator tuning positions.
The unit is clearly manufacturer marked HAMMARLUND and CA-291.
PHOTO NOTE: Earbud shown in the photographs is for size and scale reference only and is not included with the capacitor.
LIFECYCLE STATUS: OBSOLETE
HISTORICAL BACKGROUND:
Hammarlund was an important American manufacturer of radio receivers, variable capacitors, and precision RF components during the classic communications-radio era. Hammarlund components became well known for rugged construction and were used in commercial, amateur, laboratory, and military-era radio equipment.
The CA-291 is documented in period U.S. Signal Corps technical manuals as a 6-100 pF variable-air trimmer used throughout the RF and oscillator sections of military communications receivers. Its substantial mechanical construction, closely controlled plate spacing, isolated design, and precision contacts reflect the requirements of radio equipment designed for dependable field and communications service.
Original Hammarlund CA-291 capacitors are now obsolete vintage components suitable for restoration of period equipment as well as RF experimentation and collector applications.
Origin Tracker
Obsolete Stock
Legacy or discontinued stock no longer in normal production flow. Future supply may be limited.
Pipeline quantities are informational and may change after arrival, inspection, sorting, testing, or donor teardown.
Compatible Radios / Boards
Suitable for vintage radio restoration, RF tuning circuits, oscillator circuits, antenna matching projects, transmitter and receiver experiments, vintage communications equipment, test equipment restoration, and custom RF projects requiring a low-value adjustable air-variable capacitor. Buyer must verify capacitance requirements, voltage requirements, mounting dimensions, shaft configuration, terminal arrangement, and physical fit before installation.
RF Boneyard Bench Notes
BENCH TESTED
Test Instrument: DER EE DE-5000 LCR Meter
Test Mode: Cp
Test Frequency: 100 kHz
Test Points: Between the two screw terminals
Minimum Measured Capacitance: 2.14 pF
Maximum Measured Capacitance: Approximately 6.5 pF
Capacitance changes with mechanical shaft rotation.
Maximum reading stabilized at approximately 6.5 pF.
Visual inspection shows intact plate structure with age-related oxidation, discoloration, and cosmetic wear consistent with vintage hardware.
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