RCA BHF-1A

GENERAL DESCRIPTION


H.F. BROADCAST TRANSMITTER, BHF-1A - 1 KW HIGH FREQUENCY BROADCAST TRANSMITTER

photo RCA BHF-1A FEATURES DESCRIPTION

The RCA BHF-1A transmitter combines all the best features and accumulated knowledge of 25 years experience in the design, manufacture and operation of broadcast and high frequency transmitters.

The transmitter is compact, coming in ONE CABINET which requires 7.5 sq. feet (.7 sq. meters) floor space. The transmitter is completely tested at the factory. This REDUCES installation and test time in the field.

The only building requirements are:

 Floor Area-Load requirements 200# sq. ft. or 2140# sq. meters

 Incoming power and audio lines

 Outgoing R.F. transmission lines

The entire transmitter is housed in a lightweight aluminum cabinet with a steel base. This cabinet is 84" high, 33" wide and 32 9/16" deep. It is equipped with sliding doors that never extend beyond the cabinet, thus minimizing floor space requirements. The sliding doors are interlocked. AUTOMATIC HIGH VOLTAGE GROUNDING is also provided for the protection of operating personnel.

Shielding

All circuits are enclosed for personnel protection. SHIELDING is complete and no additional screening is required to SUPPRESS UNWANTED RADIATION.

Power Circuits

The lower half of the cabinet contains three power supplies. The main rectifier tubes, 8008's, supply the high voltage for the PA and modulator tubes. The low voltage rectifier utilizes two RCA 866A/866 tubes, and bias is supplied from a selenium bias rectifier. These rectifiers and the tube filaments require a 230-volt single phase supply.

Overload Protection

CIRCUIT PROTECTION is provided by HIGHT SPEED CIRCUIT BREAKERS and a plate contactor. The filament and plate circuit breakers serve as control switches. Circuit breakers in the cathode circuit of the power amplifier, modulator, and ground return of the low voltage and bias rectifier, operate the plate contactor. All plate and bias voltages are removed when an overload occurs. Provision is made for automatic and instant return of the transmitter to the air after a power line interruption of up to two seconds duration. If the power interruption is over two seconds duration, the transmitter will return to the air automatically, thirty seconds after the power line interruption is over.

Radio Frequency Circuits

The frequency source contains a type 6AK5 oscillator, type 6AG7 buffer, 6AG7 multiplier. Five crystal frequencies may be pre-tuned and are then available for instant selection with a five-position switch.

The signal is amplified by a 6146 amplifier which provides ample drive for the two type 4-400A tubes in the power amplifier. Using tetrode tubes in all radio frequency amplifiers provides high power gain and eliminates the need for neutralization adjustments.

Output Circuits

A tank circuit of the pi-filter type provides sufficient harmonic suppression and impedance matching. If a balanced transmission line is used a balancing bridge is inserted between the pi-filter and the line.

Modulator

In the three modulator stages the progrom input signal is raised to the level needed to modulate the power amplifier. A push-pull voltage amplifier and a push-pull cathode follower use two 6146 tubes each. In the Class B modulator two type 833A triodes are employed. Output from the modulator is coupled to the power amplifier through the usual transformer-reactor combination. Inverse feedback is applied from the modulator plates through an R-C network to the grids of the input amplifier.

Cooling

A single blower provides the pressure for the forced-air cooling of the power amplifier tubes and also ventilates the entire cabinet. An air-operated relay cuts all high voltages if the blower is not operating properly.

RCA BHF-1A diagram

TECHNICAL SPECIFICATIONS
Type of Emission A 3
Output Frequency Range 3.9 to 26.1 mc/s
Rated Power Output at Transmitter Terminals 1 kw, carrier
Output Load Impedance 50 to 230 ohms unbalanced
300 to 600 ohms balanced
Resistive ±JO.1R
Spurious Frequency Radiation -40 db
Frequency Stability 0.003%
Type of Modulation High-level Class B
Program Input Impedance 150/600 ohms
Program Input Level +10 dbm ±2 dbm
Audio Frequency Response ±2 db, 30 to 10,000 cps
(1030 cps, 60% reference)
Modulation Capability 400 cps:100%
50 to 7500 cps: 90% minimum
Envelope Distortion (1000 cps, 90% mod.) 4%
Noise Level, Unweighted (below 100% mod.) 52 db
Carrier Shift Up to 100% Modulation Less than 5%
Power Consumption
 Unmodulated
 40% Modulation
 100% Modulation
2900 watts, approx.
3100 watts, approx.
3900 watts, approx.
Power Factor .9
Power Line Requirements
 Transmitter
 Cabinet Lights
230 volts, single-phase, 60 cy., available also for 50 cy.
115 volts, single-phase, 50/60 cy.
Permissible Power Line Variation 5%
Ambient Temperature Range +10°C to +45°C
Elevation 8000 feet maximum
Transmitter Height 84"
Transmitter Width 33"
Transmitter Depth 32 9/16"
Transmitter Floor Space 7.5 sq. ft.
Transmitter Weight (Unpacked) 1500 lbs. (approx.)
Specifications may change without notice

TUBE COMPLEMENT
RF stages AF stages and modulator Rectifiers
Number Type Number Type Number Type
2 4-400A 2 833A 2 8008
1 6146 4 807 2 866A


THIS TYPE OF TRANSMITTER IS INSTALLED IN THE FOLLOWING COUNTRIES

ITU Country
ITU Country