Radio Transmitting Set AN/FRT-5, is designed for shore-based radio telegraph or frequency-shift operation in the frequency range 4000 to 26,000 kilocycles and with a power output of 15 kilowatts over the entire frequency range. The transmitter can be keyed at speeds up to 400 words per minute, with an average of five characters per word, and still maintain good wave form. The output circuit is designed to work into a balanced two-wire transmission line having an impedance of 600 ohms with a standing wave-ratio of 2 to 1, maximum. The principal function of the equipment is to effect communication with precision and reliability from shore-to-ship or point-to-point. The circuit is designed for A-1 emission; however, provision has been made for connecting a frequency shift keyer unit for teleprinter operation. The design of the equipment is such that satisfactory service will be obtained in any ambient temperature between the limits of 0 to 50°C (32°F to 122°F) in a relative humidity up to 95%.


Operation of Radio Transmitting Set AN/FRT-5 is based upon the principles of generating and amplifying an RF signal, interrupting this carrier signal in accordance with the intelligence to be transmitted, and radiating the interrupted RF carrier using a suitable antenna. These functions are accomplished by RF Oscillator O-92/FRT-5 or RF Oscillator O-91/FRT-5, a buffer amplifier, a first and second frequency multiplier, a driver amplifier, a final power amplifier, and an antenna circuit. If teleprinter operation is desired, FREQ SHIFT KEYER KY-45/FRT-5 is used.

The control circuit of the transmitter is fully automatic, requiring only a momentary depression of a PLATE ON button to start it through the proper sequence of operation. The automatic sequence of operation can be interrupted by the momentary depression of the PLATE OFF button. Provision is made for automatic restarting of the high voltage power supply following an overload trip-out. This restarting feature is automatically disabled for the first 30 seconds following manual start-up so that an overload will cause a lock-out during this interval. Overloads occurring subsequent to this initial 30 second period will start the automatic reclosure circuit which will in turn produce up to eight consecutive reclosures before final lock-out. Both the number of consecutive reclosures and the time during which they must occur to cause a lock-out are adjustable. A warning alarm is provided to sound an audible signal whenever the transmitter has a power supply lock-out following an overload. A pair of terminals is connected into this alarm circuit so that they are energized with 230 volts simultaneously with the operation of the alarm. This voltage may then be used to operate any external alarm-warning device which may be desired.

In the event the transmitter is not keyed for a period of 15 minutes, the entire equipment except the blowers, the oscillator and FSK heating circuits will automatically shut down. This automatic feature may be eliminated at the discretion of the operator or the time period adjusted from two to thirty minutes.


a. INTRODUCTION. - The cabinet containing this equipment is designed for forced air ventilation with adequate filtering at the intake. The transmitter may be considered in fourteen different sections as follows:

  1. Cabinet (RF UNIT T-225/FRT-5 and POWER SUPPLY PP-490/FRT-5)
  2. Exciter
  5. Buffer-amplifier, keyer stage
  6. Frequency multipliers
  7. Driver-amplifier
  8. Power amplifier
  9. Antenna network
  10. Low voltage supply
  11. Bias supply
  12. Main high voltage power supply
b. CABINET. - The majority of the components in the transmitter are an integral part of the cabinet. The RF OSCILLATOR 0-91/FRT-5, RF OSCILLATOR O-92/FRT-5, FREQ SHIFT KEYER KY-45/FRT-5, POWER SUPPLY PP-454/FRT-5, the Servo Amplifiers and the Servo Power Supply are separate chassis, which may be removed from the main cabinet for servicing. These units are mounted in a swinging rack in RF UNIT T-225/FRT-5. The blowers, relays, meters, switches, tube sockets, transformers, and other major components are mounted from or bolted to the cabinet. The frame work is of welded construction. The exterior of the cabinet is gray.

c. EXCITER. - The design of this transmitter is such that RF OSCILLATOR O-92/FRT-5, RF OSCILLATOR O-91/FRT-5 or any other source of stable r-f voltage can be used to provide excitation. Radio-frequency input to the transmitter proper is coupled by means of a coaxial line. Type RG-58/U lines and fittings are used. This coaxial input jack is located at the input to the exciter unit and is extended to the coaxial patching panel. The termination of this line is 80 ohms non-inductive capable of dissipating six watts maximum, although only 2 volts RMS in the frequency range of 2,000 to 4,300 kc is required for proper excitation.

(1) RF OSCILLATOR 0-92/FRT-5. - A crystal oscillator with a cathode-follower buffer is supplied as the standard exciter for this transmitter. This oscillator has provision for ten crystals, any one of which may be selected by means of a crystal selector switch. There are no tuned circuits in this exciter so that frequency changing consists of the selecting of the proper crystal. Output from this exciter is in the range of from two to four volts RMS into a 50 to 80 ohm load. One hundred eighty volts DC and 115 volts AC for operation of the exciter are supplied by the transmitter proper. The 115 volt input is required to supply heater and filament voltages. The crystals used are in the range 2,000 to 4,500 kc, Type AN/CR27-U in a common oven. This exciter is adequately shielded to prevent direct radiation and consequent interference to receivers operating on the same frequency during break-in or stand-by operation.

(2) RF OSCILLATOR O-91/FRT-5. - RF OSCILLATOR O-91/FRT-5 employs a MASTER OSCILLATOR which has a variable frequency range of 1000 to 1500 kc. The output of this oscillator is fed through two multiplier stages and a FINAL AMPLIFIER. The output of this FINAL AMPLIFIER is fed to the buffer of the main transmitter or to FREQUENCY SHIFT KEYER KY-45/FRT-5, whichever is desired. The stability of this oscillator is derived from a 100 kc standard, either internal or external. The 100 kc is subdivided by a DIVIDER circuit to 25 kcs and is then passed through a HARMONIC AMPLIFIER where the 25 kc harmonics are amplified in the range of 9.125 to 21.625. These frequencies are applied to one grid of a MIXER tube and a second frequency of 10 to 22.5 megacycles is applied to this same MIXER from a second multiplier stage of the MASTER OSCILLATOR circuit. The output of this MIXER contains an IF frequency of 875 to 900 kc which is amplified and applied to a second MIXER tube. A 75 to 100 kc frequency is combined with the 875 to 900 kc to make a new IF frequency of 800 kc. (The 75 to 100 kc is derived from an INTERPOLATION OSCILLATOR covering a frequency range of 600 to 800 kc and then passing through three divider circuits to arrive at 75 to 100 kc.) The resultant 800 kc IF output is divided and combined in a pair of DIODE MIXERS with a 100 kc output derived directly from the 100 kc standard. The output of the DIODE MIXERS is applied to the DC AMPLIFIERS and then to the POWER AMPLIFIERS. The signal is shifted 90° in the plate circuit of the DIODE MIXERS. This shift provides a two phase audio output, the audio frequency being dependent upon the difference in frequency of the two 100 kc frequencies. This two phase audio is used to drive the motor which adjusts the MASTER OSCILLATOR to the precise frequency. The unit is direct reading in frequency and any frequency in the range from 2000 to 4500 kc can be set up without the use of external measuring means or chart.

(3) FREQUENCY SHIFT KEYER KY-45/FRT-5. - The RF input to the keyer may be the output of RF OSCILLATOR O-91/FRT-5 or O-92/FRT-5. The input to the keyer is in the range of 2.2 to 4.2 megacycles, which is 200 kc higher than the output frequency of the keyer. As described in Section 2, paragraph 3, the frequency of a 200 kc oscillator is raised and lowered a small amount in response to mark and space telegraph signals and added to the radio-frequency output of the transmitter oscillator in the balanced modulator. The transmitter oscillator frequency is balanced out. Only the sum and difference frequencies, resulting from mixing the RF input and the 200 kc oscillator frequency, are present in the output of the balanced modulator. The plate circuit of the modulator is tuned to the lower or difference frequency, eliminating the difference frequency output. The components of the 200 kc Oscillator, Balanced Keyer and Phase Shifting amplifiers are located in an oven, the temperature of which is closely regulated at about 70°C (158°F.)

The keying circuit normally operates on polar signals, or neutral signals. In ordinary cases the negative side of the external keying circuit is at ground potential. The first limiter tube prevents negative key line voltage from affecting operation. The second limiter tube limits the maximum positive signal to the balanced keyer tube to a constant value in order to provide for variations in amplitude of key line signals. The form of the output keying voltage may be modified by series inductance and bridge capacity as controlled by a WAVE SHAPING switch. It may also be "wobbled" at about 200 cycles per second by turning on a PHASE MODULATION switch. The amount of phase modulation is varied by adjusting an associated dial. Phase modulation may tend to improve transmission under some conditions of severe selective fading.

The output voltage from the limiter tube results in mark and space signals with or without wave shaping and phase modulation and is connected to the balanced keyer tube. The full output of the balanced keyer tube, or a fraction of it, is connected to a two stage phase shifting amplifier through a point on a voltage divider selected by the transmitter MULT. FACTOR switch. The output of the phase shifting amplifier is connected to the 200 kc oscillator to increase a small amount (for example, 425 cycles for a mark signal) and to decrease a small amount (for example, 425 cycles for a space signal). The amount the frequency of the 200 kc oscillator is changed depends upon the output of the phase shifting amplifier, which in turn is influenced by the input from the balanced keyer tube.

The 200 kc oscillator is a self-excited single-ended type, the frequency of which is changed by the output of the reactance amplifier and to a limited extent by a variable capacitor designated as 200 KC OSC. ADJUST. The output of the 200 kc oscillator is frequency modulated by telegraph mark and space signals, with or without super-imposed wave shaping and phase modulation. This frequency modulated 200 kc oscillator output is combined in the balanced modulator with the frequency of the transmitter (or keyer) oscillator.

The output of the balanced modulator is amplified in two stages consisting of a buffer-amplifier and an output power amplifier. The buffer stage isolates the balanced modulator from the POWER AMPLIFIER, and also provides additional filtering of the sideband output. A ganged variable capacitor tunes the output circuit of the balanced modulator, the buffer-amplifier and the output circuit of the power amplifier stage. The plate circuit of the output amplifier includes an autotransformer for matching the low impedance of the line to the radio transmitter. Output level is controlled by adjusting the screen voltage of the power amplifier stage.

(4) POWER SUPPLY PP-454/FRT-5. - POWER SUPPLY PP-454/FRT-5 contains two individual power supplies. One power supply furnishes filament voltage, +150 volts regulated and +250 volts unregulated to RF OSCILLATOR O-91/FRT-5. The second power supply delivers filament voltage, +150 volts regulated, -100 volts and +250 volts unregulated to the FREQ. SHIFT KEYER KY-45/FRT-5.

d. BUFFER-AMPLIFIER, KEYER STAGE. - The first RF stage in the transmitter proper utilizes a type 6AG7 broad-band untuned amplifier which provides the required voltage gain between the low impedance input circuit to the grid of the first frequency multiplier. This stage is also the main keyed stage.

e. FREQUENCY MULTIPLIERS. - The first frequency multiplier, a type 807, incorporates the first tunable circuit in this transmitter. It has an output frequency range of 4,000 to 13,000 kilocycles after doubling, tripling, or quadrupling from the oscillator frequency. This stage is keyed simultaneously with the 6AG7 buffer to insure a minimum of feed through with the key open. The second multiplier, which is also a type 807, utilizes multiplying factors of one or two so that its output frequency is the required carrier frequency. Thus the output of this second multiplier is in the frequency range 4,000 to 26,000 kilocycles.

f. DRIVER-AMPLIFIER. - A single type 4-4OOA is used with a single ended to push-pull output circuit to drive the power amplifier. This stage operates at 4000 volts anode potential, which is obtained from the main power supply. Minimum circuit loss between this stage and the power amplifier is insured by the use of a single coupling circuit which is made possible by the proper selection of the mechanical arrangement of this circuit. The tank circuit is continuously variable with motor driven adjustment.

g. POWER AMPLIFIER. - Two high-transconductance forced-air-cooled triode tubes in a push-pull circuit constitute the power amplifier. The tube around which this circuit is designed is the type 3X2500A3. By utilizing this tube with its extremely high transconductance, very high tube efficiency is obtained with relatively low anode voltage. Although tube efficiency in the order of 85% or greater is readily obtainable from this tube, the design is based on a 68% amplifier (including tank circuits) efficiency. The tank circuit for this amplifier is continuously variable with a motor driven adjustment. Magnetic coupling to the P.A. input circuit is made readily adjustable for ease of tuning, and maintenance of P.A. drive. The grid circuit of this power amplifier includes provisions for coupling into the output of a Western Electric type D156,000 or equivalent twin channel transmitter. A 200 ohm balanced coaxial input is provided. One kilowatt peak power from the D156,000 transmitter is required. Means are also provided for adjusting the bias and loading of this amplifier for linear amplifier operation.

h. ANTENNA NETWORK. - The antenna network circuit, located in the PA compartment, is a parallel tuned network consisting of one variable and one fixed capacitor and two variable inductors. The degree of coupling between the PA plate coils and the antenna coupling inductors is variable and adjustment is made from the front panel. The desired degree of loading the power amplifier into the 600 ohm transmission line, for any frequency within the range of the transmitter, is adjusted by means of these controls. The inductance of the antenna coupling inductors is variable. These inductors are maintained at DC ground potential to protect the RF ammeters and power amplifier components from possible flash-over in the power amplifier tank circuit.

i. LOW VOLTAGE SUPPLY. - The low voltage supply is a single phase, full wave supply utilizing two type 3B28 rectifiers. It has an output of 600 volts at 500 milliamperes for operation of all the low-level stages.

j. BIAS SUPPLY. - The bias supply is a high current type utilizing two type 3B28 rectifiers in a single phase full-wave circuit. This supply delivers a -450 volts at a maximum current of 800 milliamperes to provide bias for all keyed stages.

k. MAIN HIGH VOLTAGE POWER SUPPLY. - The main power supply, which furnishes DC voltages to the final amplifier and the driver, is a conventional three-phase, full wave rectifier circuit utilizing 4B32 tubes. The DC filter for this main power supply is designed to reduce the peak value of the AC ripple voltage to 1% of the DC voltage. The DC output is 6 kilovolt at 4 amperes.

l. POWER CONTROL C-587/FRT-5. - POWER CONTROL C-587/FRT-5 contains a Y-delta switch for switching the primary of POWER TRANSFORMER TF-122/U to either Y or delta. A relay is provided which shorts out the primary voltage dropping resistors when the TUNE-OPERATE switch on the control panel is switched to the OPERATE position. A thermo-magnetic motor operated circuit breaker is provided in the 230 volt input line.

m. POWER TRANSFORMER TF-122/U. - POWER TRANSFORMER TF-122/FRT-5 is mounted outside POWER SUPPLY PP-490/FRT-5 bay. This transformer supplies the high voltage to the Main High Voltage Power Supply filter system in POWER SUPPLY PP-490/FRT-5 bay. This filter in turn supplies the plate voltage to the DRIVER and FINAL AMPLIFIERS.

Total Cubical Contents 212.2 Cu. Ft. (Uncrated)
Total Weight 4842 lbs (Uncrated)
Frequency Range 4,000 to 26,000 kilocycles
Frequency Multiplier Range 4 to 26 megacycles
First Frequency Multiplier 4 to 13 megacycles
Second Frequency Multiplier 4 to 26 megacycles
Type of Frequency Control Crystal or Stabilized Oscillator
Type of Emission a. A-1 (carrier ON-OFF)
b. Frequency shift
Keying Speed Up to 400 wpm
Nominal Carrier Output For Each Type of Emmission 15 kilowatts for A-1 emission, into a fixed 600 ohm load
15 kilowatts for frequency shift emission into fixed 600 ohm load
Power Factor of Equipment a. Starting (Filaments ON): 94%
b. Standby (High Voltage ON, Key Open): 90%
c. Normal Operation: 94.5%
Power Source Requirement a. Voltage: 207 to 253 volts; designed for a center voltage of 230 volts
b. Frequency: 60 cps ±5%
c. Number of phases: 3 phase
d. Input Power: Key down: 27.5 KW; Key Up: 4.25 KW
Crystal Type AN/CR27-U with fundamental frequency between 2.0 and 4.2 mc required

RF stages AF stages and modulator Rectifiers
Number Type Number Type Number Type
8 2C51

4 3B28
2 3X2500A3

6 4B32
1 4-400A

3 5R4GYW
5 5686

2 OA2
1 6AS6

10 6AK5

5 6BE6

5 6BA6

4 6AL5

2 6SJ7

6 6X4

1 6C4

2 6AG7

3 807

8 12AU7

7 12AX7

2 VR90


ITU Country
ITU Country