AN/FRT-19 Low Frequency Transmitter

Information on the FRT-19  has been posted here because there is no logical fit to any of the categories in this web site:



Frequency coverage:  30 to 300 KHz  or 30 to 600 KHz with modification.
Power Level:  15 kw output when used as a 300 to 300 KHz transmitter.
3 kw output when modified to have a frequency range of  30 to 600 KHz
Mains power:  230 VAC , 3 phase, 50 or  60 Hz
Vintage November 1953
Made by: Sierra Electronic Corp  and Sierra Electronics Mfg. Company, San Carlos, Calif.
Manual; reference NAVSHIPS 92117



a. GENERAL.--The Radio Transmitting Set A/N FRT­19 ts a shore-based, low frequency radio communication transmitter. The equipment is a continuous wave transmitter used primarily in shore-to-ship communications under widely varying climatic conditions. It . operates from a power source of 230 volts, 3 phase, 50 or 60 Hz and delivers a 15- kw radio - frequency output in the continuously variable range of 30.0 to 300.0 KHZ to a vertical radiator through the associated antenna coupler. It may also be operated as a 3 kw transmitter. Under this condition, it delivers an RF output over the range of 30 to 600 KHz.

b. KEYlNG.-- Five types of keying can be used as listed below.

(1) On-off keying (Al emission) at any speed up to and including 600 words per minute.

(2) Telegraphy modulated by an audio frequency (A2 emission) of 1000 Hz at any speed up to and including 100 words per minute and with a modulation capability of up to 100 percent.

(3) Frequency shift telegraphy (F1 emission) at any speed up to and including 240 dot-cycles per second and with a frequency shift continuously variable over the range of zero to 1000 Hz  ( ie: 500 Hz in relation to the carrier).

(4) Frequency shift telegraphy modulated by an  audio frequency (F2 emission with phase modulation) of 200 Hertz at any speed up to and including 240 dot-Hzwith a frequency shift continuously variable over the range of zero to 1000 Hz (ie +/- 500 Hz in relation to the carrier) and with a phase shift continuously variable to produce' phase modulation up to one radian (about 57 degrees).

(5) Facsimile transmission (F4 emission) with a frequency shift up to ±500 Hz with respect to the carrier frequency and with a facsimile signal frequency range of zero to 2000 Hz).



(1) FUNCTION. -- The radio transmitting set is designed to deliver an output of 15 kw of RF power to any suitable antenna system within the limits specified under Antenna

The transmitter may be converted into a low power (3 kw) equipment usable over a wider range (30 to 600.KHz). This modification is possible because the high power equipment employs the self-contained 3-kw radio transmitter as a driver, adding a 15-kw power amplifier and associated power supply to complete the components of the high power equipment.


Refer to the Complete  System Diagram in the table below.

--The entire radio transmitting set with the exception of the antenna coupler, is enclosed in a unit assembly 16 feet long, 6-2/3 feet high, and 4 feet deep. This assembly is made up of four distinct cabinets of equal size which are bolted together. The four major units of the transmitter enclosure are referred to from left to right as Cabinet I, Cabinet II, Cabinet III, and Cabinet IV. Each of these units has its own indicator and control panel in front. The terminal board for connecting the 230 volt, 3-phase, 50/60 cycle power
line is located on the bottom of Cabinet IV. There are two line voltage regulators. One is in Cabinet I and the other is in Cabinet IV. All cabinets are constructed of aluminum and have floor and top plates to provide complete electrical shielding. The transmitter is entirely closed and access is provided by suitably interlocked
locked front doors. The side and rear plates are held on the frame by captive screws. The antenna coupler kit is a separate installation, housed in its own building in the immediate vicinity of the antenna tower. Dimensions of this building are 8 feet high by 8-1/2 feet wide by 30 feet long. The 600 ohm RF r-f line from the transmitter enters one end of the building near the roof, through a pair of feed-through insulators. The 3-phase 230 volt power line enters the same end through the floor. Adjacent to it is the 11-pair lead-sheathed telephone cable linking the tuning unit with the remote control panel on the transmitter. The antenna lead leaves the other end through an insulator near the roof. The local control panel is just inside the door.

b. RADIO TRANSMITTER, T-396/FRT-19 contains the Frequency Generator, Crystal Oscillator and Keyer.

(1) FUNCTION. -- The Frequency Generator is designed to produce a highly stable carrier signal to be keyed by the keying circuits before it is delivered to Cabinet II. The output is continuously variable over the frequency range of 3.0.0 to 600.0 KHz. The crystal oscillator which contains 10 crystal units may be used instead of the frequency generator as the source of the RF carrier frequency or an external exciter may be used as an alternate method. A keyer provides five types of keying. The keying may be either neutral or polar for Al,- A2, Fl or F2 types of emission. Modulated continuous wave (A2 emission), is available up to 100% and phase modulation (F2 emission) is available up to 1 radian (about 57 degrees).

In A1 and A2 types of emission, the keying signal may be either "mark positive" or "mark negative", however, in Fl or F2 emission only a "mark positive" keying signal may be used.

Complete FRT-19 transmitter system
Front panel of transmitter
Antenna Coupler MK-156/FRT-19
Designators for the AN/FRT-19

"It looks like the whole room is perhaps the whole antenna tuner and/or final amp plate circuit. The background appears to be a series of fixed inductors held high on their supports, probably in series, and most likely wound with big Litz wire. The vacuum relays that switch the inductors in and out of the circuit for coarse tuning are probably that stuff on the floor (with the glass columns). Note the smooth rounded, and polished corona rings at the two terminals of each switch.

The coil  in the center is likely  a variometer that allows fine tuning of the circuit. A few fixed capacitors(?)  (white cylinders in the near foreground) appear to be porcelain or ceramic and provide parallel fixed caps. It appears the window-size feedthrough on the far wall goes to the antenna  That means the plate circuit must be off-photo to the right.
The variometer is found in many early receivers, like the Radiola III, when variable caps were replaced with fixed caps and tuning is performed by varying inductance of the LC tuning circuit. Much easier and cheaper to build.

Variometer are almost universally used in high power VLF and LF transmitters because building a high value variable cap that can tolerate the tuning currents (1,800A) and voltages is not reasonable. The largest antenna one can come up with for VLF or lower LF is always way too small to be resonant, so the antenna feed point looks like a large capacitance in series with a tiny it takes some "L" to get the system to resonance. Take a look at the US Navy station in Australia that runs 2 MW at
19.2 KHz. for high power VLF stations that use a similar variometer."

From an additional naval reference book " The antenna coupler includes a matching network and its control system, a 48 volt DC power supply, and a regulated 105 volt DC power supply. The antenna coupler matches the impedances of the transmission line and the antenna tower, for frequency antenna height combinations listed in table 4-1 (mot available). This is done by means of a variometer and a vari-coupler driven by motors, and several capacitors and inductors which may be switched into or out of the circuit electrically. Control is afforded by a local control panel in the helix house, and remotely by a similar panel in cabinet IV".

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Credits abd References:

1) NAVSHIPS 92117
2) Jim Fleming VE3PBJ
3) Nick England   K4NYW
5) Nick Broline W5FUA

Apr 11/20