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====== Adcock - SCR-551 Direction Finder ======
Manufactured for the U.S. Signal Corps by Press Wireless Inc.

A semi-transportable Adcock direction finder was developed for the U.S. Signal Corps in 1942. A rotating shelter with the direction finder receiver was mounted on a tower; the operator rotated around its axis while taking bearings sitting in the shelter.

The system also included three low-power transmitters to measure and test the direction finder in the field.

{{:images-small:mil-usa-scr551b.jpg?direct|Peiler SCR-551}}
===== Technical data =====
  * [[Principle]]: Adcock - DF receiver, [[IF]] kHz / FM 10.7 MHz
  * [[Operation modes]]: [[A1]] with BFO, [[A3|AM (A3)]], [[F3|FM (F3)]]
  * [[Frequency range]]: 6 x SW (2 - 20 MHz)
  * [[Frequency display]]: Analogue dial
  * [[Frequency memory]]: none
  * [[Signal strength indicator]]: Oscilloscope tube
  * [[Signal processing]]: 
  * [[Features]]: 
  * [[Sensitivity]]: [[A3|AM (A3)]] / [[Selectivity]]: kHz (-6/-60 dB)

==== Power supply ====
  * [[Batteries]]: three 6 V accumulators, two accumulators or 12 V DC are used to generate the 110 V AC voltage by means of a chopper power supply, one 6 V accumulator for the tube heaters.

==== Dimensions ====
  * 737 x 930 x 600 mm, weight 147.5 kg

==== Accessories ====
  * AN-121-A antenna system, Adcock antenna
  * Nine 6 V batteries, each with a weight of 17.7 kg
  * Telephone EE-8-A
  * Shelter HO-11-A, 237 kg
  * Tower TR-51-A with mounting FT-371-A, 496 kg
  * Spare parts, 93 kg

----

===== Operation =====
{{ :images:mil-usa-scr551-im-shelter.jpg?direct&300|Shelter of the SCR-551-B}}The SCR-551-B shortwave direction finder was used in a semi-transportable manner; a rotatable shelter is mounted on top of a tower, in which the operator sits in front of the BC-976-B direction finding receiver. When taking bearings, he rotates around his own axis with the entire shelter.

The Adcock antenna elements are arranged vertically on both sides of the shelter. The two elements M-339-A (left) and M-340-A (right) are plugged into the AN-122-A dipole collector on both sides. The length of the lower beams can be adjusted to the frequency using a crank. To receive horizontally polarised transmissions, the Adcock elements can be brought into a horizontal position. The auxiliary antenna **AN-121-A**, consisting of three elements and 220 cm long, is attached to the top of the shelter and is used for side detection.

When the 10 - 20 MHz range is switched, the antenna is electronically shortened via a relay to prevent resonance; with the SCR-551-A, the antenna shortening can only be switched to the 17 - 20 MHz range.

The receiving system is housed in a "Central Unit" **CS-121-A**.

At the top of the cabinet is the direction finding receiver **BC-976-B**. The single conversion superhet covers 2 - 20 MHz in five band ranges (2 - 2.8 / 2.8 - 4.4 / 4.4 - 6.4 / 6.4 -10 / 10 - 20 MHz). The large tuning knob is located below the frequency dial in the centre of the front panel.
To the left of this is the volume control, below which are the controls for the [[RF gain]] (SENSITIVITY) and the antenna adjustment (ANT.GAIN).

The BC-991-A oscilloscope is used to visualise the direction finding signals and is used for side determination. The adjustment elements for the oscilloscope tube are located under a cover.

At the bottom of the Central Unit **CS-121-A** is the Control Panel **PN-21-A** with the main switches for the 12 V and 6 V supply voltages, various fuses, a voltmeter for the two voltages and the corresponding switches. The loudspeaker is also located on the control panel.
A light for the azimuth dial can be plugged into the "Scale Light" socket to read the bearing in the shelter.
{{ :images:mil-usa-scr551-front.jpg?direct&300|SCR-551B}}
Various methods can be used for direction finding. For direction finding using the "Aural Null" method, the direction finding mode is activated with setting the switch next to the oscilloscope tube to the BEARING position. The band switch BAND SW is used to select the frequency range and the main tuning knob and the fine tuning knob VERNIER to tune in the signal. Now the ANT.GAIN control is turned to the left or to the minimum position and then signal is tuned to maximum again using the Vernier knob.

Now the direction finder is rotated until a minimum is found; then the SENSITIVITY control is slowly turned to the right and the AUDIO GAIN control to the left until the bearing minimum is sharp. The azimuth can now be read off.

Before using the ‘Visuall Null’ method, the oscilloscope is set up according to the instructions. SENSITIVIY, AUDIO and ANT.GAIN are turned up. The station of interest is tuned using the BAND SW band switch, the main tuning TUNING and the fine tuning drive VERNIER. The ANT.GAIN is now turned all the way to the left and the station is then tuned in again precisely. The signal is displayed as a line on the oscilloscope; it may be necessary to adjust the SENSITIVITY control until the line fits on the screen. Use VERNIER to tune to the signal maximum again and use BALANCE to set both lines to the same length. Now turn the direction finder until the signals are at minimum height, possibly readjust with SENSITIVITY. It should now be possible to find a clear minimum.

The side determination is done using the oscilloscope indications. If the operator rotates the direction finder to the right and passes over a station in front of him (i.e. in direct azimuth), the right-hand line of the oscilloscope display becomes larger; if the direction finder has passed over the station to the left, the left-hand line becomes larger; with direct alignment, both lines are the same.
If the lines are reversed, i.e. the left line is larger when the direction finder is rotated to the right and the right line is larger when the direction finder is rotated to the left, then the station is behind the operator, 180° opposite, in reciprocal azimuth. In other words, if the operator turns the direction finder to the right and the right line increases, the station being aimed at is in front of him (direct azimuth), if it decreases it is behind him (indirect azimuth).


{{ :images:mil-usa-scr551-bc-976b.jpg?direct&600|BC-976-B}}

In monitoring mode, when only searching for stations of interest, the oscilloscope tube can be switched off with the operating switch on MONITOR to protect it.

===== Technical principle =====
Direction finding receiver based on the Adcock principle.

==== Components ====
The set is equipped with tubes.

[[de:6AC7]], [[de:6AC7]], [[de:6AC7]], [[de:6SK7GT]], [[de:6SA7]], [[de:6SK7GT]], [[de:6SK7GT]], [[de:6SQ7]], [[de:6SF5]], [[de:6V6GT]], [[de:6SL7GT]], [[de:6H6GT]], [[de:6SF5]], [[de:6SF5]], [[de:6SF5]], [[de:6SF5]], [[de:6SJ7GT]], [[de:6H6GT]], [[de:6SL7GT]], [[de:6H6GT]], [[de:6SF5]], [[de:5Y3G]], [[de:5Y3G]], [[de:3AP1]], [[de:5Y3G]], [[de:1A5GT]], [[de:1A5GT]]

===== Technical documentation =====
  * [[https://www.radionerds.com/images/b/b6/TM_11-247.pdf|TM 11-247]]

==== Development ====
{{gallery>:images?mil-usa-scr551*.jpg&0&80x80&lightbox&showtitle&titlesort}}

==== Further information ====
  * [[https://www.radionerds.com/index.php/SCR-551|SCR-551]] on radionerds.com