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Aviation History
1945
1945 - 0541.PDF
MARCH 22NU, 1945 3O7 direction-finding receiver, and it can be said of it that it overcomes the majority of the shortcomings of aircraft direction-finders. The radio technicalities of this apparatus were described in the Wireless World for January, 1945. It is unnecessary to explain these details here, the chief purpose of this article being to suggest some of its practical applications as a navigational instrument. The outstanding features of the Marconator are the fol- lowing : — (a) All calculations are eliminated, a direct reading of the true bearing of the station received being given with all corrections made for magnetic variation, compass deviation and quadrantal error. (b) Automatic '' sensing '' of bearings, thereby avoiding 180 deg. ambiguity. (c) Operation independent of pilot. (d) Visual indication of bearing. The way in which this is accomplished is fairly simple to follow if reference is made to the photograph of the instrument. It will be seen that there are two scales en- graved o to 360 deg. and a third scale engraved 35 deg.-0-35 deg. The outer scale is a re- peated reading of the master compass, and is capable of rotation through -po deg., the reference point being 'tie lubber line seen at the top of the instrument. ^» The inner scale is tire loop scale and is fixed, it being traversed by the loop pointer from which can be read the loop bearing relative to the aircraft's head. In the photograph the loop pointer will be seen pointing 3. Bearingover - shot. Turn Cursortowards Needle. 2. Approachingthe Bearing — Needle about toline up with Cursor. to 10 deg. on the loop scale; it will also be seen that the loop pointer covers the outer or compass scale and is pointing to 45 deg. Thus the true bearing of the source of signal can be read direct when the loop is turned to the correct position. The method/of ensuring that the loop is turned to the correct po§itrfon is ingenious. As has been previously explained, the bearing is nowadays shown visually. The designers of the apparatus have, therefore, arranged for the visual meter to be attached to the mechanism which turns the loop pointer, and have arranged its needle to align itself with the loop pointer when in the position corre- sponding to the bearing and to read out of alignment when off this position. It will be apparent that this simplifies the operation of taking a bearing very considerably, as it is only necessary, 4. Bearing determined—Cursor and N«edle aligned. once the signal has been tnned-in, to turn the loop/drive so that the pointer moves in tlfe direction in which the visual needle is showing until the loop pointer and visual needle are in alignment. When this condition is satisfied, the true bearing of the station be- ing received may b< n ad of! the outer scale of the instrument. Due to the circuit design, merely turning the loop towards the direction in which the visual needle is pointing ensures that the "sense" is correct. The diagrams illustrate an example of the loop pointer and visual needle positions during the operation of taking a bearing. Being a centre-zero instrument, the visual meter can be likened to the ammeter of a motor car. It will be understood that if the station went off the air the operator might think he was obtaining a bearing when actually the meter was inopera- tive, since the inoperative and on-roursc or bearing indica- tions are both when the needle is in the central position. This danger has been overcome by the inclusion of a tell- tale indicator in the centre of the dial, which shows at once if the transmission ceases. Intersecting Pointers The visual indicator is duplicated in the pilot's cockpit instrument board, where it is used for homing purposes. The danger referred to with a central-zero meter is in this" instance overcome by using a double-needle type of indi- cator. This instrument is arranged to have intersecting pointers which intersect centrally when flying a homing course, and both point to either left or right if the correct homing course is deviated from. The convention now accepted is for the needles to point in the direction of the station when a deviation is made. The advantage of the double-needle instrument is that if the signal ceases both needles collapse and thereby give an immediate indication to the pilot which would not be apparent with a single- needle indicator. This type of instrument is well known in this country, and is generally known as the "drunken man." The combined mechanical accuracy of the master compass repeated scale, th<<Ioop mechanism and the Mar- conator provide an overall accuracy of 1 per cent., accord- ing to the makers. In its simplest application of taking bearings from 'which a "fix" can be obtained, the following procedure would be necessary: Before the flight the wireless operator or navigator would select suitable stations along the route
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