A Wavetrap for Long and Short Waves

Radio Record, Volume IV, Issue 42, 1 May 1931, Page 16

A Wavetrap for Long and Short

Waves

By

MEGOHM

} HE following is the result of ¥e5)i experiments made with the object of producing a wave-trap for use with all-wave or other receivers, enabling them to receive distant stations when the ‘local station is on the air, and -also enabling them after the local: has: closed down to be used for DX without interference from nearby 80-metre phone transmission. This combination trap has been despecially to increase the utitity of the "Advance" a.c. short-wave re‘eeiver (described in the 1931 "Radio Guide"); when used. to receive: broadcast, but may equally well be applied ‘to any other receiver for the purposes mentioned. When listening on the 80metre band the trap may be used to reduce local phone or c.w. interference, and thus one may receive stations that otherwise could not be received. A wave-trap of this type will function best when the resistance is kept low,.and as the coil: offers more. r.f resistance than the condenser :a conidenser. of large capacity is employed. Thus the coil may: have few. turns, and with heavy gauge wire, low rf. resigtance will be secured, iwhere. the interfering station is close at hand it is advisable. The construction of a copper shield is described for the benefit of advanced constructors and those wishing maximum efliciency, but for economy a tin box of. similar dimensions, and having a close-fitting lid at the top, may be used. With a eoat of duco enamel this looks very neat. The Shielding. (or PER sheet of 26 gauge is used, " _ the dimensions being shown on the diagrams. The bottom and two sides are bent from one piece, as being more

economical of copper.. The two ends. are then soldered in, leaving 1-16in. of the bottom -and sides projecting. Most of the soldering can be done inside unless the constructor" is a‘'‘practised hand, in which case it may be done outside. The secret of a neat: joint is to use killed acid and ordinary solder, with a clean, hot iron. Use only as much solder as can be picked up by the iron, and then spread in both directions along the joint. Clean off any -roughness with coarse carborundum paper on a small block of wood. Before soldering the front panel: in place, it should be drilled and _ the switch and condenser’ mounted in place. After soldering iri, the coils are placed and eonnected’ up;. and: then. the. back is ‘soldered in. -The top may be per--manently soldered or. not, as desired. . Phe: top: is' cut the exact length of body from back.to front, and sideways, the width of the inside plus one inch, which forms a half-inch flange down at each‘side. Ais the front edge, project ing 1-16in. The body is a sheet 20% by 73, bent as.shown, which leaves about fin. extra at B; to be finally cut off to make both of equal height. An earth wire is soldered to the back of. the shield, and run round the back of the receiver to the earth terminal. . The Tuning Condenser. "THE most suitable capacity is .0005, though .001 or .00085 may be used,

altering coil turns. to suit.. Note that no part of the circuit is connected to the shielding. Therefore, the condenser must be insulated by mounting on a strip of 3-16 ebonite 43 by 2 inches, or to suit the condenser, and providing a

space for the aerial connection which may be either a socket or a terminal, A’ large hole will be required in the front panel to clear the nut of the onehole fixing condenser, arid another to clear the aerial connection, See that nuts and screws to hold the ebonite

are placed so that they will not touch the condenser frame. Drill the ebonite first, then mark positions of holes on the copper, using the ebonite as a ‘templet. Arrange the height of the condenser so that the. moving plates are never less than fin. from the top of shield. A vernier dial is best, but a plain dial is quite practicable. The Switch. UNLESS the trap is to be used on two bands the ‘switch is not required.. Any two-way switch,.if insulated, is suitable, and requires th only the spindle is to project outs the shield, a hole being made to giy 1-8in, clearance all round. If it is desired to construct a suitable switch, the diagrams give necessary particulars. The minimum size of the ebonite is 2 1-8in. by 14in. The spindle is a piece of jin. brass rod not less than 1#in. long. ‘The top portion of the switch consists of three layers of jin. ebonite drilled. to take the spindle and form a bearing. Two bolts at each end hold the switch to the panel. At the contact end, a narrow slip of ebonite holds the main portion clear of the panel. . yo The arm of 18 brass should be short, drilled jin. and slipped over the end of the spindle and: soldered on. Between it’ and the ebonite is a 24 brass washer cut like a solder tag. At the other side of the ebonite a narrow washer or turn of 24’ wire is soldered to the spindle to prevent end play. Bending of the arm to fit well down to contacts is left to the constructor’s discretion. A stop each side is a 1-8in. screw and nut projecting outwards. A pigtail of 26 wire may be placed across the arm and its washer. The arm connects to fixed plates of condenser and aerial input. Output Connection. OR the "Advance" short-waver the trap shield is intended, to push close against the left-hand end of receiver with the output pin engaged in the aerial socket, the height being accurately adjusted., The output pin is fixed in a piece of ebonite about 2in; square, bolted to the shield in corre position with a din. hole for clearance. For other receivers a terminal utay replace the pin, but it must be remembered that pick-up of the unwanted transmission must be avoided at this point, or the usefulness of the trap will be nullified. A one-inch metal collar soldered round the output, would allow of connection being made with a short rubber-covered wire and the trap then pushed up close to the shic¢!ded receiver with the spare wire inside the circular collar. ‘ The Coils. HE 80-metre coil consists of ten turns of 18 enamelled or d.c.c. wire on a 3in. former, held together by four ," strips of celluloid, each projecting one inch on one side to act as feet. The San are spaced to occupy exactly -8in. The broadcast coil should have sufficient turns to tune 4 little above the wavelenzsth of the local station, The

more low-loss this coil is made, within reasonable limits, the better will it perform. In the original 60 turns of 24 tinned wire spaced 22 te the inch (with 46 enamel) on a 3in. former made a coil & LOver any required wavelength. To cut out 1YA and 3YA 45 furns would be > sufficient, spacing 22 to inch or using 20 d.c.c. close wound, on 3in. former. Cement to three double strips of cellu-. loid and mount as shown or as most convenient, Many experimenters will have a suitable coil on hand. Wooden Base. THE wooden base is of #in. rimu, with a 8-8 strip screwed under back and front.edges. The shield is attached by four screws through the copper, allowing it to hang over at the right to clear the projecting base of receiver. A pin projecting #in. may be put in base of receiver at A so that, using this as a _. gauge for position, the output pin will engage with the aerial socket without trouble. Using. the Trap. HE trap may be always kept in position if used. on an unselective broadeast receiver, because when not required as a-broadcast trap, the short- wave coil may be switched in, when there will be no effect upon broadcast signal strength, whether tuned against 80 metres or not.. If unwanted, 80"metre phone transmission is heard, adjust the condenser until it is cut out. This will not affect the receiver tuning. but when the broadcast coil is in use, tuning of the receiver aerial circuit will be altered. oo Performance of the trap depends to Some extent upon the receiver. Using the "Advance" all-electric short-wave receiver at a distance’of: two miles rom. 2YA, whilst the latter is transitting, 2PC,‘can be received without nyerference. Stations further removed from 2YA’s frequency are, of course, more easily tuned out. In some locations or with certain receivers, if the trap does not appear to be cutting out sufficiently, it would be advisable to try disconnecting the output pin from A, and, instead, connecting to CO. the centre of the coil, or B, a quarter of the total turns (see theoretical diagram). This refers to broadcast, and when switching over to short-wave it would be necessary to then connect output pin to the bottom of large coil. Experiment before shielding. Theory of the Trap. THE theory of, the trap is that it \ accepts the unwanted signal instead of rejecting it, and is therefore called an "acceptor" wave-trap. A circuit of low r.f. resistance is provided. by the coil and condenser, tuned to. the unwanted frequency, around which it may circulate unhindered in- ‘

stead of entering the receiver circuit, which is tuned against it, In practice no such circuit ean have zero resistance, so that a small amount of the interfering signal may enter the receiver. Where this residual signal is not a negligible quantity, another trap circuit is required to absorb it. As a rule, one stage will be found sufficient, but when a keener trap is desired, the double circuit shown in the diagram may be utilised. It should be understood that when receiving short-wave the ‘‘Advance" receiver experiences no interference whatever from 2YA at two miles distance, A Double Trap. [His trap provides at A a circuit tuned to the unwanted frequency, by-passing it to earth, The wanted signals pass through a coil of few turns to the receiver primary coil, which must be separate from and not a tapped portion of the secondary or aerial coil, The few turns at B are coupled to a circuit tuned to the unwanted signals, providing a low-re-sistance path through which the rem-

nant may circulate. With suitable coils this trap may be used for either broadeast or short-wave. Hyven when unshielded and fitted with 80 metre coils, it will cut out a nearby transmitter completely, and when shielded and carefully constructed will cut out transmission at the station, enabling signals on other frequencies to be received,

Note that ‘the receiver must have a loose-coupled r.f. stage. The best arrangement for this trap is to have one condenser above the other on a panel about 84 by 6in. minimum, B circuit at the top. Condensers may be of différent values with appropriate coil turns for each. For 80 metres ten turns. for .0005 condensér and 12 for .00015, The best coils are made of 18 d.cc. on four strips of celluloid, the wire spaced -about half its diameter on a 3in. former. The coupling coil in circuit B> requires only six turns of No.:18 d.ce., which are wound outside the trap circuit coil, separating the two with celluloid strips. While winding, strips of cardboard are placed in between these to preserve a space between the two coils. One end of this winding connects to the pin or terminal engaging with aerial connection of receiver. The coils are not heavy and may be hung behind each condenser, but the fields must be opposed, so the bottom coil may be on edge, slightly diagonally, with its central axis from side-to-side of the trap. The upper coil would then be placed horizontally with its axis vertical. For broadcast, suitable coils and condensers are used, and the coupling coil for circuit B may then be ten turns of 18 wound outside one end of the larger coil.