The receiver is built on a single-sided printed circuit board which measures 2 by 4.3 inches. The small size means that the board can be fitted into a wide variety of enclosures, which should be well screened. The main directional signal is picked up by a tuned ferrite rod (or loop) aerial and fed, via the R.F. attenuator (RV1) to a low-noise FET R.F. amplifier (TR2). A second FET R.F. stage (TR1) amplifies the signal from a vertical whip aerial and can be switched into circuit to give a heart-shape response so that the direction of received signal can be determined. This direction is not accurate, but serves to eliminate one of the two sharp null directions that the ferrite aerial provides.
The amplified signal is tuned in a Hi-Q coil (L1) and fed into the TCA440 receiver integrated circuit. This I/C includes R.F. amplifier, balanced mixer, separate local oscillator, I.F. amplifier and voltage stabilizer.
The local oscillator is tuned by L2 with VC3 and covers 2.265 to 2.455 MHz which converts the incoming signals of 1.81 to 2.00 MHz to a 455 kHz I.F. This signal is fed from the mixer through the 5.5 kHz bandwidth Mechanical filter (CF1) and back into the TCA440 I.F. amp.
The amplified signal is rectified by diode D2 and appears as audio across the volume control (RV4). Automatic gain control is not normally used in a direction finding receiver (this would mask the effect of rotating the aerial) so the gain of the R.F. and I.F. amplifiers are set by a fixed, stabilized voltage ( about 0.7 Volt maximum ) from trimmer resistor RV2.
Adjustable audio levels are fed from the volume control to a single, high gain audio amplifier which drives the headphones to adequate level. Medium to high impedance headphones are preferred.
TR4 with L3 form a simple Beat Frequency Oscillator which can be switched in to enable reception of C.W. or SSB signals. When a bearing is being taken, this is usually done on a beat note.
R11 with ZD1 (a 4.7 Volt Zenner diode) produce a stable voltage for the tuning control (RV3) to help stabilize the frequency as the battery volts drop. The complete receiver takes about 20 mA from the 4 AA cells which should give alkaline cells a life of at least 50 hours. (Each contest normally lasts about 3 hours)
Tuning is accomplished by 3 matched varicap diodes and a 100K Ohm linear pot. which is preferably fitted with a slow-motion drive. This is a much cheaper and lighter method than using an air-spaced tuning capacitor. If required, it is quite easy to substitute air-spaced tuning capacitors instead of the varicaps and a value of about 50 pF maximum would be suitable.
The ferrite rod aerial is made about 8 inches long if possible, and the coil is close-wound on the center. About 30 turns of 24 to 30 gauge would be suitable and should not be sealed until the receiver is aligned. (The inductance required is about 55 uH.) It is also a good idea to screen the rod, aluminum or copper being suitable, avoiding a shorted turn around the rod. This screen can be about the same length as the ferrite rod and should not be too close to it. If the ferrite rod is about 3/8 inch diameter, the screen should be approximately 1 to 3 inches diameter with the rod mounted centrally . The screen should be earthed to the receiver case, which must also have a connection to the receiver earth line.
The telescopic whip sense aerial should be about 2 to 3 feet long when extended, and must be insulated from the receiver enclosure.
After construction, the receiver is aligned on a suitable signal generator, if available, and the phasing of the sense signal is adjusted by the use of the capacitor C29, which is made out of two insulated pieces of wire about an inch long. It may be necessary to change the value of R1 which is provisionally fitted as 1.8K ohms. In most receivers built recently, we have fitted small trim-pots of about 10K to 22K Ohms in place of R1, and this is adjusted, together with C29, to produce the best sense operation. The sense should be set up on a signal whose direction is known, and from a distance of at least a mile.
|R1||1.8K||0.25 W Carbon||£0.03||F.|
|R2||47K||0.25 W Carbon||£0.03||F.|
|R3||4.7K||0.25 W Carbon||£0.03||F.|
|R4||220||0.25 W Carbon||£0.03||F.|
|R5||1K||0.25 W Carbon||£0.03||F.|
|R6||3.3K||0.25 W Carbon||£0.03||F.|
|R7||470K||0.25 W Carbon||£0.03||F.|
|R8||470K||0.25 W Carbon||£0.03||F.|
|R9||470K||0.25 W Carbon||£0.03||F.|
|R10||120K||0.25 W Carbon||£0.03||F.|
|R11||330||0.25 W Carbon||£0.03||F.|
|R12||470K||0.25 W Carbon||£0.03||F.|
|R13||68K||0.25 W Carbon||£0.03||F.|
|R14||1K||0.25 W Carbon||£0.03||F.|
|R15||10K||0.25 W Carbon||£0.03||F.|
|R16||220||0.25 W Carbon||£0.03||F.|
|R17||150||0.25 W Carbon||£0.03||F.|
|C25||( Not Used )|
|C29||10 pF approx||Twisted wire|
|VC1||Varicap Diode||£0.88||C. / B.|
|VC2||1 off KV1236||Varicap Diode||£0.88||C. / B.|
|VC3||Varicap Diode||£0.88||C. / B.|
|L1||154AN7A||Toko RF Coil||£0.70||C. / B.|
|L2||154AN7A||Toko RF Coil||£0.70||C. / B.|
|L3||FLCS10390||Toko IF Coil||£0.70||C. / B.|
|L4||FLCS10390||Toko IF Coil||£0.70||C. / B.|
|RFC1||4.7mH||Toko RF Choke||£0.70||C. / B.|
|CF1||CFMQ-001||Mech. Filter||£1.76||C. / B.|
|C. = Cirkit||B. = Bonex||E. = Electrovalue||F. = Farnell|
Suggested fitting components and soldering order: ( Cross each component off of the list when fitted. )
1 - CF1 Marked CFMQ-001 One mounting leg has to be removed. ( Red core towards center of P.C.B.)
2 - 2 Links (Use resistor legs)
3 - RV2 4.7K or 10K Trimpot. ( 4K7 or 10K)
4 - L1, L2 Marked 154AN7A - White core.
5 - L3, L4 Marked FLCS10390 - Blue core.
6 - R2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17.
7 - C1, 2, 3, 5, 10, 13, 14, 22. 0.01 uF
8 - C6, 9, 23, 27. 0.1 uF
9 - C12 220 uF ( Note Polarity )
10 - C4, 8 4.7uF ( Note Polarity )
11 - D2 Green Band = + OA91 ( Note Polarity )
12 - D1 Band = + 1N4148 ( Note Polarity )
13 - ZD1 Band = + BZX55C4V7 (Note Polarity)
14 - RFC1 R.F. Choke ( Marked 472J )
15 - C7 470 pF ( Marked 471 ? )
16 - C24 100 pF ( Marked 101 ? )
17 - C26, 28. 150 pF ( Marked 151 ? )
18 - C19 33 pF Polystyrene.
19 - C20 56 pF Polystyrene
20 - C21 120 pF Polystyrene
21 - TR3, 4 BC237B ( Note Polarity )
22 - VC1, 2, 3. Tuning Diodes Carefully break apart at joins. Polarity - ( 35Z xxx writing on side marked with a bar on layout. )
23 - TC1 65 pF Trimmer.
24 - IC1 TCA440 Note polarity and Make Sure that all pins are through the circuit board.
25 - C16, C17 0.1 uF
26 - C18 0.01 uF
( R1 and C29 are fitted on test. TR1, TR2 fitted after wiring up board. )
( NOTE : C25 Not Used )
|R1||Adjust on Test- approx 1 K 8||BROWN / GREY / RED|
|R2||47 K||YELLOW / MAUVE / ORANGE|
|R3||4 K 7||YELLOW / MAUVE / RED|
|R4, 16||220||RED / RED / BROWN|
|R5, 14||1 K||BROWN / BLACK / RED|
|R6||3 K 3||ORANGE / ORANGE / RED|
|R7, 8, 9, 12||470 K||YELLOW / MAUVE / YELLOW|
|R10||120 K||BROWN / RED / YELLOW|
|R11||330||ORANGE / ORANGE / BROWN|
|R13||68 K||BLUE / GREY / ORANGE|
|R15||10 K||BROWN / BLACK / ORANGE|
|R17||150||BROWN / GREEN / BROWN|
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Printed Circuit Layout of DF Receiver ( copper view )
PCB Component Layout