Receiver construction resources
Some of these links and resources below are for the more technically minded. For a basic home-build receiver kit check out Bob Titterington's 80m ARDF receiver kits. Please note that these kits are sold on a not-for-profit basis and are neither guaranteed nor endorsed by the RSGB.
144 MHz Receivers
ROX-2
David Deane, G3ZOI, tackles the problem of the declining availability of AM radio ICs by utilizing a surface mount IC designed for cellular radio. It achieves the low cost /simplicity objective by single conversion to a 10.7MHz or 455 kHz IF. The RSSI output of the SA605/SA636 chip provides demodulated AM and DC drive for an audio S-meter using the VCO of the 4046 PLL IC. The web site gives further information for constructing the receiver, which is integrated with a 'tape-beam' yagi antenna.
Junior 2
This design by Rainer Flosser DL5NBZ uses a BF961 dual gate MOSFET followed by a Siemens SO42P, a ZN414 am receiver chip and an LM386 audio amp. It is designed to be mounted at the feedpoint of the yagi antenna and has a balanced 75 ohm input for this reason. It is fitted in a 112x60x27 Eddystone diecast box. The ZN414 is now virtually unobtainable but the MK484 from Maplin is a reasonable equivalent. The article, accessed via the link above, is in German.
ROF 1 from Norway
This receiver, in broad brush terms, is a ROX 2 with a PLL first LO to give accurate frequency setting. Full details are provided.
DF1FO advanced design
Nick Roethe has designed an advanced receiver based around a dual conversion arrangement reminiscent of the HPE2. It uses dual conversion at 10.7MHz and 455kHz with a phase locked loop for the first LO. Two dual gate mosfets are followed by a TCA440 for the IFs and an LM386 for the audio. The receiver uses a crystal filter, stores up to 4 frequencies and has an audio S meter. There is, of course, a digital display. The most innovative features are auto attenuation when the S meter hits FSD, range estimation, display of the remaining time that each transmitter is on the air and a low battery warning. There is a version that can be built into the boom of a yagi and PCBs, programmed controllers etc are available.
3.5 MHz Receivers
80m ARDF Receiver
Bob , G3ORY has 80m kits available at a cost of £40 plus £3 p&p. These kits are also sold on a not-for-profit basis in order to promote 80m ARDF in the UK.
Junior 80
This direct conversion receiver uses a frame antenna in its original form. It comprises a BF254 RF stage, a diode mixer (pair BAT41), a BC549C and the ubiquitous LM386. The design is another from Rainer Flosser DL5NBZ and it uses three RF transformers. The article at the link above, is in German.
HPE 80
Click on HPE80-Neu. This is another German designed receiver marketed by Siegfried Pomplun. The original design was by Dieter Schwider DF7XU and the receiver uses a TCA440 in a direct conversion arrangement. The audio stage uses discrete transistors in the Lin complementary pair circuit that originated back in the 1960s. There is only one coil apart from the ferrite rod antenna and that is for the LO. The electric field sensor uses a piece of metal angle which also supports the ferrite rod tube. The effectiveness of the sense circuit is slightly suspect and is not as good as some arrangements, but it does work. 1 IC, 4 BJT/FETs, 1 voltage reg and 4 diodes is a low component count. There will be at least one of these receivers in use at pretty much any Continental 3.5 MHz event. The site of F1NQP has the circuit diagram
RX80 from PA0NHC
The previous reference used the TCA440 as a direct conversion receiver but there are drawbacks to this arrangement, not least of which is the problem of on-frequency radiation. This design by PA0NHC shows how the same IC can be used in a superhet scheme with very few extra components. It uses a 741 as the audio stage and this requires that the headphones (left and right ear) are connected in series. The selectivity is provided by ceramic filters (SFZ455A) and these are backed by IFTs to eliminate the spurious responses. Two ICs and two transistors but three coils (apart from the ferrite rod).
HRX80 from PA0HRX
This is very similar to the PA0NHC design but uses an LM386 for the audio stage and also simplifies some other aspects of the circuit. With two ICs, two transistors, a voltage regulator and one coil (aside from the ferrite rod), this design must have one of the best complexity/performance trade offs of any. The text is in Flemish although the circuit is pretty self explanatory.
80m Sniffer
Bryan Ackerly VK3YNG calls his receiver a 'sniffer' as if it were a low sensitivity receiver designed for close in reception only. In reality it is very much an 'up-market' offering with up-conversion to 10MHz and a crystal ladder filter. The line up comprises a BF904AR double dual gate MOSFET followed by a Motorola MC13135 and an LM386 output stage. Only the mixers and amplifiers in the MC13135 are utilized and the limiter and discriminator are unused. The receiver features a tone level indicator, exploiting the fact that human hearing is more sensitive to audio frequency than it is to level. This 80m receiver is the most sophisticated featured here but has run into problems with the availability of some of the parts.
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