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Radio Society of Great Britain
Technical Information

UK newcomers to the IARU style of direction finding are likely to want information about receivers for both 3.5 and 144 MHz. It is clear that virtually no one will already have a DF receiver for 3.5 MHz but many who currently use an FM hand held for 2m DFing will appreciate that an AM receiver is intrinsically better suited to the needs of ARDF. This page provides links to many existing designs and gives a short summary of each one. The page is restricted to details of receivers only.

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.

2m Sniffer The Australians have a disconcerting convention of referring to an ARDF receiver as a 'sniffer'. These are not low sensitivity receivers for use close in to a transmitter but fully fledged ARDF receivers. This is a fully synthesised receiver with a host of features available either as a complete unit or as a fully built and tested board from Bryan Ackerly VK3YNG.

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.

Compact 145E Jiri Marecek OK2BWN markets an excellent range of ARDF equipment. His 2m receiver is the Compact 145E. This receiver is available for about 180 plus carriage (at 2013 prices).

DF1FO advanced design Nick Roethe has designed an advanced receiver based around a dual conversion arrangement. 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.

3.5 MHz Receivers

HRX80 from PA0HRX This design uses the venerable TCA440 AM receiver IC in a superhet arrangement. IF filtering is by cascaded 4kHz ceramic filters and it uses an LM386 for the audio stage. 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 ARDF Receiver Bob Titterington G3ORY developed a kit for the PA0HRX design but this is now out of stock. These kits were also sold for 40 on a not-for-profit basis in order to promote 80m ARDF in the UK, but the difficulty in sourcing several of the components means that it is currently unavailable.

PRX80S Dieter Schwider DF7XU has designed a number of 80m receivers and his most recent offering is a version of the PA0HRX design. This is available either ready built or as a kit. The ready built version is about 125 and the kit 100 (at 2013 prices)

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 a well designed offering with up-conversion to 10MHz and a crystal ladder filter. Bryan has recognised the difficulty in sourcing many of the 1970s chips and uses easily obtainable ICs, discrete components and home woumd coils where possible. The receiver features an audio S meter although this is not to everyone's 'taste'.

Superfox3.5GX Jiri Marecek OK2BWN markets an excellent range of ARDF equipment. His 80m receiver is the Superfox3.5GX and this boasts a market leading sensitivity. This is of particular value in FoxOring competitions but less important for the other formats. This receiver is available for about 170 plus carriage (at 2013 prices).

Chinese PJ80 When these receivers first appeared, they were unbelievably cheap. Today they are rather less cheap but are not undercut by any other ready built receiver. The drawback is that the direct conversion design is around 7 dB less sensitive than a basic superhet like the PA0HRX design.