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. For the time being the page will be restricted to details of receivers.

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.

RX144 MHz ARDF (Select Union flag and then 'Technical'). This offering from F6KSJ appears to be a direct conversion receiver using an NE612 for crystal controlled conversion to 4 MHz followed by a second NE612 which converts to audio. An LM386 amplifier with low pass filtering completes the line up. The design has recently (Feb 2003) been improved by F8AZG. The first LO is now separate from the first NE612 and there is an ingenious FET switch to turn off the power when the phones are removed. The circuit is notable for the way in which it utilizes the complementary outputs and inputs of the NE602 and LM386 and a balanced connection to the antenna is employed. Full constructional details are available for the receiver and an accompanying yagi. The receiver is designed to be mounted on the boom between the reflector and the driven element.

HPE2 V8.0 This site does not give constructional information but it gives details of the latest version of Siegfried's 144 MHz receiver. This latest version has a digital readout to give excellent frequency setting. Earlier versions were available in kit form

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. The pdf file of the manual takes about a week to download on a dial-up link (the pics could surely be compressed) but this does not detract from a superb, compact receiver.

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 Titterington G3ORY has 80m kits available at a cost of £37 plus £2 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.