2m Receiver : ROX2-K

A receiver suitable for both AM Radio orienteering and the end game of FM foxhunts.
(updated 24/05/2026)

above: SCHEMATIC: VERSION B (open in new tab or save-as for larger image)

The SA605 makes for a simple AM receiver. It contains a mixer good to 500 MHz, an onboard local-oscillator (LO) transistor, 80 dB of stable IF gain, and an FM-quadrature detector. This application does not use the FM detector; instead, the received signal-strength indicator (RSSI) serves to demodulate the AM signal’s sidebands. The RSSI output provides an indication of the received signal’s strength, but if an amplitude-modulated signal is tuned to the upper or lower sideband (slightly off centre of the carrier), the RSSI acts like an AM demodulator.

In this circuit, the LO is based on a simple Colpitts-oscillator circuit. A BB135 /BB149 varactor diode tunes the LO. The LO feeds the internal mixer with RF from the feed point of a direction finding antenna. The circuit uses a single tuned circuit pre-selector at the antenna feed point. Being a simple RF preselection, there is a potential to hear spurious image and other unwanted signals (eg. airband signals), however in practice, this is rarely a problem in the normal ARDF environment.

The SA605 is a FM device with logarithmic amplifiers , this results in the AM signal being clipped and limited in strength at quite low RF input strengths. Using the volume of the AM modulation to determine transmitter direction is therefore only reliable when the received signal is weak.

The circuit includes a gain control at the mixer input which can keep the received signal at a low level as the distance between receiver and transmitter decreases.

The receiver generates an audio tone from the RSSI output, an audio S-meter or squeaker. The pitch of the squeaker will vary according to the strength of the carrier wave of the hidden transmitter.

The squeaker is therefore used as the primary detecting method. The transmitter modulation is mainly used to determine the transmitter ID (eg ‘MOi’).

The squeaker pitch varies, but the volume output is constant, pre-set by a fixed resistor (R15) in series with the output.

A positive result of the AM limiting is that it provides some protection against excessive volume and ear damage.

A dual-gang potentiometer is used for the gain control. This is to keep the volume of the signal AM modulation, below the volume of the audio S-meter, being the primary detection mode.

It should be noted that this receiver is designed to work with continuous carrier transmissions. Whilst all UK and many EU events use this mode, international championship events can use keyed carrier transmissions. The author has no experience of the receivers suitability in that circumstance.

above: BOM: VERSION B

PCB

The PCB is designed to fit a standard diecast enclosure (Hammond 1590B).
The potentiometer controls (16mm Omeg) are fixed directly to, and also support the pcb.

above: PCB Fitted. Showing antenna connection using 4mm sockets and hairpin match for a 2element yagi. (yagi description in separate post).

above: enclosure prepared for fitting PCB. nb. tags on insulated spacers.

The antenna input pads connect to the antenna feed and hairpin match and are supported on an insulated m3 spacer.

In testing prototype build, version B, it was found there was insufficient attenuation of the signal when very close to a transmitter. This resulted in the pitch of the squeaker excessively high.
An off-board switch was therefore added for extra IF attenuation when needed. eg. < 20 metres.

above: added near/far if attenuator switch

DOWNLOADS

25/5/2026 GERBER FILES VERSION C

minor updates from version B

• Incorporates header for IF attenuation switch.
• VFO coil moved slightly further from antenna input.
• C23 removed, higher gain from LM386 not required.
• https://drive.google.com/drive/folders/1bYR-hP36qO15_AofJUF_LGwXmmbaqlhr?usp=drive_link

REFERENCES

• Archive Article, EDN Access, 29/9/97 by Steve Hageman*