Recent updates

  • 04-2007 - New PCB. Controls now side mounted
  • 09-2007 - IF gain switch added
The pcb has been revised to simplify construction.
The new board is narrower, a PP3 battery will now fit within the enclosure. The gain and tuning controls are side mounted offering better protection against water penetration.
The fixing holes for the potentiometers do not require precise location in the enclosure.
Some refinement of the RF and IF gain control has made the provision of the 'brute-force' attenuator switch at the front-end unnecessary.

2m ARDF Receiver

Splash Radio Searching for old IFTs led me to disassemble a rusty AM/FM shower radio and discover an interesting looking 18 pin chip marked 'TA7613AP'.

A quick Google search revealed this chip to be a variant of the TDA1083 which is used in a well known ARDF design by OK2BWN.
The TA7613AP is a one chip AM/FM radio, complete with audio amplifier.

The 'donor' shower radio (shown left) was sold in stores in 2004/5 and the TDA1083 or the many variants has been around for a very long time so ICs should not be too difficult to obtain or salvage.

The ROX-2T uses a novel way of controlling the gain of the SA602 mixer (as found in the Elecraft K1 transceiver) but the SA612 must be directly feed by the stabilized supply voltage to the chip or a through a RFC, to minimize pulling of the VFO.

rox-2t schematic
The resulting ARDF RX is just about as simple as a superhet can get with get two RF devices, and no surface mount components, (apart from the varicap diode.)

The ROX-2T is probable not suitable for (FM) foxhunting. If the fox stops talking, there will be no modulation to detect! so it is squarely aimed at Radio O using MCW/AM transmissions.

rox-2t pcb layout

CONSTRUCTION DETAILS

The main winding L1 must be wound in the same sense for each half i.e. NOT mirrored
Use a piece of 26swg wire 100mm long, folded in half. Tin the ends and centre fold. The easiest way to remove the enamell is to boil it off, holding the wire ends in a solder blob.

INTEGRATED ANTENNA

The 3 element is based on the WB2HOL yagi described elsewhere on this web site. On the version for the ROX2 there is no hairpin loop because there is no need to match to a 50ohm feeder. The antenna is directly connected to the front-end of the receiver. The driven element is longer than shown in the WB2HOL yagi.
3 element yagi

The 2 element format is a simple close spaced antenna with no phasing elements. The gain is marginally less than the standard 3 element but in authors opinion, is just as effective in use. It has a good back-to-front ratio and its diminutive size and weight commends its use over the bulkier 3 element.

2 element yagi

The driven element connection to the ROX2 is the same for both 2 element and 3 element Yagi.

3 element yagi

HARDWARE DETAILS

rox-2t pcb layout
The enclosure is an ABS box with a RFI internal coating coating (RS part number 501-553). The box has internal ribs, some of which need to be removed using a narrow chisle. First remove two centre ribs at the rear for the headphone socket.
Ribs will need to be removed for the control pots if their body depth is greater than 8mm
Drill two 2mm holes for the driven element, Then remove the RFI coating to create an isolated pad.
Use a 10mm diameter wood drill to create the pads. If a new drill is used, blunt the pointed ends with a file to produce a wider groove around the pads.
Enlarge the holes to 3mm after creating the pads.
The finished box with all holes drilled.
A yoke carrying the driven element is made from 16mm wide oval plastic conduit, 100mm long. Drill two 3mm holes at 25mm spacing on both sides.
Open out two holes on one side to 8mm diameter (min). Try using a countersink drill. A regular drill will often rip the plastic, unless it is very blunt.
Use 3mm diameter - 10mm spacers inside the box to support the pcb. Use 8mm male/female spacer on the ouside to provide the connection to the driven elements.
Underside of the box showing the yoke peice and pipe clips fixed. Note the pipe clips will just snap over the boom with the projecting bolts located in holes in the boom. A half-height 4mm nut is used to secure the pipe clip to the ABS box

Yours truely (G3ZOI) starting out at Farley Country Park with a 2 element ROX-2T. <

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