Pa0nhc ARDFRx2m Ver 20140528-s34-p45   20160407
A completely new designed , sensitive foxhunt (ARDF) receiver for
144.0 - 144.9MHz.

Schematic Topsilk Top view Two-sides-MASK 1200DPI
for double sided home made PCB
Tune Switch
Components listing Xray Bottom view
(SMD locations)
READ construction details Antenna

Click-right on a link for downloading the file.
These masks are for making a PCB at home.

All components at the top have wires or pins.
See "Bottom view" for the location of the only 4 SMD components.

The DOUBLE SIDED PCB (123.19 x 37.846 mm2) can be mounted in a 50mm dia ALU pipe on three studs (3mm).
The screw distances are : s2-1 = 49.784 mm , s3-1 = 116.332 mm.
ALU pipe is cheap, and easy shaped by sawing, filing, drilling and gluing (use fast hardening two component glue)

At the date of publishing :
all components are cheap and easy obtainable http://www.conrad.nl/ce/ .
The crystal filter is available at  http://www.box73.de/  and others.

An experienced builder should have no big difficulties with building this project.
No hard to find AM-radio-IC is used.
Nearly all components are modern and have have wires or pins.
Only 4 SMD components are on the bottom side.
Coils L1 / 6 are wound on Conrad coil forms.
Ordering details are given.

A very weak signal of -120dBm (0.22uV over 50Ohms, m= 80%) can clearly be heard in the receivers background noise. The max. signal before blocking occurs is >=. -35dBm. The very effective and supple gain adjustment can handle this range easily. An extra input attenuator is only needed, if even stronger signals are expected. The selectivity of the prototype was abt. -80dB at + en - 250 kHz. It guaranties that weak foxes can be heard, standing nearer to the finish fox. Total current drawn is 10 to 27mA, dependent on the gain setting. One 6LR61 9V battery should last at least 6 hours. Vr1 is a very effective voltage regulator. Tuning and sensitivity remain unchanged until the battery voltage drops to below 6.0V. Much attention is paid to decoupling, and a double sided PCB is used. The receiver shows no sign of instability.


Fet1-3 are designed for AGC. For a very effective and supple sensitivity adjustment, the voltage of their G2 is varied. IC1 has no gain adjustment. This prevents worsening its good properties and "pulling" of the oscillator. 

A correctly matching, balanced input circuit, with inductive coupling and a coil wound on a ring core, proved to be impossible to design. Therefore L1 is UN balanced, matching using an easy adaptable capacitive coupling, wound on a screened coil form with a tunable VHF core. Antenna circuit L1/C1/C2 matches a 50Ohms antenna to the 1K5 input of IC1 (1:5.5). This guaranties a good sensitivity and HF selectivity. R1 leaks static to mass to prevent damage to the input circuitry, in case of using a isolated dipole.

Oscillator coil L2 is tuned below the received frequency (ca. 133.5MHz). This minimizes interferences by strong fixed stations (pagers around 155MHz). The low oscillator energy in IC1, and the carefully designed double sided PCB, prevent whistles and unsensitivity due to oscillator signal seeping into the antenna- and IF circuits. Decoupling capacitor C3 (ceramic 100pF, SRF 100 - 180 MHz) shorts oscillator and RF signals to mass. C46 (ceramic 22n, SRF 8-17MHz) is most effective at IF. For that reason all other IF decoupling capacitors are 22nF for minimum, and chokes are 22uH  for maximum impedance.

T2 keeps the 5.0V supply very constant at battery voltages above 6V. Therefore the tuning- and gain regulation voltages are very stable. The pos. gain regulation voltage is very stable, as through D1 flows a stabilized current. 

The red LED is a battery condition indicator, and becomes dark when the battery really is empty (<=6.0V).

REM: "Battery -" is free from mass. In case of a accidental short-circuit between "Battery +" and mass ("0V"), the full current runs through D2, which then immediately burns out. Take care.

The selectivity is good, thanks to F1+F2, L3,4 and 6. Rx and Ry terminate F1+F2 correctly. The pass band curve of F1+F2 can be changed by connecting a very small capacitor (<= 5pF) from the connection between F1 and F2 to mass.

Three modern dual gate MOS fets can deliver more than enough gain, and make a very good gain regulation possible. In this design, Fet2 and Fet4 are inductively coupled to the leading stage, and the gate is 180 degr. out of phase. This in favor of stability. The self inductance of the IF transformers is relatively high, in favor of high impedance and stage gain (NB: the Fets drain is a current source, Hi-Z). Ferrite beads in gate2 discourage parasitic VHF oscillations.

REM: observe correct winding directions, for correct phase. 

The AM detector Fet5 is a "infinite impedance detector". It has high input impedance, low output impedance, low loss, and very low distortion with weak signals. Fet5 should have low IDss (200uA @ Vg -1.5V to -3.5V, f.i. BF244b). The operating point (ca. 25uA) is set by R22.

Due to the relatively broad IF pass band (which makes tuning easier), wideband audio noise is generated. Narrowing the audio pass band improves the signal to noise ratio. IC2 amplifies (ca. 41dB), filters (200Hz-2kHz), and hard limits the audio to max. 3Vpp.

Direction finders (should) have no Automatic Gain Control. When listening with high adjusted sensitivity, a sudden strong signal can drive the audio output of  IC2 in saturation. To prevent damage to the listeners ears, this may NOT cause more than 85dB sound pressure in the headphone.

!! With full output of IC2 (ca. 1Vrms) the sound pressure produced by the headphone must be not higher than 85dBspl. !!

This maximal loudness can be measured using a cheap sound pressure meter. Change the value of R20 until corrected.

REM: the sensitivity of different types of headphones can vary more than 20dB (10x).
Keep the headphone and calibrated receiver together.