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pa0nhc ARDFRx2m 0141013-4 /  20170623.
A redesigned, very sensitive and selective ARDF radio foxhunt receiver for the 145 MHz band  (144.0 - 144.9MHz).
Preliminary development data.
This redeveloped version is still under test.

jIf there is enough demand, i will provide professional made PCBs.
Costs 15 Euro / piece including postage. Mail me :  

        PDF masks for a DOUBLE SIDED PCB are available below.

Be aware : "Top" and "Bottom" masks for a DIY PCB MUST be accurately (within 0.2mm) aligned, as well as the over 200 to be drilled 0.8mm holes.
You need to have a steady hand, good eyesight, a specialized drilling machine and patience.
A lot of holes function as crucial inter connection between top and bottom. In a DIY PCB they MUST be soldered on BOTH sides.
A DIY PCB has no soldering masks, with the risk of unsuspected short circuits and damages due to accidental short circuits when measuring.

Make it yourselves easy. Use a factory made PCB.

This version 2m ARDF receiver is a combination of the best properties of  my 80m ARDFrx and the former version 2m ARDFrx. The purpose is a for you easier to finish project, using less components and readily wound coils. Some components which could to be exchanged for adjustment purposes, now have a bit bigger holes. Anyone who can solder, should be able to finish this project successfully. 

        Using the high quality factory made PCBs :
- you don't have to drill 250 0.8mm holes
- all components only have to be soldered on the bottom side
- less chance of hidden short circuits due to the absence of solder masks
- top and  bottom silks make placing of over 100 components easier
- shows a nice professional finish of your project

In this design purposely NO scarce radio ICs are used. 
The three SMDs are especially selected for their availability and optimal properties for this critical application. All other components are wired or pinned for easy assembling. The coils are already wound. The cheap 10.7MHz IF crystal filter gives very good selectivity in conjunction with the highQ 10.7MHz IF coils. A cheap and light weigth screening housing can be made from a piece of 50mm ALU pipe. The PCB is fixed in it using two M3 screws. A 9V battery makes 6 hours continues operation possible. The antenna input is 50 Ohms coaxial. Switching on/off could be done by the gain potmeter. The headphone connector could be a sturdy mini-XLR.

     Receiver properties

Sensitivity :: abt. -120dBm (0,22uV / m=80%) is readable.
Blocking level  : -35dBm (equal to S9+60dB ! ).
Gain adjustment : -35dBm can be adjusted still
Selectivity : 12 kHz -3dB, abt. +/- 250 kHz -80dB.
Audio : 300Hz tot 2kHz +-3dB, slopes -18dB/oct. 
Output max. 1Vpp
The max loudness coming from your headphone MUST be adjusted for max. 85dBspl with R20.
Battery : 9V 6LR61, at least 6uur continues use.
Battery indication : LED darkens when the tuning becomes unstable due to low battery voltage.

    Schematic.
The effective and supple gain adjustment is realized by variation of G2 voltages for Fets 1 and 4. The needed adjustment voltages (+0.63V to abt. -0.67V) are the voltage drops over D1 and D2.
IC1 has no gain adjustment in favor of oscillator stability and large VHF signal capacity.
REM: as the full battery current runs through D2, it will be damaged if an accidental short circuit occurs between B+ an a PCB ground surface.

A balanced 145 MHz ring core antenna circuit with good match to 50 Ohms was not achievable. As the transformation ratio should be 1 : 5.5 , a link or tap coupling could not be used. An exactly dimension able capacitive divider is used for perfect match. L1 is tuned by its core, resulting in constant good match (a capacitor trimmer will change match during adjustment).
R1 leaks statics to GND.

A complicated more selective, but bigger and heavier RF 145 MHz filtering, is unpractical here. The local oscillator frequency therefore is abt. 133.5MHz, as mirror reception of air traffic around 124.1 MHz is less annoying than constant reception of very strong pager transmitter around 165.9 MHz. And a free running 133.5 MHz oscillator is more stable than an nearly equal 156.2 MHz oscillator. The double sided precisely designed PCB results in no de-sensibilisation, nor ghost signal reception from both low power oscillator circuits. De-coupling capacitor C3 (100pF, SRF 100 - 180 MHz) and choke L15 (1uH SRF 180MHz) are effective for VHF signals. The 22nF (SRF 8-17MHz) capacitors and 15uH (SRF 40MHz) chokes are optimal for filtering 10.7 MHz IF signals.

Audio IC2 is powered directly from the battery. All other circuits are powered from a very stable +5V. Resulting in a (for a free running 135 MHz oscillator) very stable tuning and gain setting. The LED is a battery condition indicator. It darkens when the battery voltage is down to 6V, warning that the tuning can become unstable.

High Q IF coils L3 and L6 are not damped by the Fets, as these have very high input and output impedances. Resulting in a high maximal amplification factor and selectivity. The total wideband selectivity from the crystal filter + IF coils is very good. A distant and weak fox hunt transmitter can be received without problems, even standing in the vicinity if a strong finish transmitter. 

The coupling track between F1 and F1 is wide on purpose. It forms a coupling capacitor to ground surface for the filter. A small (0.5-1.5pF) extra capacitor between F1/2 and ground could be placed to optimize the crystal filter band pass curve and/or the far away selectivity. The values of Rx and Ry depend on the type of the used crystal filter. 

    Audio.
Direction finding receivers should not have automatic gain regulation.
BE AWARE: a suddenly active strong transmitter could be ear damaging loud when listening to a distant weak transmitter.
You MUST reduce the maximal loudness from YOUR used headphone to 85dBspl, by means of R20. Its value depends on the sensitivity properties of the individually used headphone. 

AM detector around Fet5 is of the "Infinite Impedance" type. Its positive properties are : simple circuit, low distortion at very low input levels, high impedances, low current consumption. It is set to class B by means of R22. Fet5 should be a low IDss Jfet. The 10.7 MHz BFO oscillator T1 injects a little signal for demodulation of CW and SSB signals. SPECIAL : this signal injection also acts as a bias signal for FET5, enhancing its sensitivity and low distortion. A third IF amplifier could therefore be omitted, lowering power consumption an reducing component number and PCB space.

IC2 has hiZ FET inputs, 5V capabilities and rail-to-rail output. Its two stage gain is over 60dB, and has components for reducing the audio bandwidth to 150Hz to 2kHz with 18dB/oct filter slopes. Enhancing the heard signal-to-noise-ratio. REM: C32 MUST be a film capacitor. Ceramic capacitors show microphonic effects in this sensitive location.

Schema Masker
TOP copper
TOPsilk Onderdelen
&
bestelgegevens
Assemblage Masker
BOTTOM copper
Behuizing Setup