Pa0nhc ARDFRx80-7-17
(3.5-3.6MHz portable amateur radio direction finder).
A complete new design.
20140914 20170620  20170910
Last page ver : 20171013

One with ALU tube as sense antenna.

One with rubber sense antenna (BNC is shorted)

Above two prototypes on home made PCBs.
Observe how wires run to avoid influence to the IC1 oscillator area, and are glued in place.
This prevents an unstable oscillator during movement.
The minimum in the cardioid directional pattern changes to the other side, when the blue and white wires of L102 (FA3 anf FA5) are interchanged
I advise that the maximum in the directional pattern to be in the forward direction, marked by an arrow on the top of the receiver..

Test sample of a
factory made PCB.

All downloads are at the bottom of this page. DIY masks too.
However, a DIY PCB has
- NO metalized-through holes
- NO silk screens
- NO solder masks (component locations)
and therefore creates chances for soldering problems.

Make it yourselves easy. Order a nice factory made double sided PCB.

Parts lists
including ordering details
Bottom view.
(SMD locations)
parts locations
Xray Masker (PDF) Assembling
Drilling plan (PDF)
print "landscape" 
for real size.

- This receiver has a small build-in, balanced and buffered, hi-Q ferrite rod antenna. Then followed by a balanced low noise mixer, and selective 10.7MHz IF amplifier.
Even with this cheap little 50x8mm ferrite rod, it results in a sensitive and clean reception. Its own noise level is below the noise level of a silent 80m band during evening time.

- The design uses modern, readily available and cheap components, mostly wired. On the top copper are one link and two SMD trimmers. On the bottom copper are three SMD varicaps and two SMD FET's. That is all.

- Nearly all components can be ordered at "conrad.com", all others at "box73.de". Ordering numbers are given in the component list.

- Standard, ready wound coils from Neosid are used. Only the antenna coil must be wound and adjusted by you. Simple.

- A cheap (9 Euro) standard available PLASTIC (ABS) box is used for the housing. A drilling drawing for it is downloadable.
- In its bottom a 125mm x 30mm piece of ALU foil must be glued in contact with the middle supporting screw, as "grounded" screening against hand-effect.

- The small antenna rod is glued inside the plastic housing. And the sense antenna is low profile: a short piece of metal tubing, screwed on top of the box. The sense signal strength is adjustable for an optimal directivity pattern.
As an option, an external whip could act as sense antenna. 

- Reception is clean, due to the balanced input, the very selective antenna coil, the high IF frequency (10.7MHz) and a cheap but selective Xtal IF-filter (-80dB @ +- 50kHz). 

- Tuning is from 3.5 to 3.63 MHz. The smooth working gain adjustment has over 90dB control range, and is adjustable to your needs.


In stead of BF244b you can use BF256b for FET101,102,103 and FET5.

   Schematic details.
The battery current varies with gain setting between ca.16mA and 34mA. The expected alkaline battery life is minimal 6 hours. D4 protects the circuit for wrong battery polarization. The HF stages are powered by the unregulated battery supply. The oscillators, tuning- and gain adjustment, IF stages, product detector and BFO are powered by a very stable 5.0V. The red 3mm "ON" LED darkens when the battery voltage becomes to low for the 5.0V stabilizer to keep the tuning voltage stable. Due to extensive power filtering and a thoughtful, double sided "radio-communications" PCB design, all stages are parasitic oscillation-free and injection-free.

For best direction finding properties, the ferrite antenna circuit is balanced and followed by buffers FET102/103, which match the hi-Z antenna to the low-Z input of IC1 (3k). Resulting in a high Q antenna circuit, maximal signal output and selectivity, even when using this 8x50mm small ferrite rod. To preserve the good properties of mixer IC1, and to prevent "oscillator pulling", IC1 has no gain regulation. Gain regulation in the IF stages is more than enough and adaptable to your needs. Careful design of the tuning components ensures good tracking between the oscillator circuit and the antenna circuit, resulting in  constant sensitivity over the tuned band segment. For the local oscillator is a standard 7mm Neosid coil used to simplify construction

Due to the high IF frequency (10.7MHz) and the high Q of the antenna, mirror reception (17.7 - 17.9 MHz) is very unlikely. I expect no broadcast or airfield communication interference.

Excellent IF selectivity (80dB @ +- 50kHz) is obtained by crystal filter F1+F2 and the unloaded IF coils L3 and L6. The track between crystal filter parts F1 and F2 is relative wide, to create a very small coupling capacitor to mass, which sets the pass band curve of the crystal filter.

SMD Fet1 and 4 are modern high steepness dual gate MOS fets, designed for gain regulation. Ferrite beads in gates2 discourage UHF oscillations. Standard 7mm Neosid IF coils are used also to simplify construction. Thanks to the high audio gain in IC2, and the very sensitive (linear) product detector, the IF gain of the two stages is more than enough.

For maximal effectiveness, all decoupling capacitors and inductors are chosen for a SRF (series resonance frequency) of about equal to the circuits working frequency. At the output of mixer IC1, a foil capacitor C46 decouples for 10.7 / 14.3 MHz, and foil capacitor C3 for 3.5MHz. No ceramic capacitors are used here because these are to temperature unstable for this circuit.

Varying the voltage on gates2 of FET1 and 4 results in a very effective and supple gain regulation. D1 generates the positive part of the gain regulation voltage (+0.63V), which is current-stabilized by regulator VR1. The total battery current flows through D2, generating the negative part of the gain regulation voltage (-0.73V @ min. gain). If you want, the receivers minimum and maximum sensitivity could be reduced by enlarging the values of R24 and R25. 

    Detector and BFO.
The product detector is of the "Infinite Impedance" type. Its simplicity, very high input impedance and low current consumption are the wanted properties. High ohmic R22 sets its working point simply and stable into class B. 

>> The injected BFO signal from T1 "biases" this product detector, resulting in high sensitivity and good linearity <<. 

The BFO must be working for good overall sensitivity. To make CW and SSB signals audible, it generates a 10.700MHz carrier (or 10.7015 for LSB  or 9.9985 MHz for USB), which is injected into detector FET5. REM: if you only are interested in AM (not in CW/SSB), then change the crystal for one with a frequency far outside the IF pass band (for instance 12.0MHz).

IC2 amplifies the audio by 60dB, hard-limits the output to 3Vpp, and narrows the audio pass band to 150Hz / 2.0kHz, improving the signal-to-noise ratio.

        Protect  your ears from overload.

The values of R20/C39 should be adapted for 85dBspl maximal loudness in the headphone used together with this receiver.
See the headphone sensitivity table in "Setup".

WARNING: Long-term listening to audio with a loudness of over 85dBspl can cause definitive damage to your hearing.

        Example :
When listening to a weak signal, an unexpected STRONG signal will drive the audio stage fully into saturation, as it will do with your the resonating hears in the snail house of your ears (if no protection is implemented).  

Without R20 the maximal power delivered to a headphone capsule could be up to 30mW (!), resulting possibly in a sound pressure of a deafening 110dBspl (like standing near a starting jet plane).

R20 therefore limits the maximal audio power delivered to the headphone to far less than 1mW. Read in "SETUP" how to do.