Kevin's Überdyne Mk1

Kevin's Überdyne Crystal Radio

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Überdyne Mark 1
Kevin Gard, KJ4PD, Raleigh, NC
Detector: ALD11080 4-FETS in parallel

Radio: The tuned antenna is inductively coupled to a the radio via a 4” diameter coil of either 70 or 105 turns of 18 AWG. The large 105 turn 575µH coil is needed to generate a large voltage for driving the gate of the MOSFET detector for low band and is tapped at 70 turns at 340µH for high band. The loaded coil resistance is matched down to the resistance of the detector using a LCL PI network where the C is a variable cap and the 48 turn 22 AWG shunt L at the detector is about 100µH. The radio is tuned with the matching variable cap almost all the way across the BCB. Above about 1500kHz the number of turns of the main coil is reduced from 105 to 70 using a clip lead.

Headphones: iPod earpods matched with a Bogen T725.

Antenna: Square multi-turn loop. The loop antenna consists of 4 separate coils of 6 turns each. All four 6 turn loops are connected in parallel for 800kHz – 1600kHz operation and for 500kHz – 800kHz operation two coils are connected in series to form two 12 turn coils that are then connected in parallel. Sounds a bit complicated, but the reason for doing this is that the coils in parallel have higher signal output than a smaller coil of the same inductance. Two of the 6 turn coils are made with 176/46 Litz and the other two from 22 AWG. The square loop is made from two 4 foot long 6” wide poplar boards, so the length on each side of the loops are about 34’’ (a bit shy of 1m). The antenna is a tuned closed loop using a 265pF air variable cap.

Performance: This radio performed fairly well for using a loop antenna. I am pretty sure that at present the sensitivity is completely limited by the antenna. Selectivity is ok, but could be much better if the antenna was improved and the coupling better thought out. The primary coil is overcoupled to the antenna coils causing some double hump tuning, but otherwise the selectivity is better than some of my previous versions of this receiver.

Kevin's Überdyne Crystal Radio Loop Antenna

Kevin's Überdyne Crystal Radio Schematic

See Kevin's entry in the 2011 RadioBoard Contest