A shielded loopstick receiving antenna for 75 and 160 meters

A shielded loopstick receiving antenna for 75 and 160 meters

Here is the shielded loopstick receiving antenna I have been using on 75 meters. The loopstick and tuning cap are from an old AM transistor radio. The original winding was removed and the wire reused to wind 42 turns spaced 1/8 inch on the loopstick. This gives a tuning range from 1600-5000 Khz. The chassis is made from double sided circuit board material. Note that the chassis is U shaped. The loopstick cannot be surrounded by a completely closed chassis, as this would form a "shorted turn" and kill the signal.

The tuning cap is mounted on plastic spacers, so the tuned circuit formed by the loopstick and cap "float". one of the main noise reduction techniques used in this antenna is a balanced to ground condition all the way through the antenna.

 Here is a view of the preamp which uses a Motorola MC1350 differential RF amplifier chip. MC1350P.pdf The input is a three turn coupling coil. Since the chip is fully differential, no ground reference or DC bias is required at the input pins. This allows the balance to ground to not be upset. A Mini-Circuits T4-1 wide band 4:1 RF Transformer T4-1.pdf is used to convert the differential output of the chip to 50 Ohms single ended. Note the BNC connector is a plastic insulated floating one. The active connector is the black one on the bottom. The point is to keep the antenna as fully floating and balanced to ground as possible. The top connector was used to input a short (8") whip antenna to more completely cancel static by injecting a small out of phase signal into the main tuned circuit. This basically offsets any unbalance to ground. Although this works, it is very hard to adjust and the gain in performance is small. The ferrite beads help keep noise from outside the shield from entering on the power leads.

   It was found that the ferrite beads alone were not good enough at keeping noise out, so the battery pack is shielded using metal foil tape. The 4 AAA batteries will last a long time IF you remember to turn it off after use.

So how does it perform? Here is a screen shot of an HPSDR receiver connected to the transmitting dipole receiving an AM signal on 3870. Note the noise floor and the spur which is only a few db down.

Here is the same signal as received on the loopstick. Note the spur is now down about 20 db and the noise floor is lower as well. These shots were taken under (for me) quiet conditions i.e. all the grow houses are off. Under heavy QRN conditions the noise reduction is very dramatic. I am working on an outdoor shielded loop that I will post about in a few days... Stay tuned.