Loop Antenna For 3.5 Mhz

Description: C1 = 3750 pF 500 V silver-mica capacitor. C2 = 100 pF preset capacitor (Jackson C803). C3 = 75 pF variable capacitor (Jackson C809) with a knob. C4 = 12.7 pF variable capacitor (Jackson C16) with a knob. C5 = 22 nF mica capacitor. Moving coil meter rated at 250 f.s.d. 40 mm (Maplin LB808). D = HF silicon diode. R = 1 kΩ resistor (refer to text). L1 = 51/e turns of PVC-covered stranded 7/0.2 mm wire with an outside diameter of 1.2 mm, rated for 1 kV/1.5 A (refer to text). L2 = 13 turns of 16 SWG tinned wire with a 1-inch internal diameter. Feedline = 46-inch RG58 coaxial cable with a plug compatible with the transmitter. Box = ABS box type MB3,118 96 45 mm. Maplin reference LH22. Terminal blocks = 4 pieces of 12-way 2 amp terminal block, Maplin reference FE78. Spacers = 3 insulated spacers type M3, 30 mm long, Maplin reference FS40T. Spokes = 4 lengths of 8-foot 5/1-inch molded hardwood (available at DIY stores). Vertical support = wood measuring 23 x 0.8 x 0.8 inches (available at DIY stores). Wood base = plywood or similar measuring 12 x 8 x 0.5 inches with a 2.5-inch steel support bracket. The loop antenna is suitable for receiving or transmitting (10 W or less) on the 80-meter band, making it useful when an outside antenna is not feasible.

The schematic incorporates a variety of components that contribute to the functionality of the loop antenna design. The capacitors (C1 through C5) serve various purposes, including tuning and impedance matching, which are critical for optimizing the antenna's performance across the desired frequency range. The silver-mica capacitor (C1) is particularly valuable for its stability and low loss at high frequencies, while the preset (C2) and variable capacitors (C3 and C4) allow for fine-tuning of the antenna's resonance.

The moving coil meter (LB808) provides visual feedback on the antenna's performance, enabling the user to monitor the standing wave ratio (SWR) or power levels. The HF silicon diode (D) serves as a rectifier or detector, allowing for signal processing within the circuit. The resistor (R) is included for current limiting or biasing, ensuring that the circuit operates within safe parameters.

Inductors L1 and L2 are designed to create the necessary magnetic fields for resonance. L1, constructed from PVC-covered stranded wire, is designed for high voltage and current handling, while L2, made from tinned wire, provides additional inductance to the circuit. The RG58 coaxial cable serves as the feedline, connecting the antenna to the transmitter and minimizing signal loss.

The enclosure is constructed from an ABS box, providing durability and protection for the internal components. The terminal blocks facilitate easy connections for wiring, while the insulated spacers ensure that components are securely mounted and electrically isolated where necessary. The wooden support structure adds stability to the overall assembly, allowing for a robust design that can withstand various environmental conditions.

This loop antenna is particularly advantageous for portable operations or in situations where larger antennas cannot be deployed. Its compact design and efficient construction make it a versatile solution for amateur radio enthusiasts and those requiring effective communication on the 80-meter band. C1 = 3 750 pF 500 V silver-mica capacitor. C2= 100 pF preset capacitor (Jackson C803). C3 = 75 pF variable capacit or (Jackson C809), plus knob. C4 = 12.7 pF variable capacitor (Jackson C16), plus knob. C5 = 22 nF mica capacitor. = 250 f.s.d. 40 40 mm moving coil meter (Maplin LB808). D = HF silicon diode. R = 1 kft resistor (see text). L1 = 51/e turns of PVC covered stranded 7/0.2 mm wire. Outside diameter: 1.2 mm, 1 kV/1.5 A rating (see text). L2 = 13 turns 16SWG tinned wire, 1 inch internal diameter. Feedline = 46 inch RG58 coaxial cable, plus plug to suit transmitter. Box = ABS box type MB3,118 96 45 mm. Maplin ref. LH22. Terminal blocks = qty. 4 12-way 2 amp terminal block. Maplin ref. FE78. Spacers = qty. 3 insulated spacer type M3, 30 mm long, Maplin ref. FS40T. Spokes = qty. 4 8-foot lengths of 5/ 1/ inch molded hardwood (DIY store). Vertical support = 23 0.8 0.8 inch wood (DIY store). Wood base = 12x8x0.5 inch plywood or similar 21/2 inch steel support bracket. elektor electronics usaFig. 4-1 (b) 1.4 lengths molded hardwood 30" x^/ax 1/4". Varnished. 2BA holes drilled in the centre. Glued and bolted together. 2.8 off 6-way 2-amp polythene terminal blocks used as insulated wire spacers. 3.5Ve turns of PVC stranded wire (for specs see components list). 4.See Fig. 3. 5.Wood vertical support 23" 0.8" 0.8", wood stained. 6.2" x2BA bolt. 7.Box front vertical support, 4V2" x1/2" 3/4", wood stained. 8.Wood base 12" 8" xW (for similar), wood stained. 9.21/2 steel support bracket behind wood vertical support. 10. Drilled and secured with glue and c/s wood screws. Note: "= inch =2.54 cm. Suitable for receiving or transmitting (10 W or less) on the 80-m band, this loop antenna might be helpful when an outside antenna is not possible.