Loop Antenna Design. Full-wave loops are very popular antennas.
They are especially useful on 80 and 40 meters where they perform well at modest heights. These are closed loops that are one full wavelength long. Portable HF Transmitting Loop Antenna. November 12, 2004 While HF antennas are not normally known for their small size, magnetic loops can offer quite respectable performance, at the cost of some efficiency and extremely narrow bandwidth.
This tunable magnetic loop antenna is my contribution to the well established art of amateur loop making. It is small and light enough to carry while operating, it disassembles into small but rugged pieces that fit easily in a backpack or gym bag, and it can be tuned from 14 MHz to 30 MHz. Specs in Brief Frequency coverage: 14-30 MHz Efficiency: 32%@14, 54%@18, 66%@21, 78%@25, 87%@30 MHz Bandwidth: 12k@14, 20k@18, 30k@21, 50k@25, 100k@30 MHz SWR: 1.0:1 where tuned Power Handling: tested at 20W PEP Size: 21 inch square plus handle Weight: 4.5 pounds.
Screwdriver Antenna. Remotely tuned, antennas have been around for decades.
Our military and others used them during WWII for HF radio communications. Normally, capacitors and inductors are tuned by electric motors with gear boxes to reduce shaft speed and provide the torque necessary to rotate the component. Both air and vacuum variable capacitors have been tuned by this method. More ingenious are variable inductors, ranging from edge-wound coils with rollers to pancake spirals with sliding shorting blocks between turns.