Physically Tunable Electrically Small Antenna
I wanted to design a tunable and compact transmitting antenna for the 7MHz amateur radio band. The antenna is a grounded monopole 0.08 wavelengths tall, loaded to resonance with an inductor and capacitive end loading. Shortened antennas have narrow usable bandwidth. For amateur communications on the 7MHz band the instanteously occupied bandwidth is small (<3kHz voice channel), but frequency agility within a 300kHz bandwidth is desirable. I designed this antenna to overcome this limitation. The antenna is tuned to resonance at a given frequency by physically adjusting the size of the capacitive loading "hat."
Full description of this antenna and test results against a conventional quarter wavelength tall grounded monopole can be found at my ham radio website: http://n3ox.net/projects/n3oxflex/
Frequency agile electrically small antennas for low frequency bands often use expensive vacuum capacitors or mechanically adjustable inductors that are nearly impossible to improvise or reproduce if a source is not available. Instead, I set out to design an antenna that could be made entirely with hand tools and that was amenable to adaptation and improvisation.