Ham Radio Forum

Hi Everyone,

Just new to the group and doing some antenna experimenting My idea is to make compact single band, half wave end-fed antennas. I did find a circuit that was published in SPRAT magazine by GQRP Club.
It had a 20 metre end fed half wave antenna using a T68-2 core with 27 turns wire and a 20-180pF compression trimmer on a PCB, the match was "tuned" by fitting a 3.1k resistor between coax ground and the terminal, for the half wavelength of wire. Set the compression trimmer for lowest SWR, then remove the resistor and then attach the wire, retune by trimming wire with the antenna setup for use.
My question is, what formula is used to calculate values for other bands, as relates to turns on the core & capacitance?
My idea was to substitute a Mica capacitor in place of the trimmer, on a select by test principle, then just solder the selected cap in circuit, hoping the Mica cap (500 volt) would be more stable than a compression trimmer. I do have specs for this antenna for 20 & 40 metres, but no other bands. the good thing is that it needs no counterpoise.
This design looks good and must be similar to the Par Electronics antennas

http://www.parelectronics.com/end_fedz.htm

Any ideas?

73 Jack VK4JRC