How a discone works
A discone is a disc on top of a cone, with the coax centre conductor feeding the disc and the braid the cone. It behaves like a broadband vertical with a very wide bandwidth, often 10 to 1 in frequency, with a reasonable SWR above the frequency where the cone is a quarter wavelength long. It's vertically polarised and omnidirectional, which suits scanning airband, marine, amateur and public bands.
Dimensions
The design rules used here are the common ones: the cone's slant length is a quarter wavelength at the lowest frequency, the disc diameter is 0.7 times the cone's base diameter, and the gap between the disc and the top of the cone is about 0.3 times the cone's top diameter. Most home-built discones use 8 or 16 rods for the cone and disc instead of solid sheet, which works almost as well and catches far less wind.
Things to know
- A discone has no gain, about the same as a dipole. A dedicated antenna for one band will beat it there.
- The upper frequency limit is set by how neatly the feed point is built. Keep the gap and connections small and tidy.
- For transmitting, a discone is fine on amateur bands within its range, but it radiates harmonics as well as it radiates the fundamental, so use a filter.
The airband and marine channel pages list what you can listen to with one.
Questions
What frequency range does a discone cover?
From where the cone is a quarter wavelength long, upwards by about 8 to 10 times. A 100 MHz discone covers roughly 100 to 1000 MHz.
Does a discone have gain?
No. It's roughly equal to a dipole, but over a very wide range.
How many rods should a discone have?
At least 8 for both the disc and the cone. 16 gets closer to solid sheet.
Can I transmit on a discone?
Yes, within its range and power rating, but use a filter because it won't reject your harmonics.