Yet another antenna for LEO satellites...

Purpose of this paper is to show how to modify and reuse an old antenna to make a convenient traffic with LEO amateur satellites using a stationary, omni directional aerial.

The starting point..

Discone antennas are pretty popular on the market as well as easily available for small prices, especially when second hand. Very often these antennas are discarded from the first owner because considered “deaf” and thus may offer a bargain opportunity.

This is a quite common misconception: often used with scanners having poor performances in term of sensibility and dynamic range, the final end could be rather disappointing, but.. do not blame (just) the antenna!

Discone aerials, are intended to be a compact, broad band antenna, derived from bi-conical version, very popular in the EMC/EMI labs.

Illustrazione 1: Drawing of the "core" of the discone antenna, with the most important measures in evidence

The large band that it may cover, is a plus while paired with an instrumental or high grade receiver, while it may become a nightmare otherwise. The huge amount of out of band signals that this aerial can catch and bring to the input of the receiver is able to produce in the front end many intermodulation products, which in turn could be seen as “noise” or a desensing effect.

To talk with some figures on the hand, let see now how it behaves a generic discone. For this test I used a unknown brand antenna, bought for just few pounds on a flea market.

The central support of the antenna, is described on the drawing above.

The eight plus eight elements are respectively 273 and 820mm long, having 4mm diameter and the lower ring of radials has an inclination of about 60°.

To carry out some significant test, I installed it 3m high from ground on a 32mm diameter metallic pole with the all the setup placed on a quite meadow, in the countryside. To evaluate its return loss, I used my miniVNAtiny® that cover from a 1MHz up to 3GHz with reasonable accuracy once all the cables and joints are kept in consideration in the calibration.

Sweeping from 50MHz up di 1GHz I got the following curve of return loss: