While referring to broad-band antenna, from low frequency to ultra-high frequency, there are common antennas used for compromising emanation eavesdropping, e.g., active monopole antenna works in the bandwidth at 100 Hz ~ 30 MHz; bi-conical antenna at 30 MHz ~ 300 MHz; log-periodic antenna at 200 MHz ~ 1000 MHz, and
discone antenna at 200 MHz ~ 1300 MHz.
On the other hand, a discone antenna [16, 17] is wideband and can be considered as a special case of biconical antennas [18].
In [26], a lumped-element loading method was used to widen the frequency bandwidth and the resultant discone antenna was made to have a height of less than 0.25[lambda].
The antenna is built on a modification of a discone antenna that is flattened but with additions of a back cavity, a short-circuiting structure and a two-plate top structure.
It is built on a discone antenna that is flattened but with three major additions: (1) an open-end cylindrical cavity that surrounds the cone, (2) a short-circuiting structure that is made of an open-ended cylindrical wall with its upper edge connected to the top circular plate of the discone antenna through three solid metal angles placed 120[degrees] apart, and (3) a circular metal plate added above the top circular metal plate with supporting metal posts in between the two plates (forming a parallel plate structure above the top of the cone).
In other words, the proposed antenna has the discone antenna as its basic part but with a large cone angle to reduce the height.
The open-end cylindrical wall and the three angle metals form the short-circuiting structure that provides short-circuited current paths between the top circular plate of the discone antenna and the ground or the bottom of the cone (see Figure 1(b)).
