An extensible top-loaded biconical antenna is modified to improve low frequency performance while retaining standard performance specifications when needed. The biconical antenna includes a balun and a pair of conical outrigger assemblies coupled to said balun. A conducting tophat plate is removably attached to the ends of each outrigger assembly. The tophats increase the capacitance of the antenna, thereby improving its low frequency gain by 10 dB or more.

FIELD OF THE INVENTION 
This invention is related to a biconical antenna system and, in particular, 
to a biconical antenna system which can be selectively top-loaded to 
improve low frequency performance. 
BACKGROUND OF THE INVENTION 
A biconical antenna, as well as other similar tapered dipole and monopole 
antennas, including bowtie or Brown-Woodward dipoles and discones, can 
provide a very broad impedance bandwidth. However, this performance does 
not extend down into the range in which the antenna is electrically-small. 
For example, a biconical antenna with a flare angle of 120 degrees can be 
matched using a 4:1 balun to provide better than 2:1 VSWR over a 6:1 
bandwidth. However, the antenna is about one-half wavelength wide at the 
lower end of this operating band. Thus, as the frequency of interest drops 
below the operating band, the relative electrical size of the antenna 
becomes small when compared with the wavelength, decreasing the efficiency 
of the antenna significantly. 
The biconical antenna is of particular interest in applications such as 
testing noise immunity and electromagnetic emissions. To ensure that the 
results of such tests are repeatable and can be compared with the results 
of other tests using different biconical antennas, various well accepted 
standard antenna specifications have been developed. Once such standard 
biconical antenna design, defined by U.S. Military Standard 461A (Aug. 1, 
1968) is illustrated in FIG. 1. 
As depicted in FIG. 1, a conventional biconical antenna 10 used in the EMC 
industry comprises two outrigger assemblies 12 which are skeletal 
approximations of a conic surface. The outrigger assemblies 12 are 
connected to a matching balun 14 by an appropriate coupling 16. The 
outrigger assemblies are formed of ribs 13 connected between the coupling 
16 and endpoint 17 of a central support 18. The balun 14 is used to 
transfer received and transmitted energy between the antenna 10 and a 
suitable transmitter and/or receiver, respectively. The antenna 10 is 
about 1.37 meters in width and has a flare angle of 30 degrees. 
For biconical antennas of this type, it is generally expected that good 
performance can be obtained for frequencies above 100 MHz and, in fact, 
most commercially available biconical antennas complying with MIL-STD-461A 
provide excellent performance from 100 MHz to 300 MHz. Acceptable 
performance can often extends to 60 MHz. However users often attempt to 
use the biconical antenna at frequencies down to 26 MHz. Unfortunately, 
these biconical antennas are notorious for poor performance in the 30-60 
MHz range. In fact, at 30 MHz, the input match for these commercial 
antennas is so poor that input VSWR is actually determined primarily by 
line and balun losses. The poor input match results in extremely high 
"mismatch loss" and thus severely reduces gain. 
Thus, the ability of the traditional 1.37 meter biconical antenna to 
generate electric field (for immunity testing) with a given input power is 
very poor. A further consequence of the extreme mismatch is the high 
voltage at the input connector generated by the near doubling of the input 
voltage over that which would exist on a matched line with the same 
forward power. This doubling of the input voltage stresses connectors to 
the point that they often fail from electric field breakdown. 
Despite poor low frequency performance, the biconical antenna has attained 
universal acceptance in the EMC industry. The design of the 1.37 meter 
biconical antenna is rooted firmly in MIL-STD 461. Its design is very much 
standardized and biconical antennas from any of the leading EMC test 
equipment manufacturers perform almost identically. This ensures that 
repeatable measurements can be obtained without regard for the antenna 
manufacturer. In addition, the standard biconical antenna design provides 
a mechanically robust easily-transported, and rapidly-assembled device. 
Because of this, users of biconical antennas are reluctant to adopt any 
designs which depart drastically from the standard. 
Various techniques have been proposed to improve the performance of 
biconical antennas in the low frequency range. In one technique, an 
impedance matching network is incorporated into the BALUN enclosure to 
improve the input VSWR for the biconical antenna over the 30-60 MHz range. 
Because the network is incorporated into the BALUN, no changes to the 
external geometry of the antenna are required. However, the improvement 
provided by such a network is generally quite small because no amount of 
input impedance matching can change the instrinsically high radiation Q of 
the biconical antenna in the frequency range in which it is 
electrically-small. In other words, while the biconical geometry provides 
excellent performance over a frequency range in which it is of moderate 
electrical size, is simply not a good electrically-small antenna. 
Therefore, instead of using a modified biconical antenna, many user rely on 
a second alternate antenna for work in low frequency ranges. A popular 
alternate antenna is the top or end loaded dipole. Top loading provides 
improved performance at low frequencies by increasing the shunt capacity 
of the antenna, thus lowering the fundamental resonance frequency, and by 
providing a charge reservoir at the end of the antenna, increasing the 
current density near the outer ends of the antenna. 
Top loaded dipole antennas can be reliably designed to cover the 30-100 MHz 
range. Unfortunately, the top loaded dipole antenna does not provide good 
performance over the frequency range in which it is of moderate electrical 
size. A top-loaded dipole (with 1.37 meter width) antenna provides good 
performance over the 30-60 MHz range and acceptable performance up to 100 
MHz. This is a frequency range which is nearly disjoint, but also nearly 
complementary, to the 100-300 MHz operating range of the 1.37 meter 
biconical antenna. 
However, while two antennas are sufficient to adequately cover testing from 
30 MHz to 300 MHz, their use requires that operators purchase, transport, 
and store two relatively large antennas. In addition, it is often desired 
to rapidly make measurements throughout the 30 MHz to 300 MHz range. 
Unfortunately, decoupling one antenna from the measuring device, removing 
it from the testing area of interest, and replacing it with the alternate 
antenna can be cumbersome and time consuming. 
Accordingly, it is an object of the present invention to provide a 
biconical antenna which has good performance over the 100-300 MHz range of 
conventional antenna designs, while also achieving good performance over 
the 30-60 MHz range. 
It is a further object of the invention to provide a biconical antenna 
which complies with accepted biconical antenna design standards to provide 
for repeatable measurements while also being easily and reversibly 
modified for improved performance at low frequency ranges. 
SUMMARY OF THE INVENTION 
These and other objects are achieved by the present invention in which a 
biconical antenna is provided with mounts to accept removable top-loading 
"tophat" plates. The tophats increase the capacitance of the antenna, 
thereby improving its low frequency gain by 10 dB or more. For a biconical 
antenna which complies with MIL-STD 461A, gain for frequencies between 
30-60 MHz is increased by 10 dB or more. 
When the tophats are detached, the antenna operates as a conventional 
biconical antenna which complies with, e.g., MIL-STD 461A well as other 
EMC testing requirements for biconical antennas, and therefore has the 
expected and repeatable performance over the 30-300 MHz range. When 
increased performance is needed over the critical low frequency 30-60 MHz 
range, the tophats can be attached to the antenna. Preferably, the tophat 
mounting provides appropriate locating and supports to ensure that the 
tophats can be mounted in the same position each time to provide for 
repeatable measurements.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIGS. 2a-2c illustrate a biconical antenna 11 according to the invention. 
The antenna 11 comprises two outrigger assemblies 12 connected to a balun 
14 via couplings 16. The outrigger assemblies 12 are connected to a 
matching balun 14 by an appropriate coupling 16. The outrigger assemblies 
12 includes ribs 13 arranged connected between the coupling 16 and an 
endpoint 17 of a central support rod 18'. The ribs are arranged to 
approximate a conic surface and, in conjunction with the support rot 18', 
generally form a 30-60-90 triangle. 
A top-loading "tophat" plate 30 is removably attached to each outrigger 
assembly 12, preferably at the endpoint 17 of the central support rod 18' 
by a mounting assembly 32. The tophats 30 are generally flat conducting 
plates. When mounted, the tophats 30 add capacitance to the antenna, 
thereby increasing its relative diameter and improving its low frequency 
performance. Preferably, tophats 30 are mounted substantially 
perpendicular to the support rod 18'. To compensate for the increased 
bending moment produced by the mounted tophats 30, support rods 18' can be 
stiffened relative to those in conventional biconical antennas, e.g., by 
using a tubular support, as opposed to the more conventional solid rod. 
The antenna can be further strengthened by adding supporting struts 20, 22 
if necessary. 
In one embodiment, illustrated in FIG. 2d, the mounting assembly 32 
comprises a fastener 33, such as a screw or pin, which passes thorough a 
hole 35 in the center of the tophat and engages a suitable receptacle 34 
in endpoint 17 of the support rod 18'. The screw or pin can be separate 
from the tophat 30 or integrally connected. Preferably, the mounting 
assembly 32 also includes appropriate locating pins, markings, or is 
otherwise suitably shaped to ensure that the tophat 30 can be repeatably 
mounted in the same position to provide for repeatable measurements. 
The particular mounting assembly used is not critical to the invention and 
a wide variety of other removable mounting assemblies can also be used, as 
will be apparent to one of skill in the art. For example, as shown in FIG. 
2d, the top hat can be fitted with a spring-like "gripper" 33' which is 
configured to mate with the end 17 of the support rod 18 in a conventional 
biconical antenna and retain the tophat 30 in place by compressive 
friction. In this manner, tophats 30 can be provided which for use with 
pre-existing biconical antennas. Other configurations are also possible 
such as frictional mounts, engaging slots and tabs, magnetic clasps or 
even hook and loop fasteners. Furthermore, the tophats 30 need not be 
mounted directly to the end of the support rod, but can instead be mounted 
on the ribs or supporting struts by appropriate mounting components. 
The configuration of tophat 30 itself is also not fixed. Preferably the 
tophat 30 is a generally planar aluminum disk, although non-planar and 
non-circular configurations of different materials may also be used. The 
improvement in low-frequency performance provided by the tophats 30 
increases with the diameter of the tophat. Preferably, the diameter of the 
tophat 30 is at least equal to the maximum conic diameter of the outrigger 
assembly 12. 
A preferred design is illustrated in FIG. 3. The tophat 30 is circular and 
has a plurality of cutouts 36 to reduce its weight. The resulting tophat 
30 has an outer rim region 37 with supporting spokes 38. Because 
electrical charge builds up around the circumference of the tophat 30, the 
cutouts 36 have only minimal impact on the overall performance. In the 
most preferred embodiment for a biconical antenna complying with MIL-STD 
461A, the maximum conical diameter is approximately 20 inches and the 
tophat 30 has a diameter of approximately 30 inches. 
While the invention has been particularly shown and described with 
reference to preferred embodiments thereof, it will be understood by those 
skilled in the art that various changes in form and details may be made 
therein without departing from the spirit and scope of the invention.