This document describes the design, simulation, fabrication, and testing of a broadband discone antenna with an operating frequency range of 500 MHz to 1 GHz. The author theoretically designed the discone antenna by selecting design parameters like a 66 degree flare angle and 750 MHz operating frequency. Simulation in HFSS optimized the parameters, resulting in a 1 mm cone-disc gap and 76.25 mm disc diameter. A physical model was constructed and tested, with measured return loss crossing -10 dB around 890 MHz. While the simulated and measured operating frequencies were higher than the intended 750 MHz, the discone antenna design achieved the goal of operating over 500 MHz to 1 GHz.

1. UNIVERSITY OF NORTH CAROLINA AT CHARLOTTE, ECE DEPARTMENT, ECGR 4121/5121 PROJECT I 1
Design of Broadband Discone Antenna
Joshua S. LaPlant
Abstract—For all intents and purposes, the objective of this
project was to design and fabricate a broadband, electric or
magnetic, dipole that operates within the frequency range of 500
MHz to 1 GHz. This was accomplished by applying appropriate
engineering design techniques to theoretically design, simulate,
and fabricate the broadband dipole.
I. INTRODUCTION
The discone radiator can be designed and implemented
as a broadband antenna for a wide array of applications; in
particular, this vertically polarized antenna is often used for
communications and broadcasting in the VHF (30 MHz-300
MHz) and UHF(300 MHz- GHz) spectrum. In addition to the
discone’s wide assortment of applications, it is fairly simple to
fabricate, requiring less material than some of the alternative
geometrical structures (i.e. biconical).
Fig. 1. Discone Antenna Cross-sectional View with Parameters [2]
The geometrical dimensions associated with a typical dis-
cone design are shown in Fig. 1. and can be defined as such:
D = disc diameter
S = length of air gap between cone and disc
Ls = cone slant height
Lv = total cone height
α = cone flare angle
Cmin = minimum cone − upper diameter
Cmax = maximum cone − lower diameter
When designing any antenna structure, it is important to
have an understanding of impedance matching and its roll
in the performance of the structure. For the purposes of this
investigation, the coaxial cable that is used to feed the antenna
Joshua S. LaPlant is an undergraduate Electrical Engineer in the Depart-
ment of Electrical and Computer Engineering, University of North Carolina
Charlotte, Charlotte, North Carolina (e-mail: jlaplant@uncc.edu).
will be the RG-58A/U. The RG-58A/U has a characteristic
impedance of approximately 50-ohms, our target matching
impedance. Another important characteristic of the RG-58A/U
is the diameter of the outer conductor, 3.15 mm. These two
characteristics will become key to the physical design of the
antenna.
II. THEORY
Before the theoretical design of the discone antenna com-
menced, specific design variables were selected in order to
find the optimum match to a 50-ohm coaxial cable. According
to Nail [1], smaller flare-angles (α) may lead to a larger
impedance mismatch as the slant height of the cone (Ls)
approaches λ
2 . However, larger flare-angles tend to exhibit
high-pass filter characteristics as the slant height of the cone
exceeds λ
4 . Knowing these relationships, the flare-angle (α)
was choosen to be 66 ◦
to ensure a smoother standing wave
ratio (VSWR) on a 50-ohm transmission line (shown in Fig.
2.).
Fig. 2. Standing Wave Ratio versus Frequency for Different Flare-Angles
(α) [1]
In adherence to the project guidelines, the operating
frequency (fo) was selected to be 750 MHz, a mid-point
between 500 MHz and 1 GHz. The operating frequency was
chosen to be 750 MHz to produce a moderate wavelength,
thus truncating the total cone height (Lv).From Eqn.1, the
wavelength at the desired operating frequency (λo) was
calculated.
λo =
c
fo
(1)

2. UNIVERSITY OF NORTH CAROLINA AT CHARLOTTE, ECE DEPARTMENT, ECGR 4121/5121 PROJECT I 2
λo was found to be 400 mm. The slant height of the cone
is dependent upon the operating frequency by λo and some
scaling factor. Before exploring this relationship, the ratio
of the lowest operating frequency to the frequency at which
the discone slant height equals one-quarter-wavelength (K)
was determined, using Eqn.2, to be 0.667 (where fmin =
500 MHz).
K =
fmin
fo
(2)
Since the antenna will behave similar to a high-pass filter
(when exceeding λ
4 ), the performance of the discone would be
inefficient for frequencies below cutoff. Therefor the antenna
should operate at the critical/cutoff frequency, wherein the
slant height of the cone is roughly λ
4 . This relationship can be
represented in both Eqn.’s 3 and 4, where λmin was found to
be 600 mm at 500 MHz.
Ls = K
λmin
4
(3)
Ls =
λo
4
(4)
Notice that both Eqn.’s 3 and 4 result in the same slant length
for the cone, Ls = 100 mm.
The next order of design involved selecting the minimum
cone-upper diameter (Cmin). For ease of design, Cmin was
selected to be the same diameter as that of the coaxial
cable’s outer conductor (Cmin = 3.15 mm). The cone-bottom
diameter, otherwise known as Cmax, was calculated by solving
Eqn.5:
Cmax = 2Ls sin(
α
2
) + Cmin (5)
Solving Eqn.5 results in a Cmax value equal to 112.077
mm. As explained by Nail [1], the optimum values of the
cone-to-disc gap length (S) and disc diameter (D) can be
calculated by applying Eqn.’s 6 and 7:
S = 0.3Cmin (6)
D = 0.7Cmax (7)
Consequentially, the cone-to-disc gap length (S) resulted in
0.945 mm; the diameter of disc (D) was calculated to be 78.45
mm.
III. SIMULATION
The next step, and possibly most crucial, in the design
process was to simulate the antenna design parameters using
the provided software, Ansoft HFSS, by rendering an equiv-
alent discone antenna model (Fig. 3.). A series of parametric
sweeps were performed to compare and test different geo-
metric configurations of the original design to optimize the
antenna’s performance. More specifically, parametric sweeps
were performed for the cone-flare angle (α), cone-to-disc gap
length (S), and the disc diameter (D). In doing so, the E-
plane pattern was optimized by comparing an assortment of
flare-angles; whereas a parametric sweep on the cone-to-disc
gap length was performed to adjust the zero-crossing of the
imaginary part of the impedance. The purpose of performing
aforementioned parametric sweeps was to adjust the design
parameters while observing their effects on the SWR, S11,
input impedance, gain, and radiation patterns. Subsequently,
Fig. 3. HFSS Model.
the original antenna design was kept mostly the same, with the
exception of the cone-to-disc gap length and the disc diameter.
These parameters were adjusted to be 1 mm and 76.25 mm,
respectively. With these design modifications accounted for,
HFSS was used to render plots of the standing wave ratio
(VSWR), return loss (S11), input impedance (Zin), gain (3-D
polar plot), and E-plane field patterns.
A. Reflection Coefficient (S11)
The simulated reflection coefficient, otherwise known as
S11, was originally plotted between the frequency range 500
MHz-1 GHz to determine the frequency at which the S11
crosses -10 dB (Fig. 4). This frequency, which should be
somewhat close to the designed operating frequency, was
approximately 0.83 GHz. However, it was noted that the
simulation gave me a resonant spike around 0.98 GHz on
the S11 (as well as the input impedance). After a thorough
investigation, it was determined that the origin of this
simulation error could not be determined.
Fig. 4. Return Loss(S11) versus Frequency (500 MHz-1 GHz)
For the purposes of comparative analysis, an additional S11
plot was included for a sweep from 500 MHz to 5 GHz in
Fig. 5.

3. UNIVERSITY OF NORTH CAROLINA AT CHARLOTTE, ECE DEPARTMENT, ECGR 4121/5121 PROJECT I 3
Fig. 5. Return Loss(S11) versus Frequency (500 MHz-5 GHz)
B. Standing Wave Ratio (VSWR)
Although it is not entirely necessary to have a complete
understanding of the standing wave ratio, it can be of value
when analyzing the degree of mismatch, thus enabling the
engineer to determine the available power that is reflected at
the input terminals of the antenna. Furthermore, the VSWR
and S11 are related to the degree of mismatch, which is
determined by a function of the characteristic impedance of the
coaxial cable and the input impedance of the Discone antenna.
Fig. 6 shows the results for the VSWR, making note that it
Fig. 6. Standing Wave Ratio (VSWR) versus Frequency (500 MHz-5 GHz)
has a VSWR of approximately 2 at 830 MHz (the simulated
frequency at which Fig. 4 crosses -10 dB).
C. Input Impedance
The performance of an antenna is directly related to its input
impedance, becoming optimal whenever the line and load are
perfectly matched. Unfortunately, for the scope of this project,
achieving this is virtually impossible without implementing
stub-tuners to match the real part of the impedance to 50-ohms.
In an ideal situation, the imaginary part would be between +/-
5-ohms; the real part, 50-ohms. This inadequacy can be seen
in Fig. 7, which shows the simulated input impedance versus
frequency for the operating frequency range.
D. Gain
The simulated total gain, at a frequency of 830 MHz was
captured and has been included in Fig. 8.
Also, the simulated total gain in dB at a frequency of
830 MHz, has also been included in Fig. 9. Section I makes
Fig. 7. Input Impedance (Zin) versus Frequency (500 MHz-1 GHz)
Fig. 8. 3-Dimensional Polar Plot of Total Gain at 830 Hz
note of the fact that discone antennas are vertically polarized,
exhibiting an omni-directional pattern in the horizontal plane.
An extension of this characteristic for the designed discone
antenna can be seen in Fig.’s 8 and 9.
Fig. 9. 3-Dimensional Polar Plot of Total Gain (dB) at 830 Hz
E. E-Plane Field Patterns
The E-plane field pattern is a good approximate for a
dipole’s E-plane fields at the designed operating frequency
(fo). Thus, a plot of the normalized E-plane field pattern was
simulated and plotted for 830 MHz (Fig. 10). It was discovered
that the flare-angle has little to no effect on the E-plane field
pattern shape at the operating frequency. However, as the
operating frequency is increased, the E-plane field pattern’s
shape begins to shift from its circular pattern to a distorted
rendition thereof.

4. UNIVERSITY OF NORTH CAROLINA AT CHARLOTTE, ECE DEPARTMENT, ECGR 4121/5121 PROJECT I 4
Fig. 10. Simulated E-Plane Field Patterns
IV. MEASURED RESULTS
The final step in the design process involved physically
constructing the discone antenna out of a few simple materials.
Upon proper and precise construction, the discone antenna was
tested and measured by utilizing a network analyzer.
A. Fabrication
The fabrication process was carried out by obtaining copper
foil (40 gauge), RG-58A/U coaxial cable, and solder. In order
to obtain a 3-dimensional copper cone, the designed antenna
parameters were applied to a 2-dimensional sheet of copper,
ensuring the correct trigonometric relationships to obtain the
desired design geometry. Subsequently, a copper disk of diam-
eter D was cut out and prepared for final connections. By far
the most difficult step in the fabrication process was correctly
measuring and maintaining the cone-to-disc gap length S. To
perform this step, the coaxial cable insulator was stripped off
so that 1 mm of the dielectric was extending beyond the outer
conductor to act as a spacer for length S. Also, a thin layer of
Styrofoam was laid flush with the dielectric gap to prevent the
cone from touching the disc. Finally, the outer conductor was
soldered to the cone; the inner conductor was soldered to the
disc. The completed discone antenna fabrication can be seen
in Fig.11.
B. Reflection Coefficient (S11)
The discone antenna in Fig. 11 was tested and measured
using a network analyzer. The results of this were recorded and
plotted in Fig. 12. Upon further investigation, the measured
return loss crossed -10 dB at approximately 0.87 GHz. The
S11 remained under -10dB until a frequency of approximately
4.8 GHz.
V. CONCLUSION
The intent of this project was to design a broadband dipole
antenna that operates between the frequency range of 500
MHz to 1 GHz. All other design parameters, including the
type of broadband antenna, were freely chosen and calculated
Fig. 11. Physically Constructed Discone Antenna
Fig. 12. Return Loss(S11) versus Frequency (500 MHz-1 GHz)
by the user. The broadband dipole antenna that was used to
accomplish these means was a discone antenna configuration.
Initially, a theoretical design was birthed after researching
pertinent information, equations, and relationships that adhere
to the discone antenna’s geometry and performance. Next, the
theoretical design was rendered and simulated using HFSS,
ensuring that changes made to the theoretical design had
analytical/scientific basis for doing such. Finally, the antenna
was fabricated and tested to make sure the broadband antenna
design was valid.
At the onset the design process, the operating frequency
(fo) was chosen to be 750 MHz. When comparing the selected
operating frequency to the simulated and measured operating
frequencies, it was observed that the simulated operating
frequency was 830 MHz, the measured to be 890 MHz. There
are many factors that cause this, chiefly, the losses encountered
when physically measuring. Another possible source of error
could be to the materials selected in HFSS (although it
is probably negligible). Rather than using copper, like the
physical model, the material was set to PEC.

5. UNIVERSITY OF NORTH CAROLINA AT CHARLOTTE, ECE DEPARTMENT, ECGR 4121/5121 PROJECT I 5
VI. REFERENCES
REFERENCES
[1] J.J. Nail, ”Designing Discone Antennas,” Fed. Telecommunication Labs
Inc., Nutley, N.J., USA, 1953
[2]
[3] C.A. Balanis, ”Discone and Conical Skirt Monopole,” in Antenna Theory
Analysis and Design, Third edition, Hoboken, NJ, USA, Wiley, 2005,
ch.9, sec. 9.6, pp. 521-522.