Design Note
DN041
SWRA421 Page 3 of 19
3 The Half Wave Dipole Antenna in Brief
The half wave dipole is one of the simplest and most commonly used antenna structures in
the RF world. It can be made from a piece of wire, or a trace on a PCB. The basic half wave
dipole antenna is a wire or trace with a length of lambda/2 and a slot in the middle where the
antenna is fed. The impedance is around 73 ohms, and the directivity is 2.15 dBi.
4 Designing a Half Wave Dipole Antenna on a PCB
This design note describes the antenna design process for the Zlight2 reference design, a 40
mm PCB disc that is used to demonstrate the capabilities of the CC2530 when used as a
wireless LED controller. This places some constraints on the use of the basic dipole antenna
structure, the main ones being that the antenna will be placed close to the edge of the board,
and it will be curved with a radius of 20 mm. The width of the trace does have an impact on
the antenna performance, particularly the bandwidth. In this design note we will stick to a width
of 300 um (12 mils), and refer the reader to more thorough papers on antenna theory if they
wish to probe deeper into this particular property of the dipole. Placing the antenna close to
the edge means that the effective dielectric constant will be harder to determine, as a 2.5D
EM simulation tool such as Momentum will not accurately capture the effects of our finite
substrate. Changing the shape of the antenna from a straight line into a curve means that the
radiation pattern will not be the same as that of an ideal dipole.
As a first step, we simulate a close-to-ideal dipole using ADS Momentum to verify that the
simulation setup yields credible results. Then we add substrate, curvature and a ground plane
in order to model the final board more accurately. Finally, we add some length to be able to
manually fine tune the antenna length on the first prototypes. This latter step is a
precautionary one, given the uncertainty in the effective dielectric constant seen by the
antenna.
The prototype testing and optimization process involves trimming the antenna to the right
length, and then adding matching and filtering components to obtain a good trade off of high
output power at the carrier frequency and low harmonics.
5 Simulating the Antenna
5.1 Step 1: Close to Ideal
Figure 1. A 300 um Trace in Free Space
The structure shown in Figure 1 is composed of two arc segments, each with a width of 300
um, a radius of 3 m and an angular length of 0.56 degrees. The substrate definition is set to
free space above and below the trace. Figure 2 shows the simulated return loss (S11) using
ADS momentum.
