How to design the monopole PCB antenna in the shortest time?
This is a problem for us. Antenna is not the main part in the PCB, but it’s the necessary link for all main parts, lots of communication terminals integrate it. Normally we take much time on it, but get much depressed response.
Another, in the application of the antenna, some irrelevant usage will lead to serious problems. In order to avoid unnecessary mistakes, this document will give customers some design guidelines. So, at the later sections, we will describe some key issues .
Furthermore, we have to consider some simulation and measurement results. They are in the following content. The antenna sample is tested in ideal environment. But in customer application, the antenna’s performance maybe changed, because the practical environment is different with ideal environment. For the reason that will influence much.
In this application note, we intend to provide some simple guidelines/rules for using the dual band antenna. And finally it achieve optimal performances.
1. Make it clear: why using Monopole PCB Antenna
2. What is Antenna Terms
This is a relative measurement of the antenna radiates or receives RF power. We get it in a given direction, comparing to an isotropic radiator, measured by dBi or dB. If a negative number, it means that the antenna radiates or receives less than the isotropic radiator. But if a positive number, it means the antenna radiates or receives more than that.
Voltage Standing Wave Ratio (VSWR)
This is for total isotropic sensitivity. And in many systems, if normally less than -102 dBm, the performance is better.
3. How to decide the antenna dimension
4. How to check the Simulation Results
Radiation characteristics of the proposed antenna are almost omnidirectional. The antenna gain of 900MHz and 1800MHz is above -2dBi.
5. Measurement Results in Passive State
Some measurement results are listed below. For the reason that, the return loss S11 and VSWR is shown in figure 8 and figure 9 respectively. Also, the efficiency and average gain of the antenna is shown in table 1 together.
Figure8 Measurementresultof S11
Table1 Measurement result of efficiency and average gain
The measurement results of S11 and VSWR is consistent with the simulation ones, as a result the efficiency is nearly all above 70%. The result is very welcome in many applications.
6. Measurement Results in Active State
The active characteristic of the PCB antenna is tested in SATIMO system. The 3D antenna patterns of 897.4MHz and 1747.4MHz are shown in figure 10 and figure 11 respectively, and TRP/TIS of some relevant channels, like in table 2.
Figure10 Measurement 3D antenna pattern of 897.4MHz
Figure11 Measurement 3D antenna pattern of 1747.4MHz
Table2 Measurement results of TRP/TIS
The measurement patterns are similar to the simulation ones, and almost omnidirectional. The TRP of the PCB antenna is above 28 dBm for GSM900 and above 25 dBm for DCS1800. The TIS is below -104 dBm during all the channels.
7. Some Implementation Guidelines
To achieve optimal performances of the Monopole PCB antenna, some basic regulations information is valuable.
1) The housing of the antenna has to be non-metallic
2) The substrate underneath the antenna must be FR-4 PCB
3) An inhibit area around the antenna could make it work properly
4) An appropriate trace on the PCB connect the monopole PCB antenna to the RF module
5) Basic EMC rules must in any case is important to implement a PCB antenna suited for RF.
7.1 The Housing
As for any internal antenna, to avoid shielding electromagnetic wave, the housing has to be non-conductive. Every plastic/dielectric material makes the antenna resonate at lower frequencies. This design should take into account. Hence the antenna tune to slightly higher frequencies in application.
7.2 PCB Requirements
The antenna requires FR4 material (1mm thickness) underneath it to resonate in the GSM900 /DCS1800 frequency bands, so it has to be placed on a free FR4 area without any other metal as shown in figure 1.
Since substrate’s relative permittivity is an important factor in dimensioning an antenna, a typical standard value in electronic manufacturing of the PCB FR4 is recommended, which is 4.35+/-0.05. The loss tangent tanδ of the PCB, which could affect the performance of the antenna, is expected to be less than 0.02.
7.3 Antenna Placement
To avoid unexpected interferences, lay the monopole antenna in the corner of the PCB is more suitable for communication terminal.
Furthermore an inhibit area of not less than 10 mm width has to be preserve beside the antenna to make it work properly, such as in figure 12.
In order not to affect the antenna performance, no conductive part cannot place closer than 10 mm from the antenna contour.
As for any internal antenna, it is also wise to avoid placement of components which is capable to generate radiated spurious/harmonics or to pick up radio frequency interference in the close vicinity of antenna.
Figure12 Antenna placement
7.4 Ground Plane Size
Ground plane is a must in order to have a good EMC behavior. But in this case the ground plane also need as antenna’s electrical counterweight. We suggest to use a ground plane area of at least 15mm*36mm to get optimal performance.
With smaller ground plane extension, the antenna performance maybe reduce. But that depends on application layout.
To achieve optimal performance, any metal (such as ground plane, traces…) cannot adhibit to FR4 PCB. Not only underneath the antenna, but also in the inhibited area.
7.5 The Trace of PCB Antenna
Because the antenna should be in a 50ohm system. It is output load impedance should be consistent to 50ohm. To meet this requirement, the RF trace should impedance-controll. Also its characteristic impedance should be similar to 50ohm.
In this scheme, we could adopt the G-CPW trace. Then we could easily transform to any other type of trace.
The size of the G-CPW could account by TXLINE 2003 in figure 13. We determine the the RF trace’s size by the depth and relative permittivity of the PCB substrate.
Since EMI problems are always severe when using an internal antenna. So a good shielding strategy is to have only short trace (which should be less than 10 mm) on the outer layer and route the long traces on the internal layers. As a result it is better to use at least 4 layers of PCB.
Figure13 Trace impedance calculations
7.6 General Approach to External Impedance Matching
This is advisable to introduce a discrete components matching network on the PCB. So we could cope with potential impedance mismatch produced by application environment.
In figure 14, the trace matches well with the antenna, while only a 0 Ohm resistor mount
In figure 15, otherwise, tune the matching pi-network simply by changing components’ values.
In figure 16, more the schematic of the matching network show in it.
Without any further PCB layout change, the implement of matching pi-network is very easy.
Figure14 Antenna and trace are linked by a0Ohm resistor
Figure15 Antenna and trace are linked by pi-network
Figure16 Schematic of pi-network
Above all, we know the monopole PCB antenna described in this note is low-profile. It integrate easily, fabricate conveniently. Furthermore appropriate for the current radio system of GSM900/DCS1800. But we also believe that the PCB antenna recommended would provide a great help for the customer application in antenna design.