|
This 4-part report discusses antenna fundamentals and the various types of antennas used for short range devices. Fundamentals are presented along with practical design principals. It is excerpted from the report: ISM-Band and Short Range Device Antennas. This part covers examples and measurements.
Click here for Part 1: Antenna Basics
Click here for Part 2: Types of antennas used for short range devices
Click here for Part 3: RF Propagation
Test Module Schematics
In many short range applications, the overall size of transmitter or receiver modules is small compared to the wavelength. The ground plane is therefore usually part of the antenna and radiates or receives energy also. To test the behavior of various antennas under practical conditions, test modules based on the TRF4903 have been built that deliver an output power of +8 dBm. The MSP430 microcontroller is used to program the TRF4903. Figure 23 shows the test module schematics.
Click for larger image
23. Test Module Schematics.
The TRF4903 was programmed to send an unmodulated continuous wave (CW) signal with +8 dBm at 915 MHz. The matching elements L1 and C2 have been optimized by network analyzer measurements to tune the particular antennas used.
PCB Monopole Antenna Module
Figure 24 shows the layout of the PCB monopole antenna. In order to save PCB space the monopole has been bent by 90 degree. In the PCB layout, the monopole was made longer than calculated according to Table 1. This should give some room for possible manufacturing tolerances.
24. Layout of the PCB Monopole Antenna.
Using a vector network analyzer, we measured the antenna impedance on the upper pad of L1 and cut back the monopole until real antenna impedance was achieved. The antenna impedance in resonance is 35.5 Ω, which is within the theoretical value range of 30 Ω to 36.5 Ω. The mismatch loss to 50 Ω is as low as 0.13 dB in this case. No further matching components have been used; inductor L1 was replaced by a 0 Ω resistor. C2 was left unpopulated.
The radiation characteristic of the antenna module was measured in an anechoic chamber with the test module upright (see Figure 25) and flat (see Figure 26) on the turntable.
The outer boundary of the radiation patterns given in this report correspond to an effective radiated power (dipole related) of ERP = + 10 dBm; the scale is 20 dB/division.
25. Vertical Radiation Pattern of the Stub Module (Upright).
The radiation pattern is almost angle-independent, the maximum ERP is + 6.5 dBm, corresponding to EIRP = 6.5 dBm + 2.15 dB = +8.65 dBm. The TRF4903 delivers +8 dBm of output power. The maximum antenna gain is therefore +0.65 dB.
As expected, the horizontal radiation pattern has a more pronounced radiation characteristic:
26. Horizontal Radiation Pattern of the Stub Module (Flat).
The maximum ERP is +10.85 dBm, corresponding to EIRP = +13 dBm. With +8-dBm transmit power, this gives an antenna gain of 5 dB.
|
