Reducing the antenna ground plane – Questions and ansers
The reducing of the antenna ground plane or a minium antenna ground plane is a very common question to me all the time
Q: How small is small?
A: What is normal size? The lowest frequency in GSM 850 is 824 MHz. Embedded antennas for GSM have a need for 40 mm x 100 mm ground plane in average. The effect of reducing of ground plane is listed here:
The antenna efficiency will shrink. The bandwidth will be reduced as well. The centre frequency will jump.
Q: Our board, as presently configured is about 22mm x 34mm with the radio positioned at one end.
A: Bluetooth and WIFI are operating on 2400 to 2485 MHz. The centre is 2442 MHz. 2442 MHz / 824 MHz = 3. That means everything that is mentioned for GSM is at Bluetooth just one-third. The size of 40 mm x 100 mm as standard ground plane for GSM will shrink to 13 mm x 33 mm. The size of 22mm x 34mm looks ok. Think about, that you have to use a part of the 34 mm length to place the inverted F antenna on top of the PCB. Based on width of 22 mm you maybe can save the meandering of the antenna or the meander will have less bends. The meandering of the antenna reduces the bandwidth of the Bluetooth antenna. Anyhow, the bandwidth of the antenna without meander or less meandering could be already bigger than 2485 MHz – 2400 MHz = 85 MHz. The inverted F antenna can be optimised on impedance easy. Further tuning can be done by matching circuit. Anyhow, tuning by changing the antenna structure is the better option. In the end a product contains often both options. #AQA