Antenna - 434 and 868 MHz

I haven't looked into the theory, I took a practical approach and measured S11 on a selection of kit antennas we use or have used and all have only one resonance frequency.

Hi,

Ideally, the antenna element should be a quarterwave in length to achieve the maximum radiation. For 868 MHz, this is 8.6 cm and for 433 MHz this is 17.3 cm. If you use a length of 17.3 cm (433 MHz quarterwave) then at 868 MHz the radiation will be around the null point. i.e. very little radiation. If you would like to have one antenna element structure then this can be done in a couple of ways:

1. Antenna element structure needs to consist of ideally two quarterwave effective lengths from one feed point or two lengths that are in resonant with help of external matching components. 

2. Compromise the antenna length for both frequencies to say 3/8 wavelength for 868 MHz and find a good match at both 433 MHz and 868 MHz.

Otherwise you can take the standard antenna ref designs for 433 MHz and 868 MHz and just insert an RF switch. Drawbacks with this option is the switch price ($0.1) and the size consumed for both of the antennas.

Take a look at the CC-Antenna-DK since this is an ideal platform when debugging and tuning antennas in the lab.

Regards,   Richard