I am designing a 915 MHz meandered inverted F antenna (MIFA), based on DN023, for use with your CC1101 IC. I am tuning it with a cheap 'LiteVNA' network analyser.
My ground plane is larger than in the reference design (drawing below), the PCB is slightly thicker, and there is also a large battery below the board. I think this is causing the resonant frequency of the antenna to be much lower than specified in the design note.
When sized for 915 MHz (i.e. L6 = 1mm, based on DN023), the antenna resonates at ~750 MHz.
Without a matching network, I had to shorten the antenna by a large amount to get it tuned to 915 MHz:
Then, when I put it in an enclosure, the resonant frequency drops again, so I'll have to shorten it even more. This seems concerning, I don't want to lose too much antenna.
I have three questions:
- Is there a problem with using this antenna geometry here? Or is this expected due to the bigger ground plane etc, and it just needs to be shortened?
- What is better for performance: severely shortening the antenna like in the above image, or using a matching network to artificially raise the resonant frequency? My understanding is that it would be best to get the frequency as close as possible to 915MHz by shortening the antenna, then use matching components to cancel out any complex impedance (i.e. get it as close to 50Ω + 0j as possible). But if there is a better way (maybe adjusting the spacing of the grounded part of the antenna?) I'm all ears.
- If it's ok to shorten the antenna by so much, can I modify the geometry of the 'meander' to take better advantage of the space I have? So keeping the shortened length, just reducing the meander to make the antenna wider. Example below.
Thanks for your help, this antenna stuff is hard.
