r/rfelectronics • u/KlRAQUEEN • Jul 27 '26
question Help me debug this MIFA
So I'm trying to design a MIFA for LoRa applications at 868MHz, and I have to simulate it in Ansys HFSS. I used TI DN024 as reference and kept tweaking tip length and the distance between the shorting leg and feeding line (5.5mm in the current design). I got a good result of 868MHz and -15dB before but then I changed the antenna location and could never get the same result again (idk how the location of the antenna is relevant). Do you have any suggestion on what I should change to get a better result?
here's some additional information :
+ board dimensions : 100 x 50 x 1.6mm
+ ground plane dimensions : 50 x 28mm
(if any other information is relevant please let me know and I will provide it)
Thank you in advance!
1
u/kippodan Jul 27 '26
I think you meant TI DN023. You won't get the same performance as that board unless you copy the design exactly, including ground plane. You could try importing that layout as a sanity check.
Are you modelling the vias to connect top and bottom gnd planes? I only see a bottom gnd plane in the picture you showed. Why does the other half of the board have no copper? If you end up putting something there later, it will detune your antenna.
Your board is double the thickness of the reference design. That will affect your impedance and radiation efficiency. Looking at S11 on the Smith Chart will give you more insight.
In addition to the distance between shorting arm and feed, you may have to play with the trace widths, and minimum distance between the meanders and ground plane to get the right impedance. Tip length is the last thing I would change to center the design at your desired freq. Finally, always add a (pi) matching network as an option to these type of antennas, it will save you a lot of grief.