Part 1: DIY 2.45 GHz PCB dipole antenna
For some BLE (Bluetooth Low Energy) experiments, I decided to create a simple dipole antenna on a PCB. One of my hobbies is amateur radio, especially CW on the short wave bands (HB9DPA), so I’m familiar with antennas in general. What makes this antenna different is of course its size: Short wave antennas are several meters to several tens of meters long while this one should only measure a few centimeters.
My goal was to create a small PCB with a single SMA connector. A balun consisting of two capacitors and two inductors should ensure that the antenna really only consists of the two elements and that the coaxial feed line wouldn’t radiate too much. For the connection from the SMA connector to the balun, I decided to use a grounded co-planar waveguide. Making the antenna slightly longer than required, 3cm for each element to start with, should allow to shorten it as needed to make it resonant at 2.45 GHz.
For the PCB, I used a double-layer presensitized copper clad board, 1.6mm thick and made from FR4. The etched boards look like this:
As an additional challenge, I decided to use 0201 components for the first time. I’m quite used to SMD soldering, but the smallest passive components that I had used so far were 0603. It’s amazing how small 0201 really is — the components look like grains of salt!
Picking them with tweezers requires a steady hand:
Luckily, it was surprisingly easy to solder the small components. The trick is to put tin on one pad first, then hold the soldering iron in the left hand and place the component with the tweezers in the right hand, heaten up the tin and thus fix the component on the pad. To avoid that the component moves while soldering the second pad, I pressed them gently on the board with the tweezers. This wouldn’t win a beauty contest, but I’m quite happy with it:
To determine the resonance frequency, I used a spectrum analyzer with a tracking generator and a directional coupler. The initial result looked promising, but as expected, the resonance frequency was way too low, at about 1.98 GHz:
It took a couple of iterations to reach the desired 2.45 GHz resonance frequency. As I was a bit impatient, I cut off too much and had to make the antenna elements longer again, but in the end, it looked good:
The reflected power is down by about 35 dB at the resonance frequency of 2.45 GHz, which is equivalent to an SWR of about 1:1.04. The SWR remains below 1:1.2 between 2.4 and 2.5 GHz.
And this is how the final product looks like :-)
Let me know if you have questions or comments!
You may also want to read the follow-up post with some radiation pattern measurements for these dipole antennas.