I had ordered ChemE's version 3 dipole design and have some concerns. It's seems that the
wider trace on this version, or perhaps the board material has a different velocity factor. It
seems its elements (1/4 wave) should be longer (closer to 78mm than 60mm.)
I can see that capcha's design v1 pcb uses a narrow trace element. I was confused by the
60mm length as a 1/4 wave 915 Mhz element should be 78mm (about 3"). His tuning was
done presumably to deal with the velocity factor of the copper trace track with 1mm on FR4 of 0.77
which yields 78mm * 0.77 or 60mm which is what he made the trace length on his v1.0 915Mhz
PCB antenna.
I tested the Captcha v1.0 version and used it with a LORA mote. It worked very well. It worked even
better when I optimized the feed line length as discussed here to make my coax the optimum
length. Attaching Captchs dipole to my VNA I got a
VSWR of about 1.6 with an impedance of 52 ohms
at 915 Mhz. (This was using a 15cm feed line I had for testing so it was not ideal. )
Using ChemE's version 3.0 with the same feedline I got a
VSWR of 5.1 with an impedance of only about
15 ohms. at 915 Mhz. However, at 1100 Mhz, the VSWR drops below 2 and the antenna's impedance is about 50
ohms. It took some time for me to believe this so I double checked the antenna connections and tried again. The
VNA showed ChemE's antenna had a nice wide bandwidth that was shifted more toward 1.1 Ghz.
So, ChemE's version 3.0 appears to be a nice antenna, but it's tuned for 1.1 Ghz not 915 Mhz. I believe this is
due to the shortness of the elements which were calculated by Captcha for 1 mil copper trace by applying the
velocity factor for that particular trace on FR4. I believe the easiest way to address this would be to increase
the length of the elements so they are closer to 1/4 wave (i.e. 78mm ).
Here are some photo's of the tests.
https://photos.app.goo.gl/JmiqdGHLOQTF2m6m1