PCB Dipole antenna for 868MHz
Designed with Diptrace
The antenna is attached to the VNA with a 22.5cm length of RG174. On 868MHz this two half-wavelengths(VF=0.65).
The following graph shows the resulting VNA plot for this antenna:
The above plot is for the dipole tuned for 868MHz. The blue line shows an SWR of 1.1:1 at 867MHz. The dip is much wider than on 433MHz and has a bandwidth of about 51MHz (838 to 889MHz) for an SWR of 1.5:1 or less. The red line shows an impedance of 47.48 Ohms for this frequency, so a fairly close match for the radio.
- fed with one wavelength (22.5cm) RG-174
- bandwith below 1.5:1 SWR = 51MHz
- each dipole leg is 6.4cm
During testing (915MHz dipole shown in picture), the antenna was simply were left hanging down, away from any nearby metallic or organic objects.
Some general things to keep in mind about these designs:
- This is a half-wave dipole and therefore it does not have a 50 ohm feedpoint impedance. You will have some rf coming back to the radio on transmit but due to the rigid and balanced PCB based design and the accurate calibration of the length of the dipole legs using the VNA this will be minimal.
- To keep the impedance mismatch from getting worse it is important to only use feedline lengths of (multiples of) a half wavelength (compensated for the appropriate velocity factor) for the frequency of interest. See the picture in this post to see the effects of different feedline lengths: https://lowpowerlab.com/forum/rf-range-antennas-rfm69-library/small-loop-antennas-433-mhz/msg14061/#msg14061
- coax feedline losses are siginificant at these frequencies, use the lowest number of half-wave multiple lengths possible.
- I tested these dipoles on 1.6mm thick FR4 substrate PCBs with 1 ounce copper from OSH Park. Feel free to use these designs to manufacture elsewhere but it is not guaranteed that the results will be the same. Heck, even OSH Park could end up supplying you with different materials, but this is what worked for me.
The PCB design for this antenna can be ordered directly from OSH Park: https://oshpark.com/profiles/captcha
Alternatively, if you wish to make any modifications, you may download the zip file with the gerbers or the Diptrace design files from this site.