This weekend I finally had access to a VNA to tune my PCB dipoles for 433, 868 and 915MHz.
The 433 and 868 antennas were attached to the VNA with a 22.5cm length of RG174. On 433MHz this is a half-wavelength (VF=0.65) and two half-wavelengths on 868MHz. For 915MHz I only trimmed the feedline an extra 5mm to 22cm which makes it close to two half-wavelengths.
During testing, the antennas simply were left hanging down, away from any nearby metallic or organic objects.
The following graphs are the resulting plots for each antenna:
433MHz
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The above plot is for the dipole tuned for 433MHz. The blue line is SWR and shows a dip around 434.8MHz. Close enough for using it on 433MHz. The SWR (top left, in blue) shows a value of 1.11:1 for this frequency. The red line shows an impedance at this frequency of 45.62 Ohms, reasonably close to matching it with the radio which expects 50 ohms. The bandwidth of the antenna with an SWR of 1.5:1 or less is about 15MHz (from 427 to 442MHz). More than sufficient to cater for any variations in operating frequency.
- fed with one half wavelength (22.5cm) RG-174
- bandwith below 1.5:1 SWR = 15MHz
- each dipole leg is 13.3cm
868MHz
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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
915MHz
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The last plot is for 915MHz and shows an SWR of 1.29:1 for this frequency. The lowest SWR is slightly lower in frequency, but negligible for our purposes. I just left the marker at 915MHz. The antenna has a bandwidth of 43MHz (888 - 931MHz) for an SWR of 1.5:1 or less. The red line shows an impedance of 42.64 Ohms at 915MHz.
- fed with one wavelength (22cm) RG-174
- bandwith below 1.5:1 SWR = 43MHz
- each dipole leg is 6.0cm
The tests were performed with the SDR-Kits DG8SAQ Vector Network Analyzer 3E (site:
https://www.sdr-kits.net/)
(Prototype PCB dipole attached to the VNA)
Now that I have accurate measurements of how long each leg needs to be I have updated my PCB designs and uploaded them to OSH Park for the whole world to grab and reproduce:
https://oshpark.com/profiles/captcha. Just order the ones you need and OSH Park will take care of the rest.
On the 868MHz board I have also added cut marks for trimming the antenna for 915MHz use. The 433MHz board has cut marks for both 915 and 868MHz. If you are going to trim the tracks, make sure you remove the excess outside traces.
Based on the test results, I calculated the velocity factor of the FR4 for a single-layer printed dipole (track width 1mm, one once copper) to be about 0.77.
All these designs have solder pads for an edge-style SMA connector:
(Prototype board with SMA edge connector)
I ordered the SMA connector from RS with the part number 526-5785 (
http://au.rs-online.com/web/p/sma-connectors/5265785/)
In the PCB designs I have deliberately left a solder mask over of the bottom sma connector ground pad. I did this purely to minimise corrosion in case you wish to solder leads to the pads instead of using the edge SMA connector. The solder mask is fairly easy to scrape off.
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.
I mainly made these designs for myself so that I could get easy access to a low-cost and good performing omnidirectional antenna for 433MHz. I did the 868 and 915 designs as a bonus for the community as I already had everything set up and it only took me an extra half hour or so to come up with the final designs. I'm hope some of you will appreciate it.