I finally found the time to make some measurements on the printed antenna used on my sensor board, which I didn’t think was performing very well. Turns out I was right.
For this first part of the test various antenna configurations were tested by mounting them on unpopulated sensor boards connected to an Anritsu Sitemaster RF fault analyser via some added SMA connectors. This instrument is capable of making return loss measurements, which were taken over a range of 800 MHz to 1 GHz.
Return loss is a measure of the proportion of power reflected back into the transmitter (or back out of the antenna) due to a mismatch. It is given in decibels as incident minus reflected dB, and is related to VSWR. A high positive return loss value indicates a good match.
Measurements were made on the following antennas:
- A reference dipole tuned for resonance while mounted on the instrument
- The printed antenna cut to the recommended length for 868 MHz as given in the TI application note
- Wire monopoles of length 85 mm, 86 mm and 87 mm
For the purpose of these tests a target VSWR of 1.5:1 was selected as an acceptable match, and corresponds with 96% of the incident power doing useful work. The corresponding 14 dB return loss figure is marked in red on the graphs. Note that the instrument shows the return loss as a negative value, but the magnitude may be read directly. Marked in blue are the ISM band edges at 863 MHz and 870 MHz, and also the common 868.35 MHz spot frequency.
Each antenna was tested with and without a battery pack attached to the board. This is important because the PCB ground plane capacitively couples to the battery quite effectively and the performance of the antenna is drastically altered in most cases. The battery consisted of a pair of (old) AA cells in a common side-by-side holder and was attached with double-sided tape.
First up was a reference dipole made by attaching short lengths of wire directly to an SMA connector. The overall length was initially set to half a wavelength at 868 MHz (173 mm). During the test the antenna was aligned perpendicular to the plane of the analyser to minimise coupling. As expected, it was necessary to reduce the length of the antenna slightly to achieve resonance at the desired frequency. The overall length of the tuned antenna ended up being 160 mm, which is reasonably consistent with the theory, which results in a half-wave dipole being slightly shorter than a half wavelength.
The antenna exhibits a sharp resonance at the tuned frequency and has enough bandwidth to provide an excellent match across the entire 863 MHz to 870 MHz band.
The original PCB antenna was taken from the design in this (PDF) application note. The dimensions of the rest of the board are fairly similar, but likely too small to provide an adequate counterpoise. Without the battery there is a poor but measurable match in the band of interest. The actual resonance appears slightly high meaning the antenna needs lengthening slightly. The other dip seen below the 868 MHz band is present in most measurements and appears to be related to some other feature of the board, as its frequency remains fairly constant.
With the battery attached, a more realistic scenario, the antenna presents a pretty bad match (VSWR about 3:1 or worse):
A second unpopulated sensor board was modified with an SMA test port added and the printed antenna removed. Simple quarter-wave wires were connected to the test port, using the sensor PCB as counterpoise. Measurements were again taken with and without the battery affixed to the PCB.
The expected quarter wavelength at 868 MHz is 86.5 mm, so wires with lengths 85 mm, 86 mm and 87 mm were tested. The measured results were similar for each with the optimum length for the band of interest being 87 mm. Note that the absence of the battery caused the resonant frequency to increase significantly as shown in the following plot. Although it might be tempting to compensate for this by lengthening the wire, it is likely that the resulting antenna would be easily detuned.
By attaching the battery the antenna behaves much closer to the theory, which is encouraging:
And by bending the antenna down so that it is in the plane of the board the tuning is retained within the ISM band but with further improvement to the matching.
Again, note the sharp match at the frequency below the band of interest, which is presumably due to the same feature that caused the double-dip seen for the PCB antenna. Its frequency is not altered significantly by changing the length of the wire as shown in the final plot, for an 85 mm long wire. This measurement offers an indication of how accurately the antenna needs to be cut for best match.
The results suggest that a simple quarter wave wire can yield an excellent match even on a small board such as this, but that enlarging the ground plane is beneficial, even if this is only done parasitically such as by mounting in close proximity to the battery.
Once the rain and snow stops I am going to go out to the
Open Air Test Site garden to do some actual radiated performance tests which will be reported in due course, but I expect the results to back this up.
If this does turn out to be the case then I will be dropping the printed antenna in favour of 87 mm of wire for a second revision of the sensor!