Today I traveled down to Heritage Park. With a pond and trails, it is the nearest public space without much RFI I can use for testing antennas.
Also, I managed to acquire quite a few cheap 2.4GHz yagi antennas from China. These
can be found on eBay
for between 8 and 20 USD.
Without an anechoic chamber, I'm a little limited to how I can do my testing. The antenna under test is mounted at the end of a wooden 1x1 beam.
The beam is mounted to a picnic table with clamps, holding the antenna 16 inches away from the table and parallel with the table.
Measurements are provided by my RigExpert IT-24, using the python image scraper I reverse engineered earlier. The self-calibration was executed a few minutes before
beginning the test.
I have three styles of antennas, labeled A, B, and C.
Type A consists of a short coax pigtail, running into a plastic enclosure with a small metal dipole. I suppose they hoped it would
resonate with the other elements on the boom.
Type B consists of a longer coax pigtail, running into a cylindrical plastic housing, with a loop around the body of the antenna,
and some component heat-shrinked inside the loop. My assumption is that whatever components are inside the heatshrink provide an
Type C consists of a longer coax pigtail, running into a cylindrical plastic housing, with a loop around the body of the antenna, but no
components under heatshrink.
Standing-Wave-Ratio, or SWR, is the ratio of
either voltage(VSWR) or power(PSWR) transmitted into an antenna to that emitted.
SWR can vary wildly between different frequencies, so the IT-24 includes a feature to plot the SWR
of an antenna across a range, specifically between 2.3GHz and 2.6GHz, or some subsets of that range.
High SWR can come from a variety of sources, from bad cables, to poorly designed antennas,
to poorly controlled component part tolerances, to a bird nest. Antenna design is a system
problem that tends to affect every antenna of that design. Part to part variances tend to indicate
production, supply, or quality-control problems. Bird nests will be an ever-present installation location hazard.
Additionally, good SWR can
come both from the antenna efficiently transforming electrical signals to radio signals, but
it can also come from high-loss cable, where the transmitted power is simply eaten by the
cable before it hits the antenna.
...are surprising. The three designs vary wildly, but none are very stable over frequency.
This is just not a good antenna - the SWR meter was pegged, so this antenna will
always reflect at least 80 percent of the signal instead of transmitting it to the air.
This also means it will receive less than 20 percent of any signal. The IT-24 can measure
SWR up to 10:1, but this antenna is so bad I won't even try. I do notice that the coax is
secured with a zip-tie, that seems to crush the coax. If there coax were crushed to the
point where the shield and center conductor touched, it would certainly return a bad match,
but anywhere in-between good coax and crushed coax will likely increase SWR somewhat.
Anyhow, these are garbage and will be discarded.
We have a winner, but I think they could be better.
Antennas B1 and B2 both have 2 to 1 SWR or better across the WiFi range.
However, averaged over frequency, B1 is noticeably better than B2. Good quality antennas will perform similarly in similar circumstances.
Also, the frequencies with low SWR are different between the two samples, which indicates some tolerance or variability in the design, or perhaps in the test.
I am very curious what magic lurks inside the heatshrink, although with only two samples, it is unlikely one will be sacrificed.
Welcome to funkytown. These antennas are odd.
First, the cable is 75 ohm, but the IT-24 is a 50 ohm device. It is possible
this was a design choice, as 75 ohm cable tends to have lower loss than the equivalent 50 ohm cable,
and if the cable is long enough, you can come out ahead by eating a single mismatch loss, but saving
on the loss of the cable itself. It is also possible 75 ohm was chosen because it tends to be cheaper.
Next, we see high variance in SWR over frequency. This isn't necessarily bad, as long as the
high SWR frequencies are outside of our operating range. But in this case, they do impact
the 2.4GHz band.
And, just like the Type B antennas, the variance in SWR is not stable from one antenna
to another. But unlike the Type B, where frequency varied SWR between 1 to 1 and 2 to 1, the
Type C varies between over 5 to 1 and 1 to 1. This means that each of the four Type C antennas
tested will have very different efficiencies on different channels of WiFi.
The three designs tested have very different operating characteristics. Although
the eight antennas tested are not enough to be statistically significant, I will
likely select Type B for future usage and testing. Type A will be discarded, and Type C
may be used based on the channel needed.
The next thing to test is how well each antenna can receive signals from a known test transmitter.
Since I don't have a calibrated antenna, all tests would be relative to the gain of the antenna
used on the transmitter and would have no use outside of comparing the yagis to each other. However,
there are storms approaching, so there will not be time to do this test today.