Just as a word of warning, I'm neither an RF expert nor pretend to be one. I spent most of my days in the university ditching classes, hanging out at coffee shops, and attending dance rehearsals. I spent the next decade and a half pretending to work at my various jobs. I'm the furthest thing you can get from an PhD'd RF engineer. But if you're willing to accept all that and are still willing to believe me, then read on:
I made a very interesting discovery recently while I was trying to improve the range of my 2.4 GHz wireless boards. I was getting limited range out of the on-board SMD antennas that I was using and was doing testing on multiple boards to see if it was due to the variance of the discrete RF components or if it was an inherent property of the system.
The funny thing was that in my sample of about five boards, one board constantly demonstrated a high received signal and transmitter output, almost twice that of the other boards. I was keeping detailed notes of the component values on all the boards so I copied the exact same component values on another board but couldn’t duplicate the results. The complete matching circuit was identical and I even replaced the radio and the balun to see if those were causing the discrepancy.
I spent about two days investigating this and trying to get to the bottom of why that one board was constantly outperforming the rest of them. I finally found the solution and it was staring at me in the face.
On all of my radio boards, I have two antenna options. One of them is for the onboard SMD antenna and the other is for the right angle SMA connector which can interface to external antennas. You can choose between the options by moving a capacitor to either the SMD antenna path or the SMA path. The board that was constantly outperforming the others had the capacitor in the SMA path rather than the SMD path. The strange thing was that there was no external antenna connected to the SMA.
I began investigating it and found that the SMA connector was actually acting as the RF antenna. I had to say that I was surprised that the SMA was able to radiate at all, but the biggest surprise was how efficiently it was radiating. It was actually outperforming the 2.4 GHz SMD antenna.
Now I was asking myself how this could happen. The 2.4 GHz SMD antenna was designed and optimized for these types of applications, but it was getting trounced by a lil’ old SMA connector. You have to remember that I had tuned the matching circuit for 50 ohms using my network analyzer and these test boards were all able to achieve close to 99% efficiency as measured at the SMA. Hence, I could guarantee that the SMA RF path was very efficient.
A problem arises when you need to measure the SMD antenna path. Since the SMD antenna path doesn’t have a connector, you need to solder an SMA to it and then measure the return loss. No electrical delays are required when measuring the return loss since you’re just looking at the ratio of transmit energy to reflected energy. However it’s hard to get very accurate readings when doing this because the measurements will change based on where you choose your ground, the orientation of the SMA you hacked on to it, etc. So tuning a circuit with a hacked connector is a lot like fumbling your way through the dark.
Since I’m able to guarantee the matching circuit up to the point where the RF path diverges to the SMA or SMD antenna, the main difference between the two paths would be the difference between the trace lengths at the divergence. In other words, since trace length is just an inductor at high frequencies, the difference would theoretically be the same as adding an equivalent inductor into the RF path. Since I matched the trace lengths for the two paths, that inductor would be extremely small, much less than 0.1 nH according to my board characteristics.
So my conclusion is that the SMD antennas that I’m using are most likely poorly matched to 50 ohms which is why they seem to not radiate efficiently. But more importantly, I discovered that the right angle SMAs are able to radiate efficiently and it looks like its because the total length of the center conductor of the SMA is approximately 30 mm. The 1/4 wavelength of 2.4 GHz is approximately 31 mm which would explain this phenomenon.
By now, you’re probably tired of all this RF talk and want to know what my main point is. The main point is that by using the right angle SMA as a 2.4 GHz antenna, you can achieve a decent, if not good, radiation efficiency. Its more than adequate for short range wireless and I’ve been able to transmit successfully inside a hallway (read: hostile RF environment) at up to 40m with a couple of dB to spare on my Rx signal strength.
If you’re not satisfied with the performance, you can attach an external omni-directional or high gain antenna to the SMA without needing to change a thing. Hence, this interesting discovery allows people to have multiple antenna options at 2.4 GHz based on the form factor and range they want without the need for having two separate matching circuits for an SMD antenna and an external antenna. If you don’t know the importance of this, try ordering an XBee with both an SMD antenna and external SMA connector.
Incidentally, I checked Google and couldn’t find any references to using a right angle SMA connector as an antenna. Thus, I will name this antenna the “WTF Antenna”. It accurately reflects my feelings when I first saw this phenomenon.
And for those interested in the exact right angle SMA connector I’m using, it’s part number 07259 at 4UCON . However I’ve tested this out with a different manufacturer’s right angle SMA and another right angle RP-SMA connector and have seen similar results. Don’t take my word for it. Try it out for yourself :)
P.S. If this type of antenna does indeed already have a name, then please let me know...
PPS: Here's a pic of one of the boards I'm using to test. None of them have an SMD antenna and they are all communicating very well: