A few days ago I wanted to build some more PCB monopole antennas for testing with the Mesh Potato. This is really easy – you just solder the SMA connectors to the PCB, and add a few bolts. So I built 4 antennas and hooked them up to SWR bridge (described here) to make sure the impedances were OK.
They were all duds – high SWR and clearly not resonant at 2.4GHz. Huh? The first two I built last month worked fine. An hour of head scratching and ill temper followed until I worked it out.
On the recent antennas (top) I had soldered the connector to the front of the PCB, the same side as the microstrip and antenna feed. On the first antennas (bottom) I had (purely by chance) soldered the connector to the rear of the PCB. These antennas worked well.
This is a graphic illustration in how microstrip transmission line works. A transmission line (e.g. an Ethernet cable, television cable, phone cable) transports your signal from one end to the other more or less intact. A regular piece of wire will distort the same signal over any significant distance, for example you can’t use regular wire to connect your TV to your TV antenna.
Transmission line is specially designed for the signal and impedance at the source and load of the line. Mess with the design of the transmission line, and the impedance changes, and your signal gets messed up. For example if you nick or bend your TV cable your picture might get fuzzy as signal is lost part way down the cable.
Microstrip is one way of making transmission line on Printed Circuit Board (PCB). Our monopole PCB antenna contains a length of microstrip transmission line that feeds the signal from the connector to the antenna:
The actual antenna starts where the solid chunk of ground plane (blue) ends. Both the antenna and the transmission line are the same 3mm track. However when you put a ground plane 1.6mm below a 3mm width PCB track on FR4 PCB dielectric you happen to get a 50 ohm transmission line. It’s just like a piece of 50 ohm coax as far as the signal is concerned. Without the ground plane the 3mm track starts radiating (or receiving).
Now placing the connector on the ground plane side of the PCB means the connector doesn’t interfere with the microstrip. Electrically the connector ground becomes part of the ground plane:
However if the connector is placed on the microstrip side we now have a messed up stacking of conductors – we have a conductor at ground potential just above the microstrip track rather than 1.6mm away:
Suddenly the transmission line is not a transmission line any more and the energy being fed into the assembly doesn’t make it to the antenna.
Note in the last picture the microstrip track and connector ground are not electrically connected, as the solder mask insulates them. However the transmission line properties of the microstrip are messed up due to the nearby ground conductor of the connector. I also haven’t illustrated the plated thru hole for the SM connector center pin and matching PCB pad on the ground plane side.
I finally understood transmission lines after reading the chapter from Tony Kuphaldt’s on line book on AC electric circuits. This series of books is excellent and I highly recommend them. I found the link to these books on Mark VandeWettering’s brainwagon blog.