Revised via spacing


Tests of PCB antenna performance revealed a sharp drop in received power
at 2440 MHz, roughly the middle of the antenna's frequency range.

This drop can be seen in the red (ant-120A0) and blue (ant-120B2) curve
of [1].

The test setup consisted of an atusb board with discrete balun and an
antenna with only a single ground plane as the sender, and a set of
antennas (see directory ../ants/) connected to an USRP2+XCVR2450 as the
receiver. The receiving antennas had two ground planes, connected with
vias spaced roughly 5 mm apart. (See ECN0004.)

According to figure 1 of [2], the vias connecting the ground planes at
the edge of the antenna should be spaced at around 50 mil, four times
closer than 5 mm.

After adding more vias to one of the receiving antennas, sensitivity
improved drastically, as shown in the purple (ant-120C0) curve of [1].
The same improvement could be observed in a set of ten antennas that
all had shown the drop before the addition of more vias.

The remaining drop of about 10 dB in the middle of the spectrum is
probably caused by the sending antenna. A commercially produced 2.54
GHz antenna at the receiver showed a similar drop when used with the
same sender.

[1] http://downloads.qi-hardware.com/people/werner/wpan/three-ants.png
[2] http://www.ember.com/pdf/120-5052-000_Designing_with_a_PCB_Antenna.pdf


Conclusion: place vias at the edge of the ground plane facing the
antenna with a spacing no larger than 50 mil.
