Another update from Joe Lucid's Antenna Lab:
I built a new loop antenna today, with a diameter of 3.5cm. I tuned it using the VNA, but afterwards fine tuned using RSSI values. I then took a moteino and the th mote with loop antenna on a shootout through the house:
The Moteino is equipped with a rfm69hw, the th mote with a rfm69w. I set both to full power and measured RSSI values at the receiver (an espgw with dipole antenna). Thus I was transmitting with 7dBm more power on the Moteino than the loop.
Not surprisingly RSSI values in the same room with the receiver differed drastically by orientation reflecting the linear polarization of both antennas in perpendicular directions. But starting in the next room there wasn't much difference between the orientations.
Overall the Moteino produced RSSI values between 9dBm and 3dBm better than the loop, depending on the orientation of the Moteino (rotation in its main plane) and inline with the 7dBm higher tx power. The loop produced very omnidirectional radiation across rotations around its axis.
From this data the loop performed just a bit better than the moteino with less variation - much better than my first attempt and pretty respectable given the form factor. That's not to say one can be content with these results as the dipole measurements have shown that significant gains can be had over the monopole over insufficient ground plane approach.
Joe