I was impressed by the simplicity of this article:

TDOA (Time Difference of Arrival) Directional Antenna

which shows how to make a simple, but effective, directional antenna. At some point I am going to try this out.

I also thought this video:

Use a scope to measure the length and impedance of coax

does a good job of explaining how to measure reflections (and therefore lengths and termination impedances) of cables such as coax.

I reckon it should be possible to automate this process using a couple of comparators and an Arduino (or PIC). What I’m thinking is that one comparator would detect the first edge, and the second comparator would detect the second edge (the comparator reference voltages being iteratively set by the Arduino). The comparators would be configured to generate a pulse with a length equal to the difference in time between the two edges. This pulse duration could be measured by charging up a small capacitor, and then reading it’s voltage using an ADC.

The comparators that come in AVR and PIC chips aren’t fast enough, but I’ve had a look on Farnell, and you can get comparators with speeds better than 1ns for less than £2.50 (US $4). Since we’re interested in the difference between two pulses, we might actually get a differential speed much faster than 1ns, which means that possibly we could measure to a resolution of 1 inches (assuming a resolution of 250ps).

The other project I may work on is making a standalone capacitance meter. At the moment I use an Arduino and LCD shield, but I think it could be done with a PIC12F1840.