Allright, last night's experimentation with the RTL-SDR dongle on my Raspberry Pi Model B+ was pretty successful. Incidently, I forgot to mention that this worked fine with the dongle plugged directly into the Pi, I didn't need a powered hub. That's pretty cool. Previously, I had experimented with decoding ADS-B signals from airlines. I thought this might be a pretty good thing to do with the Pi. I ordered a little MCX->Female F pigtail off of Amazon for under $6 shipped, and then thought about doing a better antenna. I would have also ordered a little case for the Raspberry Pi, but all the ones I could find for the B+ seem to be back ordered. Sigh.
I know that Darren at Hak5 and whixr at tymkrs.com had build colinear antennas out of coax for this purpose. I went to review what they had done before. It's a pretty straightforward antenna to make. Darren has a nice video and writeup:
I was curious though: his discussion of velocity factor ended with... our velocity factor is 0.85. That might be true for his coax, but how do we know?
Well, we could trust the manufacturer. Or we could guess, based on the material that we think the dielectric is. But I think I'll use my MFJ antenna analyzer to figure it out. The basic idea is to take a length of coax of length L. Sweep up from the low frequency and find the lowest frequency where the coax is resonant (where it is a pure resistance, which will also likely have the lowest SWR). Say that frequency is f. if you divide 300 by the frequency in megahertz, you should get the wavelength in free space in meters. But in the coax, four times the length of your coax is the wavelength in your coax. So, if you divide that length by the free space length, you should get the velocity factor of the coax.
When I get some coax, I'll try this out. Getting this length right is probably pretty important. I might also try to run some simulations to find out how systematic changes in fabrication affect the performance.
I'll probably do a 8 or 12 element colinear. I suspect that without an antenna analyzer that can go up that high, fabrication errors for more elements will lead to dimininishing returns and ultimately maybe even diminishing performance.
Addendum: A nice video showing good construction technique...