On Wed, 18 Nov 2015 17:00:11 -0800
artie wrote:

In article 20151119002316.0a9bd48f@silent, Steve
wrote:

With a RTL SDR type USB stick on its way, I turned my attention to
looking for an antenna with high gain that would give me good
coverage of ADS-B 1090 MHz broadcasts from aircraft, one that I
could easily and cheaply make for myself.

A quick google produced loads of hits for this type of antenna.

https://www.youtube.com/watch?v=TkUYdCPFXXs

as a random one. You get the idea.

They all consist of 'accurately' cut and calculated equal elements
that reverse the phase 180 degrees for each element so the theory
goes.

Pondering over this, it struck me that although the websites do take
into account velocity factor of the coax, that is for the coax in
normal operation. Once the outer conductor, or screen becomes an
element in an array, exposed to the outside world it has a faster
velocity of propagation which, in turn means that the outer of the
coax must be longer than the inner which is there to maintain phase
on each segment. In other words an impossible antenna to make.

Those were my thoughts which have led me to have doubts about the
cheap, simple designs that abound.

Any thoughts, or have I missed something?

Steve G8IZY

You really want to visit the flightaware forums:

https://discussions.flightaware.com/...-tracking-f21/

you'll find very detailed discussions of antennas and how to get
performance out of your SDR setup for receiving ADS-B.

Yes, thank you. A quick read of that site suggests that it is full of
interesting reading. None, however, deals with the theory of operation
of the antennas. That is why I posed my question here.

Executive summary: co-linear antennas can be very good performers.
They are very precise beasts; a poorly made co-linear antenna really
sucks.

I think you have missed the point. I was talking about coaxial
collinear antennas, made from coax cable and the design problems I see.

There are a number of very good, very easy to make designs available.

You also need to consider the RF environment -- the SDR is a very
wideband beast and you will probably have to consider bandpass
filtering to prevent overloading the front end.

Indeed and quite possibly! But that is a separate issue and is not
related to my question.

All this and more in detail on the FlightAware forums!

Do they discuss antenna theory there?