On Wed, 18 Nov 2015 19:40:34 -0500
"Ralph Mowery" wrote:


"Steve" wrote in message
news:20151119002316.0a9bd48f@silent...
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 are really missing 2 things. For the coax sections, when using
the outer sections, they are the actual antenna elements and the iner
conductor are not used. Then the sections in between are using the
velocity factor of the coax for the phasing sections.

Which is why I questioned the fact that that the sections
on all these designs are of equal length as they would have to be
different.

That means (if using 1/4 wave sections) that every other section will
be almost 1/4 wave without any velocity factor correction (actually a
very small one) and the other sections will be 1/4 wave times the .66
velocity factor of the coax (or whatever the VF is for that
particular coax.

Yes, that is why I questioned the design, as all the lengths are equal.


The other thing, to get gain the antenna pattern is compressed so the
signal will be greater toward the horizon and not so much up in the
air where the planes are. Two or 3 elements may be ok,but going to a
large number may not work as well for the planes.

I'm more interested in looking to the horizon than looking for strong
line-of-sight aircraft above me. Some of the software
allows for a statistical analysis of the distances, bearings and signal
strengths achieved to be presented as a polar plot which gives one the
ability to, in effect, identify good or bad take-offs. Being interested
in VHFDX propagation, I think this would be useful to me.

Steve G8IZY