"Steve" wrote in message
news:20151119002316.0a9bd48f@silent...

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.
Hi
You are right.
I think a good idea would be to use a coaxial cable with a high velocity
factor.
and big outer diameter with regard to the wavelenght.
The funny thing is that if you use twin lead instead of coax
it doesn't work at all :-)
Another problem with colinear antenna is beam tilt with very little spacing
from the optimum design frequency.
Here the single frequency of 1090 Mhz helps a lot.
And the fact that the sources are moving too.
But this type of signal is the worst that could be choosen
to get an idea of the real performance of an antenna.
The next worst thing to do is using a spectrum display
in the evaluation.
73's