"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

In article 20151119014546.42105df7@silent, Steve
wrote:


related to my question.

All this and more in detail on the FlightAware forums!

Do they discuss antenna theory there?



Yes. look for posts by abcd567. He has a number of posts on theory
behind co-linear antennas, and common pitfalls in their construction.

Look for posts by other hams (usually identified by callsign) as well.

There's a good article from colorado state on generalized CoCo
antennas. I believe they're also covered in a number of RSGB antenna
and VHF society publications.

(while some discuss theory, a lot of what's discussed is of the "mine
is bigger than yours" variety, and people wondering why the n-element
co-co they built from salvaged wire gives them worse results than the
stock little whip)

--
Namaste--

On 11/18/2015 6:23 PM, 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


See if this helps you...

http://owenduffy.net/blog/?p=157