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

"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.

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.


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.

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.

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

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.

All this and more in detail on the FlightAware forums!

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

As ADS-B is line of sight, I would think a gain antenna is overkill
unless you are trying to get past coax signal loss without a preamp.

I would first try a simple ground plane made from 5 stiff wires and a
coax chassis socket with some decent coax and see what happens.

If that proves insufficient, the only thing not recoverable is the
5 pieces of wire.


--
Jim Pennino

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.

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.


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.

The problem is similar to non-tracking satellite antennas.

The source will be farthest away when it is close to the horizon, but a
lot of sources will be much higher in elevation so you do need some gain
in the upward direction.

Looking at existing 70 cm satellite antennas might be worthwhile.

--
Jim Pennino

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

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?

On Thu, 19 Nov 2015 01:10:27 -0000
wrote:

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.

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.


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.

The problem is similar to non-tracking satellite antennas.

The source will be farthest away when it is close to the horizon, but
a lot of sources will be much higher in elevation so you do need some
gain in the upward direction.

Looking at existing 70 cm satellite antennas might be worthwhile.


Nope! I am looking for maximum gain to the horizon, like any DXer, for
the reason of being able to identify good and bad directions from here,
using the statistical tools available.

I've had loads of advice about watching aircraft near me or overhead
which is not what I wanted as if I want to track a plane, I'll look at
Planefinder or FR24 if my 'gain' antenna lets me down!

On Thu, 19 Nov 2015 00:23:16 +0000, 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.

Here are some NEC2 models of some ADS-B antennas. We've built a few
and they work well.
http://802.11junk.com/jeffl/antennas/AMOS-3-INV-1090Mhz/index.html
http://802.11junk.com/jeffl/antennas/AMOS-5-1090MHz/index.html
http://802.11junk.com/jeffl/antennas/AMOS-5-INV-1090MHz/index.html

The front end of the typical RTL2832U SDR gizmo seriously lacks
dynamic range. If you install a high gain antenna, with a pre-amp to
compensate for the coax losses, in a location with a good view of the
sky, you're going to overload the receiver front end. You're also
going to experience some front end overload and possibly intermod from
other services using the same L band range such as TCAS radar,
JTIDS/MIDS, and DME. Cavity filters are a big help.

For starters, I suggest a very simple antenna with coverage of the
sky. There's plenty out there to hear and you won't be bothered by
overload and interference. Once you have a good idea of what software
is the lesser evil, and you find your range to be inadequate, think
about big antennas, preamps, and filters.

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.

Retch. Those are alternating coax cable collinear antennas. For a
given length antenna, they have 3 dB less gain than a collinear
antenna made with coils or 1/4 wave stubs. The coax flavor are also
end fed instead of center fed, making it difficult to put the main
lobe at the horizon, instead of a few degrees above the horizon (i.e.
uptilt). The good news is that it will have a lousy pattern and low
gain, which should reduce overload and interference problems. If you
really want to build something like that, test the antenna to see
where it's tuned with a sweep generator, directional coupler, RF
detector, and an oscilloscope. The one's I've seen, that were built
without a fixture, look awful.

Good luck.

--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558

Steve wrote:

Nope! I am looking for maximum gain to the horizon, like any DXer, for
the reason of being able to identify good and bad directions from here,
using the statistical tools available.

It would have been nice if you had started out by saying that.


--
Jim Pennino