"Jerry" wrote in message
...

"JosephKK" wrote in message
...
On Mon, 16 Feb 2009 18:49:47 GMT, "Jerry"
wrote:


"JosephKK" wrote in message
...
On Tue, 10 Feb 2009 18:12:26 -0000, "christofire"
wrote:


"Jerry" wrote in message
...

snip

Chris

Interesting article, it describes the Lindenblat array as a quartet of
coaxial horns. Not the same as a quartet of dipoles at all.


Hi Joseph

I sent an E-mail to the address shown as yours in this group. I use
EZNEC and have lots of files on various sizes and shapes of the DCA and
other OmniAzimuth and Hemispheric coverage antennas. I'd be happy to
share
them with you if you E-mail me directly.
I figure it will benefit me to see the facts and data that shows where
I
am mistaken about how the DCA better than the other hemispheric coverage
antennas for LEO use. It is likely that I have overlooked something.
maybe the DCA can be improved.

Jerry KD6JDJ

Jerry KD6JDJ

I can retrieve the email. Thanks for the notice.

I have done a bit of gedanken after the slant range issue was raised.


A


B



o H
ooo
ooooo

Let the group of "o" represent a polar view of earth,
"A" represent satellite at azimuth (highest point in the sky),
"B" represent satellite between azimuth and horizon,
"Z" represent satellite very near the horizon,
nominal orbital height 1000 miles above mean surface,
and mean radius of earth of 4000 miles.
Then range to the satellite at A is about 1000 miles, at B may be
about 3000 miles and at H would be about 5000 miles. The arithmetic
for relative sensitivity versus elevation angle can even be solved
analytically, though 4 evenly spaced angles on either side of azimuth
would be quite sufficient to plot the most even EIRP curves.
Your DCA is notably more hemispherical than that. Can you tinker it
up to get about 16 dB more at the horizon than at the azimuth?

Hi Joseph

I am not familiar with all the satellite orbits. It seems that most of
the Polar Orbiting Satellites are about 500 miles above the Earth. If the
Earth radius can be considered to be 4,000 miles, the distance to the
satellite at the horizon is about 2,000 miles. That means that, the path
los from the satellite to the ground station varies by about 12 dB as it
passes overhead.
One of the objectives of the DCA design project was to produce a pattern
with about 12 dB less sensitivity toward zenith. I attempted to achieve
that pattern shape by tilting the dipoles to minimize overhead
sensitivity. Serindipity offered me a side benefit when it made me aware
that tilting the dipoles in this array effects the dipole input impedance.
The input impedance of the dipoles in a DCA with 45 degree tilt (90 degree
cross) is about 72 ohms. The dipole input impedance in a DCA with 30
degree tilt is very near 50 ohms. Kinda lucky!!

Jerry KD6JDJ


.... but, dare I ask, what does that do to the axial ratio? Indeed, what
kind of axial ratio is being sought in this application? ... 3 dB or more?

Chris