On Sun, 24 May 2009 07:30:03 GMT, "Jerry"
wrote:


"Jeff Liebermann" wrote in message
.. .
On Sun, 24 May 2009 04:04:52 GMT, "Jerry"
wrote:

Have you considered flush antennas, like slots?

Slot antennas have the maximum lobe perpendicular to the rocket axis.
There's very little signal below the rocket. This is why tracking
stations are far away from the launch site. If the rocket were
overhead, and going straight up, there's no signal.

Slot antennas are also a power waste. You need 4 slots, run by a
power splitter, in order to insure that at least one antenna is
oriented in the direction of the receiver. Meanwhile, the other 3
slot antennas are radiating power to nobody in particular. Say goodby
to about 3/4th of your tx power. Receive sensitivity is not
affected.

Hi Jeff
Why are you so negative?? I have designed and built lots of slot
antennas that were widely used on military aircraft. The efficiency is
quite good. I am sure a smart guy like you could design a slot antenna and
locate it for those guys with the 3G rocket. There hasnt been any text
that restricts where the slot could be located. As I read the original
post, they werent receiving signal from the accending rocket. Maybe they
only want to receive data that was recorded after the rocket reached it's
peak. There are lots of information that you and I dont yet know. But,
my major question for you is "Why are you so negative?".
Jerry KD6JDJ

I don't think I'm being particularly negative or offensive. I offered
what I consider to be a more reasonable alternative (CP turnstile
antenna) and ran the path loss calculations to insure that it would
work. I also itemized why a slot antenna would be an inferior
solution. My main point is that with a vertically ascending rocket,
the antenna pattern should be primarily ahead and behind the rocket,
not perpendicular.

I've seen one paper design that used insulated fins for the 4
turnstile antenna elements. However, I haven't seen the actual
rocket. I also built a 2.4GHz antenna system for a model airplane for
a video downlink. The initial design was a slot antenna system in the
fuselage, which worked great when the aircraft was overhead, but did
badly when near the horizon and out of the antenna pattern. Two
vertical monopoles and a power splitter did much better, especially
since it worked well in inverted flight. I must admit that I haven't
designed a slot antenna system for a high power Mach 3 rocket. I did
build a small "G" motor rocket without telemetry but that doesn't
count for much.

Anyway, we're both guessing. Without a clue as to the size, geometry,
and construction of the rocket, the configuration of the ground
station antenna, the location of the ground station, and the
approximate trajectory (probably straight up), we can only speculate
as to the optimal design. My guess is that this a level 1 rocket with
an "H" or "I" motor which could barely tolerate the slight increase in
diameter required for multiple slot antnnas. Wire or fin antennas
don't add anything.


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