On Fri, 06 May 2005 19:23:55 -0700, wrote:

Thanks for all the fine suggestions.
Some general comments and answers to questions...:

1)The body of the rocket is Aluminum.

Figured that.

2)The receiving station is right next to the rocket so as it launches
it will be going directly away from the telemetry transmitter.

Not too close I hope [g].


3)The nose cone is out as it detaches and comes down via seperate
parachute at the time of recovery.

4)No roll control system so the rocket will be expected to spin
slowly.

5)1W of output power.


It looks like a cu tape dipole on the fin with some glass over the top
might be best solution, it seems no one but me is worried about the
carbon fiber.

I didn't say that. I'm not a composites man, but it seems to me that
"carbon fiber" is a pretty broad catagory, with potentially a wide
range of dielectric properties. When you attach your radiator, of
whatever kind, to a dielectric substrate the electrical properties are
going to be affected to some degree.

If by experiment (something you are without the means to do) you find
the proper length for your dipole, then when you encapuslate it in
glass fiber, you have added a second dielectric.

And then... how do you attach the transmission line? I presume that
the line will be inside a hollow fin but you have to make a connection
to a foil that, until it is covered with the overlay, is on the
outside of the fin. Even if you succeed in the attachment, at Mach 2
this is not a benign enviroment. I suspect a lot of vibration. A
soldered connection might fail. In industry, we would shake the hell
out of the thing to see what happens.


So using 1/4" cu Tape glued to carbon fiber ,how long should it be
and should the ends be round, square or pointed? (910 Mhz)

See above. Can't say.


The rocket has three fins 120 degrees apart, could I put an antenna on
two fins and get quasi circular polarization?
If so how should I drive the two antennas?
(I'm not an antenna guy so please try and be specific, ie use a 21.5cm
peice of Rg-XX)

As you've seen, being "specific" isn't going to happen. I think you
have enough trouble without trying to achieve circular polarization,
something that doesn't seem to be a requirement anyway.


I have no portable antenna test quipment for 910Mhz, but I can carry
the resulant antenna into a friends work and use a 2Ghz spectrum
analizer with a tracking generator if that would be useful to test
antennas.

No help unless you can use a reflection bridge, driven by the
generator and sampled by the SA. Otherwise, these are useful for
insertion measurements only.


If I have just one fin antenna how can I build a circularly polarized
antenna for the ground side?

The axial-mode helix would normally seem to be the antenna of choice.
They are forgiving designs and of reasonable size at 900 MHz. Do not
expect the gain that is rumored by Kraus. However, one expects that
the missile goes "downrange" so if you want recovery data down to the
ground then you might need some tracking using a gain antenna.

An option might be a turnstile over a groundplane that can give
circular polarization and a nearly omni pattern down to the horizon.
But if you achieve the 100 mile altitude and depending on when the
parachute is deployed and winds aloft, "downrange" might be quite a
distance. You need to do some link budget calculations for this.

Well, let's just kick back and relax--after all-we are not dealing with
"Rocket Scientists" here....

OHH! Wait a minute--WE ARE!!!! grin

Warmest regards,
John

wrote in message
...
| I've voulenteered to help the SDSU mechanical engineering studens get
| telemetry from their rocket see:
| http://www-rohan.sdsu.edu/~sharring/sdsurocket.html.
|
| I have all the electronics working, I'm using a commercial 910Mhz
| telemetry radio, I have every thing working except the antenna.
|
| For the last launch I burred a dipole in the plywood fin, alas
| the rocket did not launch it caught fire and burned up the fins.
| (It did not burn as far as the electronics.)
|
| The new fins are carbon fiber composite so no antenna there...
|
| The rocket will get to mach 2 so small wires sticking out will
| probably break or burn up.
|
|
| I have enough power and ground side gain that I need no gain
| from the rocket, an isotropic radiator with 3db of loss would be fine.
|
|
| Any suggestions?
|
|
| My ideas and thoughts:
|
| 1)Simple 1/4 wave vertical sticking out the bottom plate of the rocket
| near the engine.
|
| Pros:
| simple.
| Cons:
| lots of metal to block the signal and mess up the pattern.
| Not clear if the ionized exhaust will block the signal.
| Antenna pattern is almost exactly wrong.
|
| (Telemetry really needed for recovery tracking so ionization fading is
| not a deal killer)
|
|
| 2)Horizontal dipole at the bottom plate of engine.
| All the problems of #1 except pattern.
|
|
|
|
| 3)Put Fiberglass windows in the electronics bay near the nose of the
| rocket. One window on each side, Driving two hosrizontal dipoles with
| a power splitter, one dipole on each side.
|
| Pros: Easy to do.
| Cons:
| I don't know what the pattern would be like, or exactly how I shoudl
| phase the two antennas on opposite sides. (Some metal between then so
| not a clean situation.)
|
| Resources:
| It have a minicircuits SMA 2 way power splitter, and can make precise
| metal parts (0.002" or better).
| I do not have any antenna testing equipment that is any good at
| 900Mhz.
| so any suggestions...
|
|
| Paul (Kl7JG)
|
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Paul:



If you don't mind, I would have a question for you (or, if you would be so
kind as to pose it to a college there)--one I have always pondered...



Take your rocket there, if its top speed is mach II--why not--say at
mach-one, release the fins in a simultaneous-organized-precise
manner--ridding the rocket of the leading fin edge drag--and
hopefully--allowing it to reach a greater max speed/altitude...



Wouldn't the "steering" of the air drag on the sides of the body provide
enough "control" at such speeds or, even sub-mach speeds--possibly?



Of course, if that antenna is there... grin



Warmest regards,

John



wrote in message
...
| I've voulenteered to help the SDSU mechanical engineering studens get
| telemetry from their rocket see:
| http://www-rohan.sdsu.edu/~sharring/sdsurocket.html.
|
| I have all the electronics working, I'm using a commercial 910Mhz
| telemetry radio, I have every thing working except the antenna.
|
| For the last launch I burred a dipole in the plywood fin, alas
| the rocket did not launch it caught fire and burned up the fins.
| (It did not burn as far as the electronics.)
|
| The new fins are carbon fiber composite so no antenna there...
|
| The rocket will get to mach 2 so small wires sticking out will
| probably break or burn up.
|
|
| I have enough power and ground side gain that I need no gain
| from the rocket, an isotropic radiator with 3db of loss would be fine.
|
|
| Any suggestions?
|
|
| My ideas and thoughts:
|
| 1)Simple 1/4 wave vertical sticking out the bottom plate of the rocket
| near the engine.
|
| Pros:
| simple.
| Cons:
| lots of metal to block the signal and mess up the pattern.
| Not clear if the ionized exhaust will block the signal.
| Antenna pattern is almost exactly wrong.
|
| (Telemetry really needed for recovery tracking so ionization fading is
| not a deal killer)
|
|
| 2)Horizontal dipole at the bottom plate of engine.
| All the problems of #1 except pattern.
|
|
|
|
| 3)Put Fiberglass windows in the electronics bay near the nose of the
| rocket. One window on each side, Driving two hosrizontal dipoles with
| a power splitter, one dipole on each side.
|
| Pros: Easy to do.
| Cons:
| I don't know what the pattern would be like, or exactly how I shoudl
| phase the two antennas on opposite sides. (Some metal between then so
| not a clean situation.)
|
| Resources:
| It have a minicircuits SMA 2 way power splitter, and can make precise
| metal parts (0.002" or better).
| I do not have any antenna testing equipment that is any good at
| 900Mhz.
| so any suggestions...
|
|
| Paul (Kl7JG)
|
|
|
|
|
|
|
|
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