SWR is not a concern. You may be interested to know that 17:1 SWR was pretty
common in many of the satellite antennas.

As they are interested in signals as the rocket is going away, an end fed
"longwire" would do just fine.

Were it my project, I would use 4 slotted antennas cut at 45 degrees to the
axis of flight. Particularly because the rocket is not spin stabilized.
While Mach 2 was mentioned, I suspect it would exist for a very short time.
Standard Plexiglas would most likely serve well as a slot cover.



"John Smith" wrote in message
...
Well, I just a new experimenter with antennas--in the past have only
played
with "tried and true" designs....



However, I can't help but see that three cu foil horizontal "monopoles"
worked off an aluminum body of the rocket does present itself to the
mind...



Cecil is a good modeler with EZNEC, I am hoping he will find this
interesting enough to comment... and of course the OM Roy is acknowledged
top expert!



If three monopoles, each fed off a quarter matching section of thin coax,
and worked off the counterpoise of the aluminum rocket body
"counterpoise",
would present a load of say ~11 ohms (or, does it work that way? or, would
that present a load of ~36 ohms?), then a 4:1 UnUn could be used,
"backwards", to present a 44 ohm to the xmitter--50/44 = negligible SWR...
one monopole on each fin....

While NOT circular--maybe close enough to provide acceptable signal
strength...



If this is plausible, a formula for cutting a 1/4 wave as SHF, and cut a
bit
longer, then trimmed to resonance by coupling to a GDO capable of 900+
Mhz....



But, you are probably too close to launch time for extensive
experimentation... or, are some of those students' hams to assist?



But then, my mom always said I read too much "science fiction."



Maybe these other guys will apply their knowledge here and both you and I
will pick up some points...



On a side note, two crossed dipoles make a turnstile antenna, this is a
circular polarized antenna... but crossing two of those fins is
impossible... maybe the dipoles can be separated by some distance and
still
work... here I can only wonder... indeed, if you choose this, would be a
shame to leave that third fin out there, naked... grin



Whatever, GOOD LUCK!!!



Warmest regards,

John




wrote in message
...
| Thanks for all the fine suggestions.
| Some general comments and answers to questions...:
|
| 1)The body of the rocket is Aluminum.
|
| 2)The receiving station is right next to the rocket so as it launches
| it will be going directly away from the telemetry transmitter.
|
| 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.
|
| 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)
|
|
| 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)
|
| 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.
|
|
|
| If I have just one fin antenna how can I build a circularly polarized
| antenna for the ground side?
| (I presently have a 8dbd loop yagi for the receiver, H or V
| polarization, not ciurcular..)
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
| On Fri, 06 May 2005 14:29:27 -0700, Wes Stewart
| wrote:
|
| On Thu, 05 May 2005 21:43:43 -0700, wrote:
|
| 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.
|
| Phoenix, Standard and other missiles use rear data link antennas
| buried behind the rocket plume without trouble. Of course these are
| at X-band, not 900 MHz, and receive only with *really* high powered
| transmitters.
|
| 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...
|
| Well, the fact that you don't have any test equipment is a real
| downer.
|
| The best suggestion, although I think time is an issue for you, would
| be to go to a commercial vendor and beg for a "contribution". My
| former employer (Hughes) gave money, time and materials to various
| universities all of the time.
|
| The elegant solution would be a conformal patch but I imagine this is
| beyond your resources.
|
|
http://www.uaf.edu/asgp/asrp/srp4/sr...chantennas.htm
|
| It doesn't look like there is any roll stabilization so you might need
| some pseudo "omni" pattern, during flight. But if you are only
| looking for TM after burnout and during the return to Earth (I assume
| dangling on a parachute) then you will know the attitude (at least
| "up" and "down"). Personally, I think that flight dynamics data
| during the powered phase would be more interesting to engineering
| students. [g].
|
| Fiberglass window(s) with dipole(s) behind them would probably work
| okay. Two with equal power split would be fine, however, one might do
| okay too.
|
| Regardless, it sounds like a fun project. Have fun.
|