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.
|