I am just out of my field here (never stopped my comments yet grin) but,
symmetry seems to be the key here…
Others are often having greater luck than me; however, I have great respect
for Mr. Murphys’ law.
If I were to hang a long wire (even at near Ghz freqs.) off a single fin,
having constructed it from “wire” hefty enough to take mach speeds with
ease, I would worry about drag.
While that long-wire may not end up taking on the weight of an elephant, I
would take it for granted it could be measured in ounces, indeed, I expect
pounds are quite possible…
This does not suggest itself favorable to “straight as an arrow” flight…
Warmest regards,
John
"Fred W4JLE" wrote in message
...
| 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.
| |
|
|
|
|