[altusmetrum] Contact Form

Terry.w7ami terry.w7ami at gmail.com
Mon Apr 18 20:33:31 MDT 2016


JDS,

Lucky students.  I never got to do anything that interesting in 
school.   I've heard that quadrifilar helix antennas are hard to get 
matched so congrats to your class for getting it to work.

One note.  The TeleGPS is have is mounted in the nosecone clear of 
everything else.   In addition I have it mounted to a vertical piece of 
copper clad PCB to try and give the antenna some minimal ground to work 
against.   I just bought a new longer nosecone for the rocket so that I 
can increase the length of the PCB.  I need to figure out how to 
calculate the resonant frequency of the copper foil section.   I'ts 1.5" 
wide and 5 inches long currently.   Can I calculate the resonant 
frequency of that?

If I only had an antenna test range to do real measurements on instead 
of guessing.

I did a flight last fall to 9600 feet with just the vertical for 
tracking and it did a good job.   Even though it was calm on the ground 
the upper winds caught the rocket, even on drogue portion of the 
descent, and it landed a mile and a half away.   I had good data most of 
the time and knew right where to go to get the rocket.

Terry

On 4/18/2016 7:49 PM, John Sahr wrote:
> For what it's worth, one of the antenna design projects I offered in 
> my antenna class last year was a "telemetry antenna for high power 
> rockets".  One team took up the challenge, and produced a quadrifilar 
> helix that could be mounted on the skin of a 4" fiberglass airframe; 
> the idea being that circular pol was a good idea, and that the 
> interior of a high power rocket was precious/dangerous.
>
> I think I might try antennas embedded in fins as a student project for 
> next year.
>
> jds
>
> On Mon, Apr 18, 2016 at 10:39 AM, Bdale Garbee <bdale at gag.com 
> <mailto:bdale at gag.com>> wrote:
>
>     Web Customer <am-query at gag.com <mailto:am-query at gag.com>> writes:
>
>     Copying the list since I've had this question before and getting a
>     discussion going about different antenna ideas could be fun.  If you
>     aren't already a list member, see the bottom of the
>     altusmetrum.org <http://altusmetrum.org> page
>     for a link to our list server and sign yourself up!
>
>     > Comment: I have two of your TeleGPS units.  Last month I attended a
>     > local HAM swap, which is something our Rocket club attends yearly to
>     > create awareness for rocketry.  While there I thought I would ask
>     > around for a suitable “base station” antenna for my TeleGPS, which I
>     > had on display.  Someone suggested that I could get more range from
>     > the antenna on the TeleGPS, because it was/is essentially only
>     half of
>     > an antenna and that I simply needed to add an equivalent length size
>     > of wire to the ground side of the TeleGPS.  Does this sound correct?
>
>     That's a gross over-simplification, and not the whole story.  It is,
>     however, something you're likely to hear again in the ham community
>     where the "default" antenna for many is the 1/2-wave dipole.  A
>     1/4 wave
>     whip, which is what we're providing by default on TeleGPS and our
>     other
>     flight boards isn't "half an antenna", it's just an antenna built to
>     different design criteria.
>
>     Many books have been written about antenna design.  If you're
>     interested
>     in the topic, a good place to start would be the ARRL Handbook and/or
>     one of the antenna books from the ARRL or RSGB.  Note that
>     "conventional
>     wisdom" in HF space doesn't always map well to UHF, so focusing on
>     ideas
>     that others have already discovered work well at 70cm could help
>     narrow
>     your focus.
>
>     Antennas in rockets get complicated by a lot of things, including the
>     dielectric influence of airframe materials, proximity of other metal
>     like all-thread in ebays, etc.  After trying a bunch of things, we
>     think
>     the simple wire 1/4-wave whip is a pretty good choice. However, I've
>     done some "creative" things, like loading up the motor case as an
>     antenna, loading up two fins (see the 'Fintenna' heading at
>     http://gag.com/rockets/airframes/YikStik3/
>     for more details on that), and I'm currently building a 12" airframe
>     that will have full dipoles on 70cm mounted just inside the skin
>     as far
>     From the 6" motor case as I can get them.  So many choices! ;-)
>
>     > Integrated antenna aside, my original search was for a
>     unidirectional
>     > antenna that I could connect to my computer sitting on my launch
>     > prep-table, with the antenna affixed to one of my canopy legs on an
>     > extension pole.  I would then use my 3 element yagi with my
>     TeleBT and
>     > my phone while watching and visually tracking from the launch line,
>     > and just leave my computer running back at my prep table, to
>     hopefully
>     > capture the flight.  What is the best antenna for this application?
>
>     The "problem" is that you want an antenna with good performance at
>     high
>     radiation angles (since the rocket will be "up" during much of the
>     flight as seen from the flight line).  Many sophisticated ham radio
>     antennas are explicitly designed to direct most energy to/from the
>     horizon for longer distance comms and just don't work all that
>     well for
>     rockets in flight.  A simple 1/4 wave over a ground plane, a
>     patch, or a
>     quadrifilar helix on 70cm (or variants like the "Texas Potato Masher"
>     are all pretty good choices.  An even better idea, but it can be
>     harder
>     to arrange, is to get someone to back a mile or so off from the flight
>     line where they'll have a "broad side" view of the rocket through the
>     entire flight... that's often the best way to get all the packets,
>     since
>     otherwise a ground station on the flight line is looking up at a
>     sub-optimal antenna radiation angle for at least the boost and coast
>     phases of the flight.
>
>     In any case, just experiment!  The ground software shows received
>     signal
>     strength for each received packet, so you can easily see what works
>     better and worse, and it's fun to try things!
>
>     Above all, have fun!
>
>     Bdale
>
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>
>
> -- 
> John D Sahr --- Seattle WA --- jdsahr at gmail.com <mailto:jdsahr at gmail.com>
>
>
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