[GreenKeys] OT: weather balloon telemetry
Chris Elmquist
chrise at pobox.com
Wed Nov 6 12:07:38 EST 2013
Gil,
I agree that the TV twinlead J-pole may be a good option too, although
at high altitude you will be looking into the end of it with most of
its radiation going off perpendicular to you. Think of a donut on a
broomstick and you are looking down the length of the broomstick.
So, in my mind, not an optimal choice due to radiation pattern for the
application but I'm sure it will still work.
Also, be careful to decouple the feedline (with ferrites or several turns
of the feedline near the feed) of the J-Pole because it is a balanced
feed antenna and you are feeding it with an unbalanced coaxial feedline.
If you don't decouple it, then lots of RF comes back on the shield of the
coax and the radiation pattern gets even less understood. Sometimes the
RF coming back can also mess with your power supplies, microcontroller,
the transmitter itself, etc.
Chris N0JCF
On Tuesday (11/05/2013 at 10:38PM -0700), gil at baudot.net wrote:
> Hey Chris and everybody:
>
> Yes, 144.390 MHz for the APRS tracking system. Gotta get my license
> asap now. I was persuaded by those who know better to avoid the 433 or
> 915 unlicensed stuff, and just do what works for many other
> balloonists.
>
> However, we want to keep the weight to the absolute minimum, so we can
> use smaller balloons and less gas to save money. This means no heavy
> handheld radios or such.
>
> Currently, for the tracker encoder/transmitter, I am leaning towards
> the MicroTrak-400 from Byonics:
>
> [1]http://www.byonics.com/mt-400
>
> It weighs under an ounce, and just needs an antenna and a serial feed
> from the GPS.
>
> Byonics recommend their own "V6" dipole antenna (37" spring steel
> wire), but at $40 I would rather just make one, and a dipole is easy
> and cheap to make.
>
> Your turnstile crossed dipole idea sounds interesting. I have
> previously considered an x-shaped payload base using 1/4" square wood
> dowels from home depot (which seem stiffer than round dowels, and
> weight only 0.5 oz for a 3-ft piece). I have adhesive-backed copper
> tape that could stick to one side of the square dowel to make a
> lightweight dipole, or crossed dipole, and solder thin/lightweight
> RG-174 or 178 feed coax directly to it (and, for the turnstile, make
> the 75-ohm phase section out of RG-179).
>
> I have been reading up a lot on all of this the last day or so, and
> getting quite a few recommendations: a basic (horizontally-mounted)
> dipole, a simple inverted 1/4 wave vertical hanging down with ground
> wires forming a plane, an inverted 5/8 wave vertical, a patch...
>
> Some balloon guys say to use a twinlead version of a J-Pole, like this:
>
> [2]http://www.hamuniverse.com/2meter300ohmslimjim.html
>
> This sounds light, cheap, and easy to hang, and it only needs to handle
> less than 1/2 W tx pwr.
>
> I guess the big question is which antenna has the best propagation from
> way up there -- looks like we are going to attempt three different
> missions, to 20,000, 60,000, and 90,000 feet.
>
> Please pardon the OT posts, but we have a lot of hams on greenkeys, and
> I sure appreciate the advice.
>
> thx, gil
>
> gil smith
> greenkeys moderator
> [3]gil at baudot.net
>
> -------- Original Message --------
> Subject: Re: [GreenKeys] OT: weather balloon telemetry
> From: Chris Elmquist <[4]chrise at pobox.com>
> Date: Tue, November 05, 2013 3:05 pm
> To: [5]gil at baudot.net
> Cc: [6]greenkeys at mailman.qth.net
> So, you are going to downlink on 2m? 144.390?
> You might consider a crossed-dipole (sometimes referred to as a
> "turnstile") antenna which will give you circular polarization and
> reduce fade if the balloon is spinning. It will be 3dB down from a
> linear polarized antenna on the ground but if there is any spin, it
> will
> not go through peaks and valleys as the two antennas go in and out of
> polarization alignment.
> It will then also be "flat" on the bottom of the balloon (would look
> like
> a "+" as you look up at the bottom of the balloon) and then won't stick
> in the ground when it comes down but instead be a little like landing
> gear :-)
> Here's one discussion,
> [7]http://www.west.net/~jay/turnstile.html
> Chris N0JCF
> On Tuesday (11/05/2013 at 08:49AM -0700), [8]gil at baudot.net wrote:
> > OK folks, after some good info from several of you, it does sound
> like
> > using 144.390 MHz APRS tracking would be the best bet for this
> balloon
> > project.
> >
> > This means I need to study for a ham test now, but they seem to be
> > offered frequently enough, so I guess I will need to start learnin'
> > some new stuff!
> >
> > Back to my question about antennas: what to hang under the balloon?
> > It has been pointed out how it needs to be omni. I suppose the
> > simplest would be a 1/4 wave hanging down, and suspended by three or
> > four angled ground wires that connect to a simple
> > three-or-four-radial-dowel frame or something. The antenna would
> > collapse on the ground (of course we hopefully have GPS coordinates
> for
> > the descent, so we can find it).
> >
> > Any other antenna recommendations? Dipole? Patch? Nothing commercial
> > -- something I can fabricate out of lightweight wire or foil.
> >
> > thx, gil
> >
> > gil smith
> > greenkeys moderator
> > [1][9]gil at baudot.net
> >
> > References
> >
> > 1. [10]mailto:gil at baudot.net
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> --
> Chris Elmquist
>
> References
>
> 1. http://www.byonics.com/mt-400
> 2. http://www.hamuniverse.com/2meter300ohmslimjim.html
> 3. mailto:gil at baudot.net
> 4. mailto:chrise at pobox.com
> 5. mailto:gil at baudot.net
> 6. mailto:greenkeys at mailman.qth.net
> 7. http://www.west.net/~jay/turnstile.html
> 8. mailto:gil at baudot.net
> 9. mailto:gil at baudot.net
> 10. mailto:gil at baudot.net
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--
Chris Elmquist
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