wexpeter <[email protected]> writes: > Have used Telemetrums over the last few years - very happy with the > product.
Thanks for the kind words, Peter!
> A university rocketry club my son is in is considering a carbon fiber
> HPR. They are considering 900MHz — I am trying to convince them to get
> the license and use the Telemetrum. Question is what others have used
> for an antenna. My assumption is it has to be external if carbon
> airframe. Single external wire? Other suggestion? Clearly some gain on
> the ground will be important.They are thinking possible max altitude
> of 50K. Any suggestion greatly appreciated.
>
> Peter Wexler
> WA2COM
> LUNAR/NAR/Tripoli
I'm going to CC our mailing list on this reply in case others want to
chime in, because the whole issue of what to do for antennas on
predominantly CF airframes is a fairly frequent question to us.
My personal preference is to include a non-CF section in the airframe.
For example, make the walls of the electronics bay out of glass, or use
a glass nosecone, into which the various antennas can be places. The
reason this is a big win is that you're actually worrying about *2*
antennas, not one .. the GPS patch needs a clean look at the sky or you
won't get a GPS lock, and the signal from the UHF downlink antenna
obviously needs to be able to propagate out from the airframe.
In our original TeleMetrum design, we were forced for several reasons to
use a GPS patch antenna with an integrated preamp that had a short piece
of small diameter coax wrapped around the board plugged into a U.FL
connector on the board. With those boards, remoting the GPS antenna was
at least theoretically pretty easy. You could disconnect that cable,
put in a U.FL extension or U.FL to SMA adapter cable, and put the
amplified GPS antenna of your choice somewhere else in the airframe.
But in practice, this often turned out to be a real pain. The U.FL
connectors aren't really designed for many insert/remove cycles (think
10'ish), adapters are expensive / heavy, as are standalone amplified GPS
antennas. Thus, not many people actually did this...
In our current products, all of the GPS antennas are passive patches
soldered directly to the board. The front-end on the u-blox MAX-7Q GPS
receivers we're using now is sensitive enough that this works out pretty
well, and the inclusion of a SAW filter plus *very* carefully designed
layout keeps the UHF transmitter from swamping the GPS receiver input.
The downside is that remoting the GPS antenna has gotten a lot harder
with current products like TeleMetrum v2.0 and TeleMega.
We no longer offer a purchase-time option for putting an SMA connector
on the board instead of a wire whip antenna, but the footprint is still
there, and I still sell the right SMA connectors. We've observed that
downlink signals from a simple wire soldered to the board are often much
stronger than when you put a "commercial" antenna on an SMA .. this
is partly because most "rubber ducks" are actually really lousy antennas
(including the ones we used to sell, sadly!), and partly because the
design of the PCB is such that a simple 1/4 wave whip on the end of the
board gets an exceptionally good signal "launch". So, if you really want
to remotely mount a UHF downlink antenna, it's not hard to do, but we
don't usually recommend it. For a good example of when/why you might
want to do it, have a look at my write-up for YikStik3, particularly the
description of my "fintenna" and the associated photos taken during the
build:
http://gag.com/rockets/airframes/YikStik3/
http://gallery.gag.com/rockets/YikStik3/Build/cimg1949.jpg
The performance of the fin-can downlink on that airframe, once I
re-tuned everything with the Pro75 6xl case in place, was
*outstanding*. I really wish I still had that airframe... on its maiden
flight it set new personal speed and altitude records for me. But, like
so many others, it was lost in the fire.
Bottom line? It's *possible* to remote antennas from our products, but
it's not something to be taken lightly. By far, the better approach is
to include an RF-translucent section somewhere in the airframe to house
the board(s).
There are so many different "900 Mhz" products out there that I can't
speak to them without more details, but you're unlikely to find anything
operating on that band that gives a more satisfying result than using
one or more of our products... ;-)
I hope this helps!
Bdale
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