This is a good resource as well. http://www.jcrocket.com/gps-tracking.shtml
Especially the relative tracking range estimates. Terry On Thu, Feb 26, 2015 at 12:49 PM, Bdale Garbee <[email protected]> wrote: > 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 > > _______________________________________________ > altusmetrum mailing list > [email protected] > http://lists.gag.com/mailman/listinfo/altusmetrum > >
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