On Sat, 23 May 2009 16:40:22 -0700 (PDT), mr1956
wrote:

I am looking for some help developing a properly tuned piano wire
antenna for an all metal experimental rocket.
This particular vehicle currently uses a Digi International 9Xtream
100 mw transmitter which operates using Frequency Hopping Spread
Spectrum from 910 to 918 MHz.

Would that perhaps be one of these models? If so, which one?
http://www.digi.com/products/wireless/point-multipoint/xstream-module.jsp
http://www.digi.com/register/procregistrationnothankyou.jsp?urlredirect=/pdf/ds_xstreammodule.pdf

During this first flight we observed a range of about 10 miles, about
half of the published range of this transmitter. The RF system is used
as a downlink for GPS data and we did not get a good radio link after
ignition until the rocket had descended to about 50,000 feet. Maximum
velocity was about Mach 3 which is why a conventional antenna cannot
be used as it will be melted by friction.

Ummm... let's do some path loss calcs first. I'll assume that you're
using a matching XStream radio on the ground, with a moderate size
yagi antenna pointed in the general direction of the sky.

Rocket end:
xmitter +20dBm
tx coax loss 0dB
tx ant gain 0dBi (monopole 1/4 wave)
path loss (to be calculated)
rx ant gain +12dBi
rx coax loss 0db
rx sens -110dBm (at 9600 baud)
I'll assume a minimum fade margin of 20dB.

Adding up all the gains, losses, fade margin, and guesswork, I end up
with a maximum path loss of 132dB. Plugging into a handy online
calculator at:
http://www.terabeam.com/support/calculations/free-space-loss.php
I get a maximum range of 25 miles. It should work.

However, the weak link here is that I'm doing quite a bit of
guesswork. For example, you really do need a +12dBi (tracking?) yagi
antenna to make this work. If you use a simple dipole, with a gain of
perhaps 2dBi in place of the yagi, your maximum path loss just dropped
to 122dB which yields a maximum range of only 8 miles, which is
roughly what you're getting.

You may also be getting a major interference at altitude, which does
not appear on the ground. From 5 miles up, the rocket is going to
hear plenty of 900MHz interference from all over the country. Each
signal will be fairly low level due to the distance, but there will be
lots and lots of signals up there. Try your rocket radio from a local
mountain top before launching to see if the receiver can handle the
interference. Since it's a frequency hopper and not a direct sequence
spread spectrum radio, it probably will, but it doesn't hurt to test.

Incidentally, the previous back of the envelope path calcs are the
BEST case model. There are always additional losses and reductions.
For example, few manufacturers seem to meet their published
specifications. The antenna gains really require a full NEC2
simulation which includes the metal rocket body. Things only get
worse.

For the second version, we are thinking of using a slightly smaller
diameter piano wire for a 1/4 wave length antenna as I think this may
be more suitable for a system using an artificial ground plane. In
addition, I plan to terminate the RG-178 coax directly at the antenna
and seal it with potting compound. This new antenna will be mounted
totally on the exterior of the airframe and angled back only about 10
degrees to give a better radiation pattern.

You're reinventing the wheel. Look at photos of the original Explorer
I satellite (with Von Braun and others holding it over their heads).
There are 4 wires radiating from the tubular rocket casing forming a
circularly polarized double dipole "turnstile" antenna. Since it's
since one antenna is always visible, there's no blocking by the
missile body. Since it's circularly polarized, there's no positional
nulls and peaks.
http://history.nasa.gov/sputnik/expinfo.html

However, I would really like to properly tune this new antenna and was
wondering if anyone in this group has any ideas as to how to do this.

Cut it for 1/4 wavelength at about 921MHz. Be careful when cutting
the coax phasing sections to get the velocity factor calcs correct.

Doing a tuned RCL circuit at this frequency is proving to be difficult
using discrete components, to say the least. I do not have an SWR, but
do have an RF millivolt meter as well as the means to measure the
capacitance between the antenna and metal airframe.

900MHz VSWR meters that work at 100mw are not common. You would have
problems using one anyway because the xmitter does not generate a CW
signal suitable for easy testing. Methinks you'll do better dragging
the rocket somewhere that has a network analyzer or suitable pile of
test equipment for characterizing the antenna.

Any help will be appreciated as I would hate to fly this thing again
and not get it back because our RF downlink had insufficient range.

C. Newport

I think you'll find that the tracking 900MHz yagi antenna on the
ground is the key part of the puzzle. Did you try driving the rocket
5 to 10 miles away and testing if it can be heard from the ground
station? If you run this test, it's easiest to run between mountain
tops to insure line of sight.

Good luck.

--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558