"Helmut Wabnig" hwabnig@ .- --- -. dotat wrote in message
...
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.

The first flight used a .062" diameter length of piano wire cut so
that the exposed length was about 1/2 wavelength. The wire antenna was
isolated from the metal airfame with a small nylon penetrator and
connected to the transmitter via about a 12" length of RG-178 coax.
The coax was terminated at the antenna via two small jumpers (soldered
to the center conductor and shield). The shield was grounded on the
metal airframe transforming the entire rocket into an artificial
ground plane (the antenna was also swept back at about a 45 degree
angle to reduce drag.

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.

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.

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.
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.

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


Better length is about lamda/4 or lamda 5/8.
Lamda/2 is way off.

Do not use too thin wires, the thicker the better.

For reception on the ground station use a helix antenna,
because that antenna is polarization independent.
The antenna must be pointed in the direction of the object,
helixes are very directional.

http://hamwaves.com/antennas/inducta...al_antenna.jpg
http://www.rac.ca/tca/2006-01%20Heli...alculator.html
http://jcoppens.com/ant/helix/calc.en.php

If direction is a problem,
use a quadrifilar as ground station antenna.

http://homepages.ipact.nl/~pa1are/QHA.html


w.

Hi Helmut

It may be that you are not familiar with a ground based antenna with
similar caracteristics to a QHA, but muich easier to build at home. The
DCA is much less critical to make work properly than a QHA.

Jerry KD6JDJ