On May 24, 9:48*am, "Jerry" wrote:
"mr1956" wrote in message

...
On May 24, 12:04 am, "Jerry" wrote:

"mr1956" wrote in message

....

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

Hi C

Have you considered flush antennas, like slots?

Is is practical to use antenna directivity at the ground station to
increase the "range"?

It might be advantageous to use Linear Polarization on the rocket and
Circular Polarization on the ground.

For impedance measurement at L band, it is possible to build a slotted
line using plumbing supplies. That presumes that you already have access
to a signal generator and a detector to record the signal generator's
output.
A home built slotted line will have difficulty regestaring low VSWRs.
But, once it is known that the load impedance is close to the line
impedance
(low VSWR), a directional coupler can be used to match more precisely.

I have built a "plumber's delight" slotted line that works well at 130
MHZ. The high quality directional couplers are affordable on eBay.
Note - If I can do it, it cant be too complicated!

Jerry KD6JDJ

I have looked at slotted or flush wrap around antennas but integrating
one into the existing design would be difficult due to the wall
thickness of the metal. Something like that would have to be mounted
flush and I would be worried about the wind blast peeling the thing
off. *Certainly, there is much that can be done but I am unfortunately
not Wallops Island and have limited resources like anyone else.

* Hi C

* It looks like your "wire" antenna might actually be OK for your project.
You have demonstrated that it works when the rocket is nearly 10 miles away
when you hoped for 20 miles. * Evidently, the antenna doesnt need to
function while the rocket is accending.
* Are you able to improve the ground based antenna * The equipment you now
have would need only 6 dB improvement to provide the 20 mile range.
* I'd like to know more about your ground based antenna system.

* * * * * * * * * * * * *Jerry * *KD6JDJ * * *(who has lots of small
diameterTeflon dielectric coax *to donate to a real project)

Jerry:

The "ground station" is not much; just a small receiver with a
standard 1/2 wave dipole attached using an sma connector. The
receiver is battery powered and connected to a laptop using a standard
serial cable. We had thought about using a Yagi during the previous
flight but it just got left by the wayside with everything else that
had to be done.

The basic telemetry and downlink system is manufactured by Ozark
Aerospace (Erik Hall) but uses a Digi Int. 9Xstream radio modem
(www.ozarkaerospace.com and I use the ARTS TX-900G and RX-900). I
actually built my own transmitter and GPS system using a 5 watt data
radio and Garmin GPS with a small Li-Ion battery pack. But that system
was "lost" during a not so successful flight in Texas a few years ago.
Hence, for this design I opted to buy something off the shelf that
already worked.

Most people using this system have fiberglass airframes so for them it
is a simple matter to use a commercially available antenna mounted
inside of the airfame. But this is an all metal design so I had the
telemetry board made so I could connect my own antenna. FYI, the
rocket is 15 feet long, 6" in diameter and weighs over 100 lbs empty
and close to 200 lbs. on the pad. It is all aluminum, except for the
nose cone which is laminated birch. I'm using metal for the airframe
because it just easier to work with and the first 8 feet of the rocket
is metal anyway as that is the motor case.

The published range for this Digi Int. transmitter is 20 miles, though
people doing amateur high-altitude balloon flights have reportedly
observed even better reception from high altitudes (i.e., over 80 K
feet). This rocket did about 85,000 feet last September at Black Rock,
Nevada so it is doubtful that we will get GPS data at apogee anyway
due to the COTS limitations. But I'd like to get as much as possible
and will probably eventually convert to the 9Xtend modem which goes up
to one watt xmit power.

Curt