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

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

Maximum
velocity was about Mach 3 which is why a conventional antenna cannot
be used as it will be melted by friction.

Would seem to be at odds with:

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.

At Mach 3, I seriously doubt you could hold it as erect (80 degrees to
the fuselage) as you might try with smaller wire (in fact, I would say
it would be plastered down).

If your telemetry is directly below its ascendance, you might try a
full-wave or longer antenna trailing below (if it can withstand the
propulsion heat). If that is not possible, look into an inverted F:
http://www.qsl.net/kb7qhc/antenna/In...%20F/index.htm
(it doesn't have to be a thick antenna).

73's
Richard Clark, KB7QHC

"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

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

On Sat, 23 May 2009 21:45:15 -0700, Jeff Liebermann
wrote:

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

More photos:
http://www.redstone.army.mil/history/explorer/EXPLORER.html
http://www.redstone.army.mil/history/explorer/Explorer%20One%20Satellite%20Diagram.jpg


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

On Sun, 24 May 2009 04:04:52 GMT, "Jerry"
wrote:

Have you considered flush antennas, like slots?

Slot antennas have the maximum lobe perpendicular to the rocket axis.
There's very little signal below the rocket. This is why tracking
stations are far away from the launch site. If the rocket were
overhead, and going straight up, there's no signal.

Slot antennas are also a power waste. You need 4 slots, run by a
power splitter, in order to insure that at least one antenna is
oriented in the direction of the receiver. Meanwhile, the other 3
slot antennas are radiating power to nobody in particular. Say goodby
to about 3/4th of your tx power. Receive sensitivity is not
affected.

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

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.

"Jeff Liebermann" wrote in message
...
On Sun, 24 May 2009 04:04:52 GMT, "Jerry"
wrote:

Have you considered flush antennas, like slots?

Slot antennas have the maximum lobe perpendicular to the rocket axis.
There's very little signal below the rocket. This is why tracking
stations are far away from the launch site. If the rocket were
overhead, and going straight up, there's no signal.

Slot antennas are also a power waste. You need 4 slots, run by a
power splitter, in order to insure that at least one antenna is
oriented in the direction of the receiver. Meanwhile, the other 3
slot antennas are radiating power to nobody in particular. Say goodby
to about 3/4th of your tx power. Receive sensitivity is not
affected.

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

Hi Jeff

Why are you so negative?? I have designed and built lots of slot
antennas that were widely used on military aircraft. The efficiency is
quite good. I am sure a smart guy like you could design a slot antenna and
locate it for those guys with the 3G rocket. There hasnt been any text
that restricts where the slot could be located. As I read the original
post, they werent receiving signal from the accending rocket. Maybe they
only want to receive data that was recorded after the rocket reached it's
peak. There are lots of information that you and I dont yet know. But,
my major question for you is "Why are you so negative?".

Jerry KD6JDJ

"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

mr1956 wrote in news:943a9bbd-214b-43b2-ac31-
:

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.

So you designed for a half wave antenna fed with 12" of RG178.

Lets suppose for a moment that the antenna has a feedpoint Z of say, 2000
+j0 ohms. The line will transform that to 5+j14 at the tx end, and with a
loss of 4.8dB (ie 33% efficiency). The tx is unlikely to develop is rated
output power into such a load, so there will be some further reduction.

Yes, an antenna of half the size (ie a quarter wave fed against the metal
rocket body) might well work ten times as good.

Owen