On May 24, 3:41*pm, "Jerry" wrote:
"mr1956" wrote in message
...
On May 24, 2:59 pm, "Jerry" wrote:
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.comandI 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
Hi Curt
I strongly suspect that you can use the antenna you now have on the
rocket. There is a possibility that you hadnt optimized the pointing of
the
stub on the base station. You might be able to get enough system gain by
using a simple Turnstile Antenna at the ground station ( Wikipedia ).
If you decide you want more signal than you get with a Turnstile, it would
be easy to build a 900 MHz Helix or Crossed Yagii.
It seems very important to use a Circularly Polarized antenna at the
ground station.
Jerry
I am familiar with the Helix antenna and the Yagi, but not a "crossed
yagi." Am I correct in assuming that the crossed yagi is simply two
yagis with the reflectors at right angles to each other (so that it
looks like a cross when viewed from the end)?
Directionality on the ground station is an issue though. Once the
rocket goes out of sight (it was only visible for about three seconds
of a nine minute flight), pointing the receiving antenna in the right
direction and elevation would be guess work. I know that the yagi must
be much more directional than the 1/2 wave dipole used before but how
much I do not know. I would imagine it is determined by the number and
spacing of the elements. I also think that having the transmitting
antenna more perpendicular to the airfame will help during apogee and
descent.
Curt
* Hi Curt
* I figured that your current telemetry system is demonstrated to be nearly
good enough. * From the info I now have, it seems that you need only 6 db
additional gain to be able to double the distance between the rocket and the
ground station. * Since you used linear polarization on both the rocket and
the ground station, it is possible that you will get adequate signal by
making the base station antenna "Circular".
* If the rocket accends to 20 miles and the ground station is close to the
place where the rocket started, the apogee point could be 2 miles from
"straight up" and still be less than 5.7 degrees from straight up. *I'd
suggest using a base antenna with a pattern much wider than 11 1/2 degrees
so the antenna needs to be pointed up, not aimed.
* I'd expect there are other rocket amateurs using simple Turnstile Antennas
for their base station antenna.
* * * * * * * * * * * * * * Jerry * KD6JDJ
Jerry:
By "circular" I assume you mean either a helix or crossed yagi so that
the receiving angle is uniform around the axis of the antenna?
Your assumptions are pretty close. The launch pad was about 1.25 miles
from the ground station (to the north I think) and the rocket landed
about 2.5 miles east of the receiving station. If I remember
correctly, we got a good link down to about 4,000 feet during the
descent on parachute. So if I fabricate or purchase an antenna with a
sufficient receiving pattern, point it generally up but maybe slightly
towards the pad, it should work. We were lucky that day as the winds
aloft were not too bad. In addition, the rocket comes down fast, at
about 75 feet/second so as to minimize drift. Everything was built for
strength because I knew it would hit pretty hard.
Let em play with some of the online calculators and see if I can come
up with a helix design and do some searching for a crossed yagi.
Curt