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Author Topic: Model Rocket Foxhunting  (Read 7393 times)

Posts: 11


« on: February 02, 2010, 10:39:43 PM »

Hello all,

I am on a search and rescue team and we are always looking for new ways to train.  I came up with the idea of a "downed pilot scenerio" and we had to find the pilot by foxhunting the "locator beacon".  My idea was to take a model rocket and mount a transmitter onto the rocket, launch the rocket at a low angle off into the distance and have the team radio track the rocket.  Since the whole team are hams, it makes it even easier.  Does anyone know of a place to order a transmitter/kit that is small enough to be placed into the payload section of a model rocket, and light enough to permit takeoff?

Posts: 13019

« Reply #1 on: February 03, 2010, 09:26:06 AM »

Just two topics down the list from this one, titled "70cm RDF beacon"
or something like that.  Another source is this one:

Radio beacons have been used to track both model rockets and
balloon launches.  The latter usually go much further, and can be
tracked in flight.  (If they include GPS telemetry you can check
the accuracy of your bearings in real time.)

If you are surrounded by mostly public land, the rocket approach
might work.  You've got the cost of the rocket, of course, and
need to make sure it doesn't head in a populated direction, since
the parachutes don't always open as planned.  A balloon can
often be arranged to make a more gentle landing, but may come
down in a mess.  In either case you have to consider the possibility
of it landing in a tree, power lines, a lake, or on private property.

Our approach is to get an old ELT (often one where the battery
packs are no longer available) and change the crystal to the practice
frequency of 121.775 MHz.  A volunteer then sets it out and the team
has to find it.  That way you have more control over where it ends up.

Posts: 11


« Reply #2 on: February 04, 2010, 10:10:55 PM »

Thanks for the advice!  Space is not the problem.  We will be doing this exercise on my property of 40 acres.  I wanted to use the rocket approach because since we are all trained in man tracking, someone placing a transmitter before hand would leave a trail.  I wanted to simulate this to as real of a situation as possible where the only thing we had to rely on was our eyes, and the RDF antenna.

Posts: 13019

« Reply #3 on: February 05, 2010, 09:05:00 AM »

In that case a compressed air spud gun could probably have enough range
to cover the area.  That makes it cheaper to run multiple launches in an
afternoon, since any one hunt shouldn't take more than 15 minutes to
20 minutes.

With a 40 acre parcel you're not talking about long distances - less than
half a mile maximum, or a quarter mile if you start in the center.  It doesn't
take much power to cover that distance:  a simple overtone crystal
oscillator and perhaps a one-stage amplifier is sufficient.  That is quite
easy to build yourself - you can have several spares (in case one doesn't
survive the landing) for the cost of a commercial unit.  Here is one example,
though it probably is more complex than necessary:

I've used the 9th overtone of a 16 MHz crystal successfully in such a
circuit.  You can also operate the crystal on the fundamental (or a
lower overtone) and extract the desired harmonic through a filter.  There
is a whole range of computer baud rate crystals that multiply up to
147.456 MHz that are commonly available and work well for this (such as
18.432 MHz, 16.384 MHz, 12.288 MHz, 8.192 MHz, etc.)  You can also get
canned crystal oscillators that run off 5V or 3.3V on either a subharmonic
(so you extract the desired frequency with a tuned circuit) or specially
programmed for your desired frequency.  For example, a 49.14 MHz oscillator
from Mouser that multiplies to 147.42 is available off the shelf for under
$5, and you can get one programmed to your desired frequency (though
you may have to settle for 1/3 of the desired output, since many parts
are limited to 50 or 70 MHz) for another dollar or two.

Ramsey Electronics used to sell a 2m "wireless mic" kit that was about
1" x 2" and contained a crystal-controlled transmitter.  We used a number
of these for low power hidden transmitters - including a dozen or more
hidden around a school yard on different frequencies.  Maximum range
was perhaps 100', but could be extended with a better antenna and/or
a simple transistor amplifier.  (And if the signal is very weak, that is even
better training for locating a real ELT!)

I've got a set of the 30mW 2m MicroHunt transmitters and they work great
over areas that size (and larger).  But for aerial launching and the resulting
potential impact, I'd consider something a bit more expendable.
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