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Author Topic: Foxhunting with FRS radios  (Read 10971 times)

K4MNJ

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Foxhunting with FRS radios
« on: September 29, 2004, 06:04:58 AM »

I want to get the Scouts more interested in ham radio.  One idea is to have a foxhunt where the scouts bring their personal FRS radios, then build yagis and search for the FRS fox.  

Is it possible to build a yagi and couple it with the built-in FRS antenna?  What is the best method to couple the yagi with the FRS radio?

Is this legal as long as we do not transmit on the FRS with the yagi?  

Do you have any other suggestions for getting younger Scouts interested and motivated with Amateur Radio?

73 de K4MNJ
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WB6BYU

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Foxhunting with FRS radios
« Reply #1 on: September 29, 2004, 12:25:14 PM »

Probably the simplest approach (since the antennas are
fixed to the radios) is to use body shielding: hold
the radio right up against your chest and turn around -
when the transmitter is behind you, your body will
block the signal and the received signal strength will
drop.  (This is a great way to have a transmitter hunt
using 2m HTs, since almost everyone has one.  It is
also quite entertaining to watch the pirouettes!)

One problem with this approach is that the signal will
often be too strong to hear the null - this limits how
close you can get to the transmitter and still get a
good bearing.  A quick solution is to make a cardboard
tube large enough for the radio to slip inside, and
cover it with aluminum foil.  Something about a foot
or more in length should work, longer may be better up
to a point, especially since you can't take the antennas
off the radios.

Tie a string on the radio and lower it into the tube
until the received signal gets noisy.  Then repeat the
body shielding trick:  with the radio the right depth
in the tube, the null should be quite distinct.  (On
an unmodulated carrier, the back ground noise will be
easier to detect that the complete lack of signal.)


If you wanted to make a beam, the easiest approach would
be to use a reflector, either a flat plate or a corner
reflector with the radio mounted directly at the feedpoint.
An old metal trash can lid with a handle might be ideal
if you could still find one around somewhere.  W4RNL
has some good info on optimum reflector sizes (for
example, http://www.cebik.com/pcr.pdf) but something
about 2' square would be a good start.  Fortunately
the exact dimensions for the reflector aren't too
critical, and it can be made from hardware cloth, sheet
metal, an old BBQ grill, or cardboard covered with foil.

The problem with the beam is how to detect the strongest
signal, since the FM demodulator is designed to remove
all amplitude differences from the output.  This is
why I think hunting the null (where the signal drops
into the noise) is more practical with these radios.

Perhaps we should have a separate thread for discussing
the use of DF with Scout and other groups, separate
from the technical discussions?  One local Scout group
here is using 80m DF receivers and having a great time!

N4ZOU

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Foxhunting with FRS radios
« Reply #2 on: October 03, 2004, 07:17:20 AM »

The Yagi solution is very simple. Simply use the entire little hand held radio and it's antenna as the driven element! Most all the new FRS radios have a jack available for connecting an external headset with Microphone and speaker so you would simply hook up the headset and bring it away from the antenna. A small bit of aluminum forming a tube around the external antenna on the FRS radio can be used as a variable attenuator.  
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KC0KBH

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Foxhunting with FRS radios
« Reply #3 on: October 04, 2004, 04:55:41 AM »

Another great use for those seemingly worthless little radios.  If they sell FRS radios $10/pair, then how come they can't make ham HT's so cheap?  They could be put on some discrete simplex frequency, where no one would care about the TX audio quality.  But this article has given me a reason to get another pair.
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KC8VWM

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Foxhunting with FRS radios
« Reply #4 on: October 12, 2004, 01:42:28 PM »

It's a great idea and I would certainly encourage this activity, but modifying the antenna on an FRS radio makes it (unusable) as a part 15 device from a legal context.

I was somewhat curious how one would be able to take signal readings using an FRS radio without any metering on the device?

An alternative method of foxhunting would be to give a list of random "clues" over the air at random intervals. Kind of like playing hide go seek but with radios.

Good luck and keep up the great work in promoting Amateur Radio!

73

Charles - KC8VWM
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WB6BYU

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Foxhunting with FRS radios
« Reply #5 on: October 18, 2004, 12:08:15 PM »

 Since I had a break this weekend while the weather
was nice, I did some experiments on using FRS radios
for DF.

   I set out a transmitter 1/10 mile away and tried to use
body shielding:  I could just get a null indication with the
antennas cross-polarized.  Using a 24" square sheet of metal
for a reflector gave no change in signal strength: the meter
was full scale in all directions.  So this tells there may
be two steps in any search:  long distance, where the signal
is weak (generally half scale on the meter or less), and
close in, where the signal is full scale regardless of
where the antenna is pointing.

   
   Depending on terrain and reflections, the point where
the hunt changes from one to the other may be between 1/10
and 1/4 mile.  This is just a guess - I haven't gone far
enough away to confirm it.  In the long distance hunt,
some sort of beam antenna incorporating the receiver will
be the weapon of choice.  I like the idea of the metal trash
can lid with the receiver mounted about 4" in front of it,
but a simple yagi can be made from 1x2 wood with a gap
to mount the receiver in place of the driven element.  This
is fairly straightforward, and will be fun to experiment with
in the future.

   However, even with good bearings to start with, it is
difficult to find a transmitter if you can't take bearings
within 1/4 mile of it.. (Though it might get you within
shouting range of a lost hiker.)  So I focused my efforts
today on how to handle the close stage of the hunt within
the normal parameters of no modifications to the receiver,
plus general convenience, availablilty of materials, and
cheapness.

   On 2m we often use body shielding:  hold the receiver
close to our chest and turn slowly in a circle.  When the
signal is weakest, the transmitter is behind you.  If the
signal, is too strong, take off the antenna.  If it is
still too strong, add a "waveguide beyond cuttoff attenuator",
that is, a cardboard tube covered with aluminum foil.  
Lower the receiver into the tube until the signal gets
noisy, then use it with body shielding in the normal
manner.  So I grabbed my "attenuator" and tried it with
the FRS radio.

   First problem, of course, is that the antenna doesn't
come off.  This means that you need more attenuation than
we do for 2m to get close because the receiver is at full
sensitivity.  (Sometimes on 2m the solution is to tune off
frequency, but I didn't test an adjacent channel.)  My tube is about 3"
x 5" and 3' long, and nowhere in the tube did I get the
received signal to drop out when I was 30' from the
transmitter.  Part of the problem may have been that the
tube was too large, so I tried a length of 3" aluminum
stove pipe.  Better, but still not very good (perhaps
because the connection along the seam was not reliable.)  
3" metal flexible duct was better, the type sometimes used
for dryer vents.

   In general, you want the smallest size material that
your radio will fit into for maximum attenuation.  The
best one I found was a piece of small downspout (about
2" x 2.5") with what appeared to be a soldered seam.
This was about 30" long, but probably could be used shorter
(especially if a metal cap was soldered over one end.)
My radio also fit inside some 2" thinwall PVC pipe, which
could be coated with aluminum foil.  I suspect you could
make something by bending hardware cloth around a wooden
frame - the biggest problem would be making a good
RF-proof seam.

   While looking around the barn for other materials, I
grabbed a couple of empty tin cans.  My first thought
was to solder 2 or 3 of the right diameter together to
make a long tube with a closed end.  (Which requires
soldering, but is otherwise cheap.)  Then I saw that
there was a smaller can nested inside one of them,
and the light bulb started blinking...

   The two cans are 5" x 3 1/8" diameter, and 4 1/2" x
2 7/8" diameter, or so.  No, I don't know what came in
either of them.  But I put the receiver upside down into
the smaller one, then slipped the larger one over the
top.  The received signal is reduced as the two cans
are nested further together.  Using the two cans I could
take a bearing from less than 30' away, though I had
to work at it, holding the cans tight against the
bottom of my rib cage.  That is probably close enough
to find a person talking on a radio if they aren't
too well hidden.  Soldering pairs of cans together
may give more attenuation.

    This might work even better if the cans have some
sort of sliding contacts to seal the joint better.
I'm sure that the ends of one can could be cut and bent
to do this, or thin strips of metal could be soldered
on the outside of the smaller can.  Some holes punched in
the bottom of the outer can will make it easier to
hear the receiver audio.

    So we have a couple reasonable options.  What do you
have in your garage that you can use?  Who wants to take
the next step in the experiments and try it out?

    Some thoughts on the hidden transmitter:  the best
apparoach (especially for Scouts) is to have someone
sit in one place and talk on the radio.  I'm not sure
how well they will handle continuous transmit, so
intermittent transmissions are probably good.   You will
also want to check the power consumption vs. battery
capacity before you start a long distance hunt - I suspect
the standard rechargables won't last too long.  But a
6V gel cell or a pack of C/D Alkaline cells should give
plenty of operating time.

WA6BFH

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Foxhunting with FRS radios
« Reply #6 on: March 20, 2005, 12:52:50 PM »

I would encourage you to open up your FRS radio and see how it might be modified. I’m betting that there is a pin on the hybrid I.F. chip in the radio that will provide Log S-Meter drive. You can then also remove the antenna, and replace it with a BNC connector.

Show the kids how they can use their radios with the ‘body shielding technique’ but, they will all soon want to modify their radio’s also, so as to make them better useful!

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WB6BYU

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Foxhunting with FRS radios
« Reply #7 on: March 21, 2005, 02:16:04 PM »

WA6BFH:

I strongly discourage the modification of FRS radios.

First and foremost, they are no longer legal for FRS use if fitted with a removable antenna.

Second, the modification would be somewhat different for
each brand/model of FRS radio.  Since many of the chips
used are custom-designed for this application, there is
no guarantee that the signal strenght voltage would be
brought out to a pin if it is not needed in the circuit
design.

Third, it would be a very sophisticated Boy Scout who
would be capable of working on a tightly-packed SMD board
with custom IC's, let alone the issues of how to bring the
DC level out of the case and display it on a meter that
will survive rugged use.

Yes, there are a number of alternative approaches that
could work around these issues, but if one were going
to dedicate a radio to DF receiving, a scanner probably
would be a better choice to start with.

K8MHZ

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Foxhunting with FRS radios
« Reply #8 on: September 16, 2006, 06:29:32 AM »

Please note that the purpose for FRS is communication between family and friends and is NOT meant for experimentation.  The FCC gives radio privileges on certain bands for learning and experimentation and FRS is NOT one of them.  In short, fox hunts and opening up FRS radios are not intended uses for FRS and the FCC will frown on it.

That is what ham radio is for.  If you want people to have fox hunts and tear radios apart get them to study for and pass the Tech test.
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N6JSX

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Foxhunting with FRS radios
« Reply #9 on: October 28, 2007, 08:37:04 AM »

FRS will be hard, it's due to the extensive multi-path that 465 MHz creates.

There are a few considerations that want to make when  doing FRS RDF'ing.

Try to get a radio that has a visible "S" meter, earphone jack, and a removable antenna. The reasons are obvious.

An items that you cannot do with a FRS radio that our HAM HT's can do is the VFO function that allows you to tune away from the T signal giving you XX amount of off-frequency attenuation beyond body shielding or coax step-attenuators.

Attenuation is the key to getting near the Hidden T. If you cannot modify the FRS then I'd use a (metal) tin-can with a lid that is big enough to put the whole FRS radio inside - cut a 1/2" slot the length of the can then insert your FRS into the can and close the lid. The slot will give you some amount of directivity and the metal-can provides XX amount of attenuation in all directions but the slot.

I'm working on a few Yagi Sniffer designs - one to slip over the existing mounted FRS rubber duckie, another that you connect to the radio via antenna connector. The key is NOT gain but in high F/B ratio - you use the low gain to get in the area then flip the antenna around  using the back of the yagi with a known amount of F/B attenuation.

Since FRS yagi's are so small the "interferometer antenna technique" can be used for the FRS RDF antenna system (with 3-5 element yagi's). That would use two yagi's at once - one yagi is flipped 180 degrees - the two signals coming into the yagi's will be 180 out of phase, the more perfect the bearing/phase opposites are the deeper the signal null of the T into the FRS RX. This antenna system wouldn't be that large/heavy to carry by hand - but you need to connect the antenna coax to the radio and may even need to put the radio inside an RF shield container to keep from overloading the radio circuitry.  Built creatively with PVC you could rotate the two yagi's from Vertical, Diagonal,  to Horizontal & 180-out-of-phase for the NULL to in-phase increasing the GAIN.  

This "interferometer" technique negates the need for in-line coax signal attenuator (reducing cost/complexity).

I'll be posting my designs on my RDF Group site.
http://groups.yahoo.com/group/RDF-USA

N6JSX/0
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