[FoxHunt] Fox Transmitter Plans?

Wed, 12 Jun 2002 22:48:44 -0700

Hi Jerry,

I just did a check on what it would take to change the frequency to
145.565, and it looks like a crystal change and a small programming
change will do it. This is basically the same unit I had in Georgia at
the US ARDF Championships ... except we were able to reduce the noise in
the signal :)! We also have a voice transmitter that will transmit up to
about a one minute message, and then repeat. While almost ready, I want
to do a bit more testing on it before offering it for sale. The specs
except for current draw are pretty much identical to the CW transmitter.

And for several others that wanted a bit more information on this
transmitter:

This module is a complete FM transmitter and controller built on a
circuit board 3/4" x 1 1/4", and is completely built from surface mount
components. This unit was mainly designed for transmitter hunting, and
one of the main design criteria was to keep the price of the unit low
... I think we have succeeded :)! I do expect to raise the price though
in about a month as the current price just covers the parts and
development costs ... barely.

The standard program takes a CW message (MOE, MOI, MOS, MOH, or MO5),
repeats it 10 times, and the callsign ends the transmission. The cycle
will repeat itself in about 4 seconds. Depending on the length of the
callsign and the CW message, the cycle for MO? signals runs between 55
seconds to perhaps 65 seconds. Because all of the above is hard
programmed in, user modifications are not possible.

The transmitter uses an OTP PIC 12C508 microprocessor and the ICS307-02
Programmable Clock Source as the actual frequency generator. The
modulation is done by varying the crystal used to control the ICS307,
and seems to work quite well! We've done a little testing on the
units and it looks like the output is fairly clean with the 2nd and 3rd
harmonics down 30 dB from the main signal. It has a power draw of about
33 mA while transmitting and about 5 mA in the off state. The deviation
seems to be running in the area of 1 - 2 KHz.

Depending on the output filter configuration, the transmitted power can
be varied. The current configurations are either 1 mW or 10 mW. I built
mine at the 1 mW level since I wanted to be able to put on simple low
power hunts where the user could still use body shielding to get
relativly close to the transmitter. The range can range from 100 yards
or so up to 1/4 mile or better depending on the antenna, output power,
and of course the terrain.

Frequencies other than 146.565 can be programmed in, but because of
limitations with the ICS307-02, some frequencies will generate a great
deal of background noise.

The unit has an onboard 78L05 voltage regulator, and testing with a
power supply shows that the unit will still function down to about 5
VDC. The alkaline 9VDC batteries seem to have a capacity of about 580
mAH. Since I *hate* to waste batteries, I generally run them down to
about 5 volts or so before I toss them :).

The PIC processor uses the internal RC clock. Because of that and some
program limitations, it would be difficult to get the exact same cycle
on multiple transmitters. And of course there will most likely be some
timing drift although that hasn't been measured or checked yet.

One modification I've thought about is adding an LED in series with the
power leads to give an indication of what state the tranmsitter is in.
The downside would be a somewhat reduced battery life due to the .6 VDC
drop though the LED.

Another possible modification would be to cut the trace from the PIC to
the crystal, and modulate the crystal with an external signal. *NO*
testing has been done on this module though, and it is just a thought.

Marvin, KE6HTS

Gerald boyd wrote:
> 
> Marvin I would be interested.
> 
> How much addition cost to get one on 145.565?
> 
> In town we have lots of QRM on 146.565 from an over modulated simplex group on
> 146.580.
> 
> However the standard unit on 146.565 would work ok outside the Albuquerque
> area.
> 
> Jerry
> 
> Marvin Johnston wrote:
> 
> > I have a postage stamp sized 2M MCW transmitter that can put out about
> > 10 mW that transmits on 146.565 we are starting to sell as a "kit" and
> > *currently* sells at $25.00. Designed as a small, low power ARDF
> > (Amateur Radio Direction Finding) transmitter, it is powered by a 9V
> > battery and can probably transmit for about 16 hours or so. The standard
> > transmitter module is complete and programmed with MOE, MOI, MOS, MOH,
> > or MO5 and ends with the users callsign. The module needs to be
> > connected to power and the antenna for a working 2M MCW transmitter.
> >
> > We are in the process of checking power output, frequency stability,
> > etc. and will be gathering more data as we get more of these modules
> > working and checked out. Give me an email if you are interested.
> >
> > Marvin, KE6HTS
> >
> > J Craswell wrote:
> > >
> > > I am thinking of adding a little QRP MCW transmitter to my monster model
> > > rocket before I try flying it with my 1255MHz ATV system *Just in case it
> > > fails.  What I want is a 2 metre FM transmitter that I can send MCW with.
> > > Nothing too fancy.  Anyone have plans for such a beast?
> > >
> > > 73 de Jay W0VNE
> > >
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