> I was thinking of designing some fox transmitters/tracking beacons for
> the various ham bands from 50 through 928 mhz (or higher), and would
> like to get everyone's thoughts and opinions. This would be a "blue sky"
> type engineering project so I'd like to hear what people really want.
First, I presume you have researched what is currently available.
Here are some existing designs that may give some ideas: http://homepage.ntlworld.com/daburleigh/atx80boxs.jpg http://homepage.ntlworld.com/daburleigh/atx80pcbs.jpg http://www.users.bigpond.net.au/vk3yng/foxhunt/80m_fox_or/80m-fox-or-tx.html http://www.silcom.com/~pelican2/PicoDopp/MICROHUNT.htm http://www.marecek.sky.cz/ http://www.qsl.net/ve2emm/pic-projects/mt525fox/mfx525-e.html
(KE6HTS has some low power transmitters similar to this, but
without the final amplifier.)
Or for controllers to plug into existing transmitters: http://www.byonics.com/piccon/ http://www.qsl.net/ve2emm/pic-projects/mtfoxctr/mtl_fox.html
Also WB8WFK (firstname.lastname@example.org
) is working on a set that
includes AM or FM modulation and some other interesting features
Besides the frequency range, it may be good to consider why any of
these would not work for your needs.
> Some of my own questions are:
> 1. Size, does it matter? Weight, ultra miniature?
I certainly like to use small transmitters. I have a set of the
small 2m ones from KE6HTS (about 30mW, and the board is half the
size of a 9V battery) and have built a few of my own over the
years. These are much more convenient than my ammo boxes when I
want to do a quick demo or test some equipment, also easier to
carry a set of 5 of them at time when I set them out. For
longer hunts, they can be put in a box with a larger battery
(and an amp if needed). Because such transmitters are often
very specific to the particular hunt circumstances, the best
approach seems to provide the parts that others can put together
> 2. Enclosed case, or open circuit board?
Open circuit board, but designed to fit a commonly-available case.
(Or available in the case as an option.)
> 3. How much RF output power? Adjustable?
Something around 10 to 20mW is a good level when hunters are using
body shielding to find it. Can be heard a couple hundred yards away
across a park, but still huntable reasonably close. This is a great
way to get people started with transmitter hunting. And for many
situations (balloon launches, practice sessions, hunts in small
areas) this is plenty of power and batteries last a long time.
The next level up I find myself using is around 1 watt (often an
HT.) This has been enough for most of our county-wide 2m hunts
(though some could have used a bit more power!) I have some of
the Hamtronics synthesized transmitter boards and it is difficult
to turn the power DOWN to this level without a significant increase
in the level of spurious outputs.
So an exciter board at, say, 20mW and an amp board that would bump
that up to 1 to 2 watts is a good start. Then folks can add an HT
amp if they need more power... or just use their mobile rig with a
noisemaker plugged into the mic jack.
> 4. DC power source? Should it run from a single AA cell? 12 volts?
> Or something in between?
Design the exciter to run from +5 volts through a low drop-out
regulator, then it will work from 6 to 15 volts input. For
small size, this could be a 9V battery or a pair of lithium camera
batteries. Higher power will run from the main battery rather than
through the regulator, allowing the use of 12V gel cells or packs
of rechargable or alkaline cells.
> 5. On/Off keying? FSK? Tone modulated, Or FM for voice?
> 6. If it's FM, should it be Line level, Mic level, an onboard
> Voice Recorder Chip? Any or all combinations?
One thing you might consider is making the audio generator unit
separate from the transmitter itself. Circuits like the PicCon
are popular because they can be plugged into the mic jack of any
radio - this is usually a less-expensive option than buying a
transmitter just for hiding. I often see old HTs like the IC2AT
available at a hamfest in working order for $25, and it is
difficult to build a synthesized transmitter for that price.
Tone modulation (with programmable sequences) makes the transmitter
distinctive to the hunters. A voice recorder chip provides a
simple way to record a callsign (and various other announcements
and/or annoying noises) without the bother of programming CW.
I like having the option to record two different sets of audio,
with one repeating for most of the time and the second played at
the end of a transmission for the ID.
The modulation will depend on the purpose of the transmitter. Most
VHF/UHF hunts will be on FM, so that is a common option. The
international ARDF hunts specify AM tone modulation with a keyed
carrier. This can be accomplished with a switching transistor in
the power lead to the final amp for both keying and modulation.
The 80m transmitters used for ARDF all use CW.
The Montreal Controllers by VE2EMM have outputs for audio, PTT,
and key. (PTT is on for 1 minute, key goes on and off with the
tone keying.) I use a 5-pin DIP header to add ground and a spare
pin that can carry receiver audio back to the controller (because
I used the same cables I had made for my PK232.)
> 7. Any particular frequency or bands?
International ARDF uses 2m and 80m. The British like 160m, and
10m used to be popular for evening hunts when the band tended
to be closed. 2m is probably the most common band for hunting,
but 6m, 220 and 440 are also used at times. And some folks hunt
ATV on 1215 MHz. There may be other uses, such as telemetry or
beacons that you might want to consider in the design.
> 8. Should it be radio commandable, to remotely turn it on or off?
This is easy if you have an HT for the transmitter, since it
also provides receive capability. But if you are building the
transmitter, you would have to build an additional receiver to
have this ability.
A simpler approach is to have a timed turn-on delay. Most of
my controllers have this option but I've never used it, as I
never trust that it is really set to come on properly. So I
use a toggle switch to disable the transmitter while letting
the controller run to maintain timing. After setting the
transmitter in place I turn it on. Seems to work for me.
> 9. Connections to the unit? Fixed connectors? Flying leads? Solder pads only? Or some combination?
Connectors on the box, solder pads on the circuit board. Or
perhaps a header on 0.1" spacing so wires can be soldered
into individual holes, or a header can be used with a connector.
I like the connectors for being able to swap parts around when
something fails, but reliability is better with soldered wires.
> 10. And anything else anyone can think of?
FREQUENCY AGILITY: crystal control is good for simple rigs, but
the cost can get excessive if you can't use standard frequencies.
Colorburst crystals have become the norm on 80m, but a second
frequency for the finish beacon sometimes takes some scrounging
through old junkboxes to find a suitable crystal. There are a lot
of baud-rate crystal frequencies that multiply up to 147.456 MHz,
but any other frequency in the 2m band is more difficult to
find. (Having an audio source that can be plugged into an existing
radio simplifies this greatly, since most folks already have a
synthesized radio for the frequency they are using.)
Unfortuantly there isn't a common 2m frequency for the whole US.
I give talks and demos in different areas, and it is hard to find
a frequency that is acceptable in this corner of the state. So
some type of frequency agility is required. The ICS 525 chips
used by VE2EMM and KE6HTS are a good start, as they can be set
by DIP switches or soldered jumpers. The programming is a bit
quirky, so better done on the workbench rather than in the field.
The next step up is a full-fledged synthesizer with keypad entry
or digital switches.
Perhaps a reasonable alternative would be the "programmable
oscillator" modules available now from many standard suppliers.
These are the usual "canned" oscillator modules programmed to
your desired frequency (well, within 15kHz or so). These are
relatively inexpensive (maybe $5 or so?) so it would be possible
to have a couple options in your pocket. They are usually
available up to 100 or 125 MHz and have a pretty good odd
harmonic output: if you order one for a odd sub-harmonic of
your frequency, then a double-tuned circuit should be able
to extract the desired frequency and pass it to the following
amplifier stage. This might be a good solution to building a
circuit board that would work on multiple bands. The oscillator
would be in a socket, then the filter and following amplifier
(perhaps an MMIC) would be chosen for the desired band.
Actually the ICS525 would probably be even better, since it will
generate a wide range of frequencies with a single non-critical
crystal between 5 and 20 MHz. Rubbering the crystal provides
fine-tuning of the output frequency, an option that is not
available with the canned oscillators. The board could be made
to permit either DIP switches or soldered jumpers, depending
on how often the frequency was going to be changed. Since
this also puts out a square wave it should be easy to extract
harmonics on various bands (perhaps with an active multiplier
on the higher bands.) A single board design should work
from HF up to at least 440 and maybe higher.
The other option would be to design a synthesizer that operates
over a smaller range of frequencies and beat it against a crystal
oscillator to put it in the desired frequency range, or the
synthesizer could be designed so different crystals could be
used to set the output frequency range. You still may need
some multipliers for the higher bands, but you should be able
to standardize the board design.
ON/OFF timing. Most transmitter hunts I've been on haven't left
the transmitter on continuously. This saves battery power (and
heating in the final), as well as giving those who get lost or
stuck in a ditch a chance to call for help. Having a variable
ON and OFF times would be good, whether it is set by trimpots
on a 555 timer or digitally programmed in a microcontroller.
SYNCHRONIZATION. The ARDF competitions typically use a set of
5 transmitters on the same frequency, each of which transmits
for one minute and is off for 4 while the others cycle. And
for practice sessions it is helpful to be able to adjust the
number of transmitters in a cycle. (There are also standard
CW sequences for each transmitter in the cycle.) Generally
these are synchronized before being hidden, and should be able
to maintain synchronization within a couple seconds over 8 hours
(though I know of some folks who would like to set them out
the day before and have them maintain synchronization for 24
hours or more.)
REFINEMENTS. These are some clever featuers that OK2BWN used on
the transmitters at the last World ARDF Championships. First was
an extra tone element added at the end of each transmitter cycle
when the battery voltage was above some threshold. If this tone
disappeared, then you had about 15 minutes of battery life left,
so you'd better hurry and change it. The transmitters had internal
batteries as well as external packs, so it was possible to change
the external pack while the transmitter was running without
interupting the synchronization. Jiri's transmitters also were
capable of operating on both 2m and 80m, and used a DIN plug
for both antenna connector and bandswitch simulataneously. So
the plug for an 80m antenna was wired to tell the transmitter
to transmit on 80m. With no antenna plugged in, the transmitter
didn't transmit, through it kept synchronization and programming.
So switching to a backup transmitter was simply a matter of
moving the antenna plug from one to the other, and there was no
danger of transmitting into the wrong type of antenna.
Hope this helps to give you some ideas. And if you
come up with a design, I may be interested in some.
- Dale WB6BYU