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Geoffrey S. Mendelson · November 13th 07, 05:04 PM posted to rec.radio.amateur.homebrew

wrote:
Thank you for your response.

You're welcome, I hope this helps.

Is this for licensed or unlicensed operation?

Eventually licensed.

Ok, then I would start off with what is called in the U.S.
Family Radio Service "walkie talkies". They are cheap,
easy to buy and availble almost anywhere. In the U.S.
they operate on 464 mHz, in most other places 446 mHz.

They have a relatively short range. Stated range is 5 to 10
miles, but unless you are on a mountain top talking to someone
on another mountain top, it's more like 1km or less.

However they can be used without a license and as I said,
cheap. Good to experiment with.

I plan to replicate approximately 10 to 15 transmitter units and one
receiver. This project is for a volunteer organisation.

Ok, then I would look into making a controller board or interface
on a printed circuit board. There is plenty of design software
around and you may be able to find a shop that does low quantity
boards for amateur use.

When I said portable, I was referring to it being independant of an
external power source or external mounted ariel. It's ideal size would
be similar to that of a book, preferably a cellular phone, however
cost is the main consideration.

FRS units have permantently attached ariels. Ham radios have removable
ones so you can start with a "rubber ducky" type and upgrade to a
dipole. A 2m antenna is 38 inches long, around 1 meter.

There is no operating time requirement, the only time related
requirement is that the power source lasts long enough to transmit a
simple message.

A battery pack with AA batteries will run one for several hours of
sending short messages. You can use disposable batteries or
NiMH rechargables for longer battery life. NiMH batteries lose
their charge over a week or so, and have problems "floating"
on a charger, so if they have to sit in a box waiting for
an emergency, alkeline disposable batteries are IMHO better.

I was thinking AM or FM however CW is an option.

Cheap handheld units are normally FM, this includes FRS type,
70cm and 2m ham radios and 49mHz unlicensed ones.

27mHz CB type radios are AM (and expensive ones SSB), but in the U.S.
are restricted to voice only by law. They do have a wider range in
most cases.

As for the controllers, the question is exactly how "off the shelf" do
they need to be? If you have someone on the project who can figure out
how to add the switching logic and a someone to modify the operating
system for you, discarded iPods would be perfect. There are lots of
them lying in drawers, being too good to throw out and too expensive
to fix (mostly battery replacement).

You could also do it with old pocket PC's, Palm Pilots, etc. Probably
more easily.

What ever you do, try to get 20-30 of them before you start, several
will fail or be broken in development and it's difficult to start
over again because you ran out of parts.

Whichever is the cheapest component wise and irrespective of the power
requirements, is portable and can transmit at the desired range of 5
km to 10 km.

That will have to be found out by testing. 10km in London, New York, or
Jerusalem will be very difficult, while 10km in a flat area will be
easy.

Geoff.
--
Geoffrey S. Mendelson, Jerusalem, Israel N3OWJ/4X1GM
IL Voice: (07)-7424-1667 U.S. Voice: 1-215-821-1838
Visit my 'blog at http://geoffstechno.livejournal.com/