

DIY: Cheap, ultra low-power radios that communicate over thousands of miles - elimisteve
http://www.nycresistor.com/2010/06/26/crossing-the-pond-with-100-milliwatts/

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euroclydon
I wonder if this is similar to what Paul Lutus did to communicate from the
open seas back to his Oregon home during his sail around the world?

Here is an excerpt from his book:

My ham radio link is working better than expected. Before I started this sail,
I spent some time installing and testing ham radios and computers in both the
boat and my house in Oregon. I wanted to be able to write a message here,
transmit it by radio, and print it on paper in Oregon. At the Oregon end,
because a normal person (not a radio nerd) has to be able to use the system,
there's a simple "message screen" on display. A person just sits down at the
computer and types a message, then presses a key that saves it. The next time
I make contact I collect the messages.

[http://www.arachnoid.com/sailbook/Chapter_2_--
_Oregon_to_Haw...](http://www.arachnoid.com/sailbook/Chapter_2_--
_Oregon_to_Hawaii.html)

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joshfinnie
It should be noted that he is on the 30m Amateur Radio band. I would not
suggest doing this unless you are a licensed ham of general or better.

That begs a good question. How many hackers here are also hams? I'm W1OFZ.

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coffee
"I would not suggest doing this unless you are a licensed ham of general or
better." Why? Is being licensed required for this?

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a-priori
Yes, the 30m band is allocated to amateur radio operators by the ITU, and this
is enforced by the authorities in each member country (e.g. Industry Canada in
Canada, or the FCC in the US).

In practice, especially on bands this low, it's mostly unenforceable. Just
don't be a jerk: keep your power low, your bandwidth small, and make sure no
one is using the channel before you smear your transmission all over it.

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HeyLaughingBoy
Reading this thread is giving me the bug again :-)

I might just go into the basement and put together a simple 40 meter receiver
tonight just to see if there's any activity there and maybe build a small
transmitter later.

ISTR that the FCC went to non-expiring licenses. My last one is from the late
80's/early 90's. Anyone know what the likelihood is that I still have a valid
license?

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jws
3600 miles, 100 milliwatts, 5Hz of spectrum near 10MHz, rooftop mounted
dipole, $25 of parts to make radio (including obligatory Altoids tin) => 0.05
bits per second.

[Edit: to lose that extra zero. Thanks Joe.]

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joe_bleau
Make than near 10MHz.

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th0ma5
hey there i'm kd8mek, i wrote a blog piece about software radios and such the
other day <http://verily.posterous.com> there are a billion of these qrp small
radio blogs out there, all kind of people dreaming up all kinds of things

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rmason
While it is interesting I can't really get excited because to me a QSO is a
two way conversation. Far more interesting to be running a few watts and be
having two way conversations with people instead of being merely captured on
their computer screens.

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Luc
Why do they transmit a square wave, instead of just short and long dashes of a
single frequency? Only the top of the square wave is the signal, the bottom is
the 'negative' of it.

~~~
noonespecial
Because it would be very difficult to decide if your box was not transmitting
or just unable to be heard at any given time. The square, spread over a large
time allows you to say with confidence, "yes I have received and yes its the
bottom/top of a dot/dash".

~~~
Luc
Are you involved with this as an amateur, or taking a stab at coming up with a
reasonable answer? I would like to know the actual reason, and so far Google
hasn't been helpful.

I am starting to think the square wave is an artefact from the circuit design.

~~~
HansSummers
Hi I am the designer of this circuit ( <http://www.hanssummers.com/qrsskit> )
and can answer that question. I've just seen the thread on this.

There are various possible "modes" in QRSS and certainly plain on/off keying
as you suggest, which is in fact just very slow morse code, is one of them.

The mode I am transmitting here can be known as FSK/CW. CW (Morse Code) in a
Frequency Shift Keyed (FSK) way. As you say, the top level is the "key down"
ordinary morse code.

The reason for this is NOT just an artefact of the circuit design! The circuit
is designed this way for a reason! I could just have easily designed it such
that it keyed the signal on and off, like real morse code but much slower.

There are several reasons why FSK/CW is more suitable than plain on/off
keying. 1) and 2) below are the most important.

1) Readability: Many kinds of interfering signals on the band are just a plain
carrier. It might be some mixing product coming out of your computer monitor,
or a harmonic signal of some oscillator in your neighbour's TV, or plain
carrier transmissions from great distances. A weak carrier, drifting in and
out of visibility, can look VERY much like a weak on/off keyed CW signal in
QRSS. Experience of QRSS experimenters has shown that it is much easier to
read the "squarewave" FSK/CW style transmission. It appears to be more
resilient to interference and weak signal propagation conditions.

2) Chirp. This is a temporary shift in frequency which occurs on key down, in
an on/off keyed CW transmitter. That makes the end result much harder to read,
less resilient to interference, and just plain ugly. See some examples at
<http://www.hanssummers.com/qrss/qrssqrv.html> . This "chirp" can be very hard
to get rid of in such simple circuits. It may only be a few Hz of frequency
shift but it is very ugly and visible on QRSS transmissions. Having the
transmission continuously ON and just shifting its frequency slightly, avoids
the chirp problem.

3) Drift: as the oscillator components and crystal heat up, the frequency can
drift a few 10's of Hz. On key down, components will be transmitting some
hundred or so milliwatts of power and can slightly warm up, changing the
frequency ever so slightly but visibly. Again, a problem that is not so easy
to resolve in such a simple circuit. Keeping the transmission permanently ON
solves that problem too as once the steady state thermal equilibrium is
reached, everything stays there.

I hope this helps to explain it - you can read more about my QRSS experiments
at my web page <http://www.hanssummers.com> and let me know if you have any
more questions

Hans <http://www.hanssummers.com>

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joshu
I wonder if this could be combined with UWB-like principles: you encrypt the
clock at both ends and only look at the signal at certain timestamps.

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Kilimanjaro
Hmm, interesting.

Makes me think about an alternet for when they switch the internet down.

