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When
I a little kid I always thought it would be hilarious
to have a portable radio transmitter to interfere with
the neighbour's radio channel. All in good fun, of course.
I
guess, in some ways, that interest in radio lead to
my acquisition of an amateur radio license, call sign
VK8NPF. A word of warning - an amateur radio license
does not mean that you may do whatever you like on air!
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This
is a radio! |
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I thought
we should cover some legal issues before we dwelve into
FM transmitters themselves. The communications authority
allows the use of the FM broadcast band for anyone,
provided any transmissions are low power and don't "cause
harmful interference".
In other
words, transmitting at 500W right outside your neighbour's
shed is not a great idea. But transmitting at 1mW for
'personal' use may be OK - depending on where you live.
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There
are schematics for FM transmitters all over the internet,
ranging from low power devices which can fit in your
pocket to huge monsters which draw more current than
your household supply can provide.
I
decided to go with a small FM transmitter kit, which
saves the hassle of RF issues on homemade PCBs, winding
inductors etc.
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This FM
transmitter has an output power of around 5mW from what
I can tell. Depending on terrain, that's probably sufficient
to cover about 50m of ground.
I wanted
this baby to be as portable as possible, so managed
to jam everything in this small jiffy box along with
a 9V battery and a frequency adjust knob. I also managed
to find an old transistor radio extendable antenna,
which I mounted in the jiffy box at the lower right.
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Here
is the completed transmitter, with extendable antenna
(~70cm). The kind of shonky black wheel is the frequency
adjust knob which allows quick changing of frequencies.
This
little guy works rather well. Once, with a little tuning,
it managed to transmit over the neighbour's radio. Needless
to say, this was a bit of fun for all! |
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