Jack,

Just a few FYIs...

On Sat, 10 Jan 2004 19:46:55 -0500, Jack Smith wrote:

>...<
> I'm familiar with the ITU's international radio bandplan, which almost all
> countries follow to some degree or another. You're in NZ, and I think you
> may well find that licensing a 30 MHz bandwidth pulse system in the 400 MHz
> band is somewhere between difficult and impossible.

I don't think it's anywhere near difficult - I think it'd be absolutely impossible!  :-)  Apart from the
interference within the 30MHz it's using, the harmonics would spatter all over other bands, including
television.  The authorites wouldn't even consider allowing it - certainly in the UK, and NZ seems to have a
very similar licensing system.

>...<

> In the US, the Family Radio System (460MHz) comes to mind.

I think the range may defeat this - I don't think the power allowed would give *reliable* comms at 2km.  The
Motorolas I have quote "range up to 3km" which means I'd expect it to be reliable in clear space up to 1km,
then start dropping out after that.  Incidentally, it's called "PMR446" and operates on 446MHz in the UK &
Europe, with 500mW as the max power.  I think the US is more generous, perhaps 1W?

> In most places in the world, the standard split between F1 and F2 for systems
> in the 400-500 MHz range is either 5MHz or 10MHz. In the US, it's 5MHz, but
> I seem to recall some UK systems operating with 10MHz splits.

UK amateur radio repeaters in the 70cm band (430-440MHz here) use a 1.2MHz split - since the band is only
10MHz wide, a 10MHz split wouldn't be possible.

> Depending on the power level, you may be able to get by with separate TX and
> RX antennas. Or, you may need a cavity duplexer or at least a couple cavity
> filters to prevent the receiver from being desensitized by the transmitter.

There's a device called a "circulator" which, from memory, has three ports and passes signals between them in
a circular fashion, so signals from the Tx would pass to the aerial, signals from the aerial pass to the Rx,
and ne'er the twain shall meet, which is what you want.  They're a lot smaller than a cavity duplexer, but
I've never seen them for sale, only described in technology briefing documents.

Cheers,

Howard Winter  G1BYY
St.Albans, England

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