>
>The coupling loop scheme is one way, another is a T connector with a
tiny
>capacitor in the "arm".
>
>It doesn't take much coupling to make this work.
>
>Taking 2.4GHz WiFi/Zigbee/etc as an example..
>
>100 m through air has 80 dB loss (so radiating +20dBm gets you -60 at
the
>receiver)
>
>RG-213 is about 50 db/100 meters at 2.45 GHz
>
>So your coupling doesn't have to be all that great.
>
That's correct. Routeing the RF control signal along the coax will
deliver far stronger signals than an over-the-air link, which wastes
almost all the signal power in unwanted directions. In practical terms,
a high SNR means that almost any coupling method will work. For WiFi,
you can make an excellent coupler using two quarter-wave 'twigs' of wire
(only 1.25in long) spaced a short distance apart... and it really can't
get much simpler than that.
>I speculate, but have not tried, that coupling to the *outside* of the
coax
>would probably work: all you're trying to do is get somewhat better
than
>free space, and launching the wave is a sort of non-critical thing.
>
I wouldn't even try that. The outside of the coax will always deliver a
weaker control signal - and even worse, it will act as a long-wire
antenna for every other competing WiFi signal in the neighborhood. The
inside of the coax gives a stronger signal on a shielded 'private line'.
73 from Ian GM3SEK
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