At 01:42 AM 7/20/2005, Ian White G/GM3SEK wrote:
>I'm starting to lay out a new HF/VHF home station on a 1-acre site...
>and can already see the numbers of 200-500ft control cables expanding
>out-of-sight.
>
>Obviously there is a basic minimum of links that have to be hard-wired:
>RF signal lines, AC/DC power rails and ground bonding. But there seems
>no reason why a lot of the simple control wires (eg antenna select and
>direction switching) couldn't be converted into radio links.
>
>What I had in mind is a pair of radio modems that provide "virtual
>wires". So each modem would have a big array of terminal strips -
>whatever goes into one terminal strip, comes out the other. (Obviously
>there has to be a power and some jumpering at the remote end, but you
>see the basic idea.)
>
>Does such a product exist? I've only seen units providing say 4 relay
>closures, which are far too few. This application really needs a minimum
>of 16 "virtual wires", with capability to add further blocks.
Another approach, in addition to those already suggested, is to use
Wireless Ethernet remote printer interfaces. However, these don't
necessarily expose low level bit twiddling in a way you'd like.
A couple practical concerns with wireless links which I've encountered.
If you're putting this widget out next to the antenna, it will be
potentially subjected to some strong EM fields. Aside from direct damage
things, you may find that the run of the mill remote control receiver
stuff and the wireless RS232 kinds of links have fairly "wide open"
receivers. The strong out of band RF from your transmitter will block the
receivers. I was fooling with some 2.4 GHz stuff next to a HF antenna (a
monopole over a ground plane) and found it almost impossible to get it to
work reliably. Sure, one CAN build filters, but to get the required
isolation is really, really tough. You're needing 100 dB plus isolations,
I think, and that's hard to do, especially if you have power wires, etc.,
especially in a control kind of application.
I have contemplated building some sort of "reverse bias tee" kind of thing
that could couple a 2.4 GHz signal in and out of the main coax, as well as
feeding DC power. However, I'm not sure you could get it to work
practically (however, the idea of having ethernet at the far end is awfully
attractive!).
As Tom W8JI had pointed out, there's an elegant simplicity in using DC
signalling voltages on separate wires. Cat-5 cable is awful cheap, and
gives you 8 wires. With +,off,- kind of things, you can send 3^7 possible
states (=2187 states) with 8 wires. Relays are tough, reliable, cheap as
surplus, etc.
If you want slightly fancier, you can use variable currents (so that wire
resistance doesn't bite you) and use a bargraph type chip (or, easier to
build, get one of those bargraph/LED PC boards) to turn voltage into
selection line. A resistor turns the current in the loop to voltage at the
receiving end. At the sending end, you use resistors and a 3 terminal
regulator to make a constant current source. (Regulator In goes to DC
supply, resistor goes between output and load. Control/Gnd terminal goes to
load side of resistor. You also need a resistor from Control/Gnd to real
ground for the bias current)
You might seriously consider using optical fiber. It's about the same cost
(if not cheaper) than coax, although the terminations can cost more ($25
each! ouch), but it does turn up surplus.
_______________________________________________
See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather
Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions
and ask for Sherman, W2FLA.
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