Monday, November 1, 2010

Fixing Remote Extenders

The remote extender (partly to control my Sony XDR-F1HD tuner from all rooms in my house) saga continues.  I finally worked up the nerve to take apart one of these little quasi-pyramids (the Radio Shack version RF Receiver).  I had read elsewhere on the web that the circuit board is not directly connected to the antenna.  Some guy wired it straight to the antenna and range increased from 8 feet to 50 feet or whatever else he needed.

He was right, sort-of.  Actually, the antenna is connected through a very tiny capacitor, a few picofarads, but it doesn't look like a capacitor, it looks like a piece of zip cord, with one wire of the pair being connected to the circuit board, and the other wire being connected to the antenna, furthermore most of the rest of the zip cord was removed, leaving only about 1 inch in which the two wires overlap.  Thus there is no direct copper connection to the antenna, just a 1 inch stretch where the two wires are in proximity.  This forms a small capacitor, though also with self-inductance, possibly more significant than the capacitance.

It's not clear why they did it this way, but it's clear it's not a manufacturing mistake (as the other poster suspected), this was clearly done intentionally.  Hooking the antenna up with a straight piece of wire did seem to improve reception a little.  But the biggest improvement for me came from relocating the antenna to a very high position on the outside of a kitchen cabinet.   For temporary (which really won't last long) it is held in place with clear duct tape.  I need to nail it in place this week because the duct tape is now sagging after two weeks.  I can also staple the wires, and get the top fitted back on the remote extender with some modifications to the plastic case with a drill or dremel.

One possible reason why the antenna is hooked up indirectly is to minimize RFI emitted by the unit.  One can imagine that the original designer didn't factor that in well enough, but when they were trying to get FCC approval they added this little hack to the design to get it to pass inspection.  Remember that I am modifying the receiver and not the transmitter, but even receivers emit RFI from their superheterodyne oscillator.  Then they didn't bother to change the "100 foot" range claimed specification, even though it could not longer meet that.

Another alternative is the funky piece of zipcord is actually a "balun" which reduces standing waves in the antenna.  In this case, removing the balun and wiring the antenna directly should reduce range, not increase it.  In my case, it did seem like it did increase range slightly even keeping antenna in same place, though I could be mistaken.  But moving the antenna to a position I can't move the whole unit is better still.

I did pull out my 500khz-10Ghz RF meter to check it out.  The RF produced by the modified remote extender receiver is just barely above the background.  I can find much greater RFI generators in my house that are presumably within FCC specifications. So I think this modification is not likely to cause actual interference for anyone.

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