On Thu, 08 May 2014 17:51:16 -0700, Jeff Liebermann
wrote:
On Thu, 08 May 2014 19:08:40 -0500, John Fields
wrote:
Voila! passive repeater, - probably with some gain - nice clean
install, and your warranty stays intact since you haven't even
touched the holy RX.
Passive repeaters, with omni antennas don't work. The problems is
that the small "illumination area" (my term) of the two antennas
results in a huge overall loss. Let's play with the numbers. I'll
use 0dB gain for all the antennas and for the inside to outside
passive repeater losses to make the math easier.
Garage door opener receiver end.
Sensitivity 1 uv/12dB SINAD = -107 dBm
Antenna gain on receiver = 0 dB
Distance to passive repeater antenna
is 10 meter. Path loss at 433 Mhz = -45 dBm
http://www.qsl.net/pa2ohh/jsffield.htm
Inside Antenna gain passive repeater = 0 dB
Loss between inside and outside
antennas = 0 dB
Outside antenna gain passive repeater = 0 dB
Distance from outside passive repeater
antenna to remote control is 10 meters.
Path loss at 433 Mhz = -45 dB
Antenna gain on remote control = 0 dB
Transmit power (about 100 microwatts) = -10 dBm
Remote control transmitter end.
So... if we start out with -10 dBm of signal and have -90 dB of total
path loss through the passive repeater, the signal level arriving at
the receiver will be -100 dbm, which 3 dB above the receiver
sensitivity. For PWM modulation and a crude demodulator, my guess(tm)
is that at least 6 dB SNR is required to demodulate the data. That
puts the passive repeater scheme about 3 dB short of working.
---
A cursory look through Google finds:
http://www.terra-wave.com/shop/89096...l27hveoggt6rf4
so the 5dB gain for the pair of antennas - all the rest of your
numbers staying the same - puts the scheme about 2dB long on
working.
Not an awful lot, but it's over the edge and for less than $20 for
the antennas, in my book it's probably worth trying.
---
However, had there been no passive repeater over the same 20 meter
distance, the path loss would have been -51 dB, resulting in a -61 dBm
signal level at the receiver, which is 46 dB above the receiver
sensitivity and 40 dB above my guess(tm) as to the operating
threshold. That should work just fine. More simply, the difference
in overall path loss between this 2 hop passive repeater and a direct
shot without the passive repeater is:
45 + 45 - 51 = 39 dB
---
OK, but that garage door and all the aluminum foil around the house
is a big attenuator killing what would be a direct shot.
---
There are a few situations where passive repeater do work and are
useful, but extending the range of a garage door opener is not one of
them. However, if you want to make it work, you'll need at least a
directional antenna at the receiver, pointed to inside passive
repeater antenna, which should also be directional. If the approach
is along a fixed path, a direction antenna for the outside antenna
will also help. The total gain of these three antennas should equal
the 39 dB path loss difference in order to get the same performance as
a direct shot without the passive repeater.
I've build passive repeaters like this for situations where any signal
is better than none at all.
---
Sounds just like Jim's situation.
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The catch is that if there's any signal
leakage from a direct path that bypasses the passive repeater, there
will be cancellation,
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And enhancement
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nulls
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and peaks
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dead spots
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and hot spots
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and irate clients.
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I don't think that'll be a problem in this instance and, anyway,
it's better to have loved and lost than never to have loved at
all... :-)