"Jeff Liebermann" wrote in message
...
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.

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

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. The catch is that if there's any signal
leakage from a direct path that bypasses the passive repeater, there
will be cancellation, nulls, dead spots, and irate clients.





At 200 m (down the street, why he is doing this) the FSPL is 71 dB. The Tx
antenna will have lots of loss buried in the car body. The Genie opener just
has a short (15 inches) length of wire hanging down for its antenna. No real
400 MHz ground plane so I doubt it will be 0 dB.