"Jeff Liebermann" wrote in message
...

On Mon, 23 Jul 2012 09:43:10 -0700, "Wayne"
wrote:

Military ELMR equipment is authorized for 380-399.9 MHz.
Garage door openers that operate low power unauthorized in the same band
are
sometimes messed up.

Wonder if the military or the garage door opener company will win...

http://www.theday.com/article/201207...307239975/1018

snip

#Drivel: One of my customers was having problems with their garage
#door opener. They said they could easily close the door from the end
#of the driveway, but could not open it again from the same location.
#When I arrived, I found that they had an aluminum garage door, with
#the receiver mounted behind the door. Between the metal door and
#aluminium foil backed insulation in the walls and roof, the garage
#made an effective screen room. I moved the recevier and wire antenna
#to a better location, which fixed the problem.


snip--
#Jeff Liebermann
#150 Felker St #D http://www.LearnByDestroying.com
#Santa Cruz CA 95060 http://802.11junk.com
#Skype: JeffLiebermann AE6KS 831-336-2558

My own situation was that the remotes in the cars lost range. You could
open the door if the car was practically touching the door, but not much
farther back.

Using various test gear, I found an interfering signal that was just a few
feet away. The remote opener screwed on the outer wall had failed in the
"on" position and was closer to the receiving antenna than the openers in
the cars.

The Genie engineers were quite interested in getting that failed remote in
their hands to examine the problem. They were amazed that I located the
problem, but it's all in the test gear.

Wayne
W5GIE (exiled to W6)

On Mon, 23 Jul 2012 15:12:53 -0700, "Wayne"
wrote:

My own situation was that the remotes in the cars lost range. You could
open the door if the car was practically touching the door, but not much
farther back.

Using various test gear, I found an interfering signal that was just a few
feet away. The remote opener screwed on the outer wall had failed in the
"on" position and was closer to the receiving antenna than the openers in
the cars.

I've seen something like that in action. The problem is in the way
the rolling code works and a rather dumb design decision in the remote
key fob:
http://en.wikipedia.org/wiki/Rolling_code
http://www.atmel.com/Images/doc2600.pdf
When you press the key fob button, some remotes just continue to spew
codes like a machine gun. I think (not sure) that my 2001 Subaru key
fob does this. The problem is that the system compares the received
code with up to 256 anticipated codes that the remote might send. The
idea is to catch up in case the key fob was accidentally pressed and
the receiver missed the transmission. However, if you send 256 codes,
the transmitter and receiver go out of sync and must be
re-synchronized. I actually tried it by pressing the button 256 times
and found that I could no longer unlock the car. It is my
understanding that modern chips have a better algorithm that doesn't
have this problem.

When your transmitter in the outer wall stuck on, it probably ran out
of codes fairly quickly. After that, it was no better than a jammer.

The Genie engineers were quite interested in getting that failed remote in
their hands to examine the problem. They were amazed that I located the
problem, but it's all in the test gear.

Nicely done especially since you had no idea what was causing the
problem.

Wayne
W5GIE (exiled to W6)

I live in W6-land and methinks it's is paradise (except for the
government, traffic, taxes, gas prices, etc).

--
# Jeff Liebermann 150 Felker St #D Santa Cruz CA 95060
# 831-336-2558
# http://802.11junk.com
# http://www.LearnByDestroying.com AE6KS