On Thu, 08 May 2014 20:54:18 -0500, John Fields
wrote:
A cursory look through Google finds:
http://www.terra-wave.com/shop/89096...l27hveoggt6rf4
I do wish you would use a 433 MHz or 315 Mhz rubber ducky antenna.
I'll make it easy. Put a pizza platter on Jim's roof and ceiling. Add
a 5/8 wave antenna with a suitable matching transformer, and I'll call
it about 5 dBi gain. At 10 meters from the garage, that will probably
work. At 200 meters, no way.
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.
Sure. Like I said. Add some gain at any of the antennas to make up
for the 39 dB difference in path loss between the direct versus the
passive repeater, and it will work just fine. However 2.5dB gain per
antenna is not going to make up for 39 dB of loss. Granted, you can
run the signal levels down to the minimum detectable signal level, and
possibly make it work. However, don't use my figures for doing that.
I picked 0dB losses and 0dB gain antennas simply to illustrate the
large difference in path loss between the two schemes. If you want a
more accurate calculation, I can grind out the numbers. I'll need
some details from Jim Thomson, such as the height of the garage, the
height of the garage door opener, and the size of the garage.
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.
I think differently. Using my numbers, the only way to compensate for
the difference in losses is with antenna gain. Well, maybe increase
the transmit power or put an RF amp in front of the receiver.
OK, but that garage door and all the aluminum foil around the house
is a big attenuator killing what would be a direct shot.
True. However, that doesn't make much difference if the antenna is
outside the shielded garage. I consider that a necessity, whether
using a passive repeater or a direct shot.
Sounds just like Jim's situation.
No, it's not. The situations that work is where there is absolutely
no signal via the direct path. The basement of an emergency center is
one. Another is 4 floors down in an underground garage. Jim's
shielded garage may be very lossy, but is still leaky enough that
cancellation (nulls) between the incident and passive repeated signals
will be a problem.
The catch is that if there's any signal
leakage from a direct path that bypasses the passive repeater, there
will be cancellation,
And enhancement
Only if you're lucky. The problems with such calculations as mine is
that they define the best possible conditions, where everything
behaves exactly as predicted. Reality tends to suggest that this is
most often not the case. Losses are never better than predicted.
Power output and rx sensitivity are never up to spec. Path losses
always have some extra obstructions that raise the losses. And, I
haven't even thrown in Fresnel zone diffraction. There are no
optimists in the RF business and antenna problems tend to be very
strange:
http://people.csail.mit.edu/bkph/images/theres_your_reception_problem.jpg
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... :-)
Believe me, I have loved passive repeaters since I discovered one
hiking in the hills above Big Bear Lake in southern Calif. When we
finally reached the top of the mountain, there was a huge billboard on
top. No advertising or embellishments. Just a billboard. I
discovered later that it was a passive reflector for the microwave
links used by AT&T to cross the country. I thought it was cool.
Somewhat later, I blundered into periscope antennas, where a 45 degree
reflector in the shape of an ellipse was perched on top of a tower,
with a dish antenna pointing straight up at the reflector. Again I
thought it was cool.
Then, I discovered that ecology and fiber optics had conspired to
remove the billboards from the mountain tops. The FCC also took a dim
view of the signal splatter produced by periscope antennas and
effectively banned them. So much for my first love in antennas.
At various times in my checkered career, I've toyed with various forms
of passive repeaters, and found little in the way of success. The
math shows why, but I was more than willing to ignore the calculations
in the hope that they might be wrong. When Wi-Fi took hold, I
immediately resurrected the idea in a 200ft fir tree, pointing one
dish at the WISP (wireless ISP) and the other down to my house. As
long as I didn't mind climbing the tree to realign the antenna every
few weeks, it worked fairly well.
I never make the same mistake twice. 5 or 10 times is more my style.
--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060
http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558