passive reflectors

Mon Jun 10 10:15:36 EST 2002

Hi Tony, Robert and group,

Actually, Robert it right about the inverse square rule.

However, "passive repeaters" do work (subject to the laws of physics).  They
can be thought of as "earth flatteners", rather than "range extenders".
Their best application is in cases when you can summarise the link as:  "We
would have HEAPS of signal on this link path, if only that great big
building | hill wasn't in the way".

The best example of a passive repeater that I've seen, is on a ridgeline
above the town of Karratha, in the Pilbara region of WA.  Two grid pack
dishes are mounted back to back, interconnected only by a section of
waveguide.  No electronics on site, no solar panels, no 240V.

Down in the town (on the coastal plain), there's a microwave dish at the
exchange, pointing at the passive repeater.   Over on the other side of the
ridgeline, at a distance of about 3km (from memory) is the other end of the
link, to service the Light Industrial Area.

To get over the ridgeline would have required ridiculously tall towers at
each end of the link.

Unfortunately, I don't have any performance figures for this configuration.

I have also used the passive principle to provide multi-channel VHF radio
coverage underground over a leaky feeder system, where there was heaps of
signal outside, and none in the underground structure.

As a guide, the passive systems work best when placed as close as possible
to one end of the circuit.  The middle of the link path is the worst-case
scenario (that inverse square rule again!).

Hope this helps,

Chris vk6kch

p.s  Robert, I'm an RMIT alumni, so you just _know_ this all has to be true,
coming from your competition :-)

----- Original Message -----
From: "Tony " <tonylist at wwwbiz.com>
To: "'robert scholten'" <rscholten2000 at yahoo.com>;
<wireless at lists.samba.org>
Cc: <eqeeu at yahoo.com>
Sent: Monday, June 10, 2002 7:03 AM
Subject: RE: passive reflectors

> Robert
> I think you need to get out and try this yourself :) Coming from a
> background of doing licensed link this can me done. While this will not
> work in all cases, if does work for shorter link. Using ISM equipment we
> have made this work at 3 mile.
>
> Sincerely
> Tony
>
> -----Original Message-----
> From: wireless-admin at lists.samba.org
> [mailto:wireless-admin at lists.samba.org] On Behalf Of robert scholten
> Sent: Sunday, June 09, 2002 6:02 PM
> To: wireless at lists.samba.org
> Cc: eqeeu at yahoo.com
> Subject: passive reflectors
>
>
> This passive reflector stuff is baloney.  It's basic inverse-square law.
> Consider a setup with 2 dishes, e.g. 24dB Conifers.  Beam widths are 7.5
> and 10 degrees (-3dB widths).  Area is about 0.5m^2.
> Imagine you have a really good link with two 24dB dishes over 5 miles =
> 8km.  Let's say you get 1 nanowatt (i.e. -60dBm).  I've managed about
> -80dBm from a good site with two 24dB dishes, one site on the side of a
> small mountain, the other on the plains, length about 20km.  That
> translates to -73dBm for 8km, but let's be generous to Cringely.
> Now let's put a passive reflector at that point, instead of a single
> dish.  I.e. a reflector consisting not of Pringles tins, but good 24dB
> dishes with zero loss cable between them.  Ignore the fact that both
> dishes in the reflector will radiate (i.e. instant 3dB loss).  So the
> second dish re-radiates 1nW of power into a "cone" of 7.5 x 10 degrees.
> That power is spread over an area of approx. d*tan(a1)*tan(a2) where a1
> and a2 are the angles, 7.5 and 10 degrees.  At 1000m (again, let's be
> generous to Cringely and assume 1.5 miles is 1000m!).  The power is
> spread into 23000m^2 so at our final 24dB dish we have 0.5/23000 * 1nW =
> 2e-14 watts, or about -107dBm.  More realistically you should use -73dBm
> at the repeater, include at least 3dB loss in the repeater, add another
> 3dB because he's 1.5 miles away from the mountain (2km), and come up
> with a very optimistic -119dBm.  The best cards (Cisco 350's) are good
> down to about -94dB so it's pretty much irrelevant anyway.
> How big would Cringely's pringles tins need to be to make it work?
> Optimistically he'd need an extra 11dB, i.e. 5dB on each side, so 30dB
> (about 1.5m^2).  More realistically he'd be needing a good 25dB, 12.5dB
> per side, 9m^2.  Dishes of 3.4 m diameter (about 11 feet).
> Forget it!
>
>
>
> To: Eqeeu <eqeeu at yahoo.com>
> Cc: wireless at lists.samba.org
> Subject: Piercing a mountain? Help!
>
>
> Eqeeu writes:
>  > Hi
>  >
>  > I want to setup a 802.11b link between my house and a
>  > friends, the only problem is that there is a large
>  > mountain (I think its Ainslie mountain) smack in the
>  > middle of our two houses and its impossible to get
>  > line of site.
>  >
>  > Is this a problem?
>  >
>  > Would we have to go around or over the mountain with
>  > an extra repeater WAP somewhere, or is it possible to
>  > use two high gain 24dBi+ directional antenna and
>  > "pierce" straight through the centre of the mountain?
>  >
>
>
> An article on doing passive repeaters is at :
>
>
>  http://www.pbs.org/cringely/pulpit/pulpit20020207.html.
>
>
> I think you can get more doco. from there.
>
>
> wes
>
>
>
>
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