Re: multiple radios one antenna

"PlAyDoE" <playdoe420@xxxxxxxxx> hath wroth:

I am starting up a WISP, and have limited tower space. I am covering
about a 15 mile area with about 20 remote AP's. Each AP will be
accoiated with about 80 customers at 2mbps down and 512kbps up. I
really don't want to put up 10 diffrent antennas (1 antenna for 2 AP's)

So i Was woundering if it were posable to run 2 AP's through 1 antenna
on the same channel, using a spliter or somthing. I am trying to
maximize the amount of bandwith i get off of one antenna.

Combining antennas is very common in the cellular industry. It can be
done with 2.4Ghz but the cost of the necessary combiner, ferrite
isolators, and filters, will cancel any cost benifits. In addition,
the equipment will need to be installed near the antennas, not near
the radios, resulting in a rather large box on the tower.

It's easy to combine several antennas on a single transmitter. Power
splitters are cheap and easily installed.
http://www.hyperlinktech.com/web/signal_splitters.php
However, note that they describe these as:
"...are used for connecting more than one antenna
to a single radio."
and no mention of multiple radios on a single antenna. What happens
is that these provide insufficient isolation between radios. Without
isolation, when one radio transmits, the other receiver goes comatose
from overload. Another problem is intermodulation mixing. When two
transmitters are on the air at the same time, they mix, create garbage
on the sum and difference frequencies, and cause the site manager to
pull your plug as a source of interference. Incidentally, if you're
in a managed radio site, that require that you submit your engineering
drawings to the site manager for approval before installation, they
will probably reject any form of crude combining.

If you're deperate and must combine radios, you need to build a system
similar to what the cellular companies use. Instead of 0.8 and
1.9GHz, you do the same thing on 2.4GHz. You'll need the combiners
listed above. You'll need one bandpass cavity on each radio tuned to
it's transmitter frequency band thus forming a diplexer. That limits
you to channels 1, 6, and 11 with a maximum of 3 radios per antenna.
Each filter is 25MHz wide. You can't have two radios on the same
channel on the same antenna. That won't work no matter what you do.

Bandpass channel filters:
http://www.hyperlinktech.com/web/band_pass_filters.php
http://www.rflinx.com/products/filters/

You probably should install a ferrite or hybrid ring isolator on each
transmitter, to prevent RF from going backwards into the transmitter,
mixing, and re-radiating as garbage. I couldn't find one specifically
made for Wi-Fi at a reasonable price. Oh well.

So, to combine the maximum of 3 radios, you need:
1 3 way splitter $60
3 8pole cavity filters $130/ea $390
Mess of cables $100
Outoor box $ 50
======================================
Total $600

My guess is the ferrite isolators will be about $100/ea. You'll need
three. Treat these as optional for now.

All this to save you the cost of 2 antennas worth about $100/ea.
If tower space is at a premium, this may actually be a decent
investment.

Incidentally, none of this stuff comes without signal loss. Each
cavity will eat about -3dB. The splitter will be about -1dB. All the
cables and connectors will gobble another -2dB. If you install a
ferrite isolator, it will burn another -2dB plus some more connector
losses. Add it all up and you'll have between -6dB and -8dB of loss.
To put this in perspective, -6dB loss means your effective range is
cut exactly in half.

Short conclusion: Don't do it and get some experienced engineering
help. Small WISP systems are easy. Anyone can do those. Wi-Fi does
NOT scale gracefully and big systems are a mess. That's why WiMax was
invented.

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

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