RadioBanter - View Single Post

Jeff Liebermann[_2_]

On Mon, 8 Dec 2008 11:17:10 -0800, (Dave Platt)
wrote:

A not-uncommon scenario, I think. I've seen APs which put out a
signal that has useful strength for blocks, and yet you have to be
within about 100 feet of them to establish contact with a typical
client system.

About a year ago, I was trying to find the source of a very strong
2.4GHz signal in the downtown Santa Cruz mall area. Wi-Fi
communications among the various coffee shops and hot spots was
ummm.... a challenge. I eventually traced it down to a tall
residential hotel and a 2.4GHz cordless digital phone running what I
guess was a 10 watt power amplifier. Instead of a cell phone, this
person decided to provide his own cordless phone service that covered
the entire downtown area. You could see it anywhere but as is
predictable, his useful range was limited by the handset power, which
was well less than 10 watts. I'm not going to go into detail on
exactly what happened, but it should be sufficient that he's now aware
of the problem he was causing and is off the air.

Be the first in your neighborhood to dominate the airwaves:
http://www.ssbusa.com/kunamp1.html
Cool... 50 watt linear on 2.4GHz. Now, we're cooking.

This same issue is significant in other bands, as well. My area's
ham-radio VHF/UHF repeater coordination group has a firm principle...
a coordinated repeater's transmit coverage and receive coverage should
be consistent. Having an ultra-high-powered transmitter simply causes
interference well outside the repeater's practical use range.

Good plan but there are problems. Most hams these daze use walkie
talkies with perhaps 1 watt of TX RF. The typical repeater is running
perhaps 10 to 40 watts out (after the duplexer). The walkie can hear
the repeater almost anywhere, but when trying to talk, they drop in
and out all the time. The mobiles, which run more power, are usually
well matched to the repeater's tx power. I've suggested adaptive tx
power control (to preserve battery power) on our 2 meter repeater, but
nobody wants it.

Having overly-sensitive receivers can also be a problem, albeit a
lesser one, as it means that the repeater can be "keyed up" by remote
stations too far away to hear the repeater properly. It's less of
a problem, though, as most repeaters use CTCSS tone squelch these days
and won't respond to signals intended for co-channel repeaters with a
different CTCSS tone.

We had a co-channel user that was running carrier squelch. It took
only about 7 years of constantly pounding on the trustees before they
would install PL. They lied on their NARCC application claiming they
had a functional PL system. The experience taught me a few things,
one of which is that hams generally make lousy RF neighbors.

I don't think this is an issue for 802.11
access points at all.

No, it's not for most home systems. There's a similar coexistence
problem with mesh networks and municipal networks. Neither of these
scale very well. They work ok with a small number of repeaters, but
rapidly foul up as the usage, traffic, and number of repeaters
increases to the point of mutual interference. Details and a rant on
request.

There's also a problem with wi-fi and omnidirectional antennas. These
pickup and spew inference from all direction equally well. Same with
reflections and multi-path. If the main area of operation can better
be covered with a directional antenna, then it should be used. It may
create hidden nodes, but those can be handled by enabling flow
control.

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