On Sat, 23 Apr 2005 11:22:20 +0100, Nick <(E-Mail Removed)> wrote:
It was not obvious that you had not purchased anything yet. On the
assumption that you might have some existing equipment, I asked for
the manufactory and model numbers. (Hint: Tell us what you are
trying to accomplish and what you have to work with).
>Most of these ADSL modem
>wireless routers only have one antenna.
If you look carefully at the various boards, most of them come with a
diversity switch and two antenna connectors. That's because the
MiniPCI boards found in many wireless routers are also use in laptops
and client radios, where the packaging determines whether one or two
antennas are used. I'm looking at some wireless PCI cards and note
that the 2nd antenna is terminated with a 50 ohm chip resistor.
There's nothing in the design of an "ADSL router" that requires either
one or two antennas.
Also, permit me to disuade you from purchasing an "all in one" unit
with ADSL modem, router, and wireless in one box (even if they are
somewhat cheaper). If you move, and your new location has a cable
modem instead of DSL, you get to throw the whole box out. The
wireless part tends to add a few acronyms every few months making
obsolescence a real problem. However, the worst issue is over the
location of the box. The modem and router parts want to live near
where all the wires come together. AC power, phone, and CAT5 LAN
cables all want to live near the floor, under a desk, in a closet, or
in the basement. However, the wireless part wants to live in the
open, up high, with a maximum view of the coverage area. These are
incompatible. You can place the "all in one" unit on the floor and
use a coax cable to relocate the antenna(s), but the coax losses are
horrendous. Anyway, I suggest you purchase seperate boxes for the DSL
modem, the router, and the wireless access point.
>The Belkin seems to attract the
>least number of negative comments on the shopping sites and it has two
>antennas, so I guess it is try it and see.
I now have 3ea older unreliable Belkin 802.11b access points. I don't
have the model number handy. The problem was that Belkin does not
seem to be interested in updating the firmware in older products. If
you look at the previous generation of Linksys, Netgear, and DLink
products, you will see continuous product development well after the
product is considered to be obsolete. Belkin and some others do not
bother to do this. The reliability issues I had with Belkin are
apparently known, but because development stops immediately after the
product release, the issue becomes permanent. I suggest you keep
looking.
>The fallback would be to
>connect a WAP to the router via a crossover cable and use that to feed
>the external antenna. I imagine this would overcome the "throughput
>problem" you mention below.
Methinks it would be nice if you describe exactly what you are trying
to accomplish and what you have to work with. There are far too many
topology options in wireless to provide universal solutions.
>Well the yagis are already fitted both ends from a soon-to-be-defunct
>community wireless project. The LOS is only about 30m.
At 30 meters, almost any external antenna will work. You should have
a good strong signal with your unspecified gain yagi antennas.
However, there's always a way to screw things up. One of my
installations involved a pair of 19dBi dish antennas at about 100
meters. Signal was very strong and I was getting about 25Mbits/sec
thruput. No problems until someone installed an access point that was
directly in line with one yagi, but somewhat furthur behind the yagi's
intended target. Because it was in line, the gain of the antenna made
the interference problem much worse. Oops. They refused to move to
another channel (because the others were polluted) so I reduced the
gain of one of the yagis and repositioned it somewhat until the
interference was somewhat less of a problem. Final thruput was about
15Mbits/sec with no interference and about 10Mbits/sec when the
interfering access point is live. Watch where you point high gain
antennas.
>Elsewhere on
>the network we have used small patch antennas to good effect. They seem
>to work better than the 7-ele yagis, particularly in a multipath
>environment. I don't really understand this because I would have
>thought a narrower beamwidth would work better in the latter case and
>the patch antennas are much smaller than the yagis by volume!
Working better measured how? More signal? Better S/N ratio? Better
thruput? What are you measuring?
I much prefer patch (panel) antennas over yagis. It's not just the
cost per dBi of gain issue, but also bandwidth, sidelobes, and
sensitivity to environmental issues, that make the patch superior.
The yagi's rotten sidelobes and f/b ratio will cause reflections and
interference pickup. Also, the yagi is MUCH more sensitive to
mounting issues than a panel antenna (with a solid ground back
surface). Yagi's also have a practical limit on gain. For every 3dB
of gain on a yagi, the antenna becomes twice as long. 15dBi is about
the practical limit for yagi's, while panels will go to 19dBi and
dishes to 24dBi. (Yes, I know there are higher gain dishes, panels,
and yagi's, but if you read the fine print, they will be very narrow
bandwidth and not cover the entire 2.4GHz band).
It's been a while since I've played with the numbers so let me do some
digging (mostly on the fab-corp.com web pile)
Type specified -3dB bw Cost Cost per
gain dBi degrees $US dB gain
Patch
Rootena RT24LP14 14dBi 35 $42 $3.00
Maxrad Wisp 13dBi 35 $39 $3.00
Dish
PacWireless 15dBi 19 $35 $2.33
PacWireless 19dBi 17 $41 $2.16
Yagi
Maxrad 15 15dBi 30 $59 $3.93
Antennex 14.6dBi 30 $65 $4.45
Yep, the yagi is still the most expensive (per dBi gain).
>> Yes, with a very minor catch. If you're using wireless in your house
>> from perhaps a laptop, the thruput between the laptop and the
>> neighbors will be horrible. That's because the access point has to
>> switch between the side and outside antennas. Some, not all, wireless
>> routers and access point take excessive time to do this.
>That could be a benefit from a security POV. There is no requirement
>for client machines in the two properties to exchange data. I didn't
>realise that the WAP actually switches between the two antennas. I
>would have thought they would just be connectected to the electronics
>via a hybrid splitter.
Splitters and hybrid combiners have loss and loss is a bad thing. If
the path length to the two antennas is different, there's a real
chance that the signal phase at the two antennas will be different.
If 180 degrees, they will cancel. The result will be a rather nasty
series of nulls. There are systems that do use combiners and
splitters, but only with isolated antennas, where there's no chance
that a given signal will be heard by both antennas. On the other
foot, the switch does not have any (major) interaction between
antennas and can be run without worrying about creating nulls (i.e.
dead spots).
I'm still trying to decode what you are trying to accomplish. If you
are worried about security between wireless clients, then please
consider the WRT54G router. It has a feature misnamed "AP protection"
which is really "client isolation". No wireless to wireless bridging
is allowed.
>They must scan between the antennas synchronously with the tx/rx packet
>stream. Clever stuff.
No, it's much simpler than that. The access point stores which
antenna heard a specific MAC address successfully last. If it fails
to hear another packet within a specified time (varies from 100msec to
several seconds), then it switches to the other antenna and waits for
the resends. Note that it does NOT work like a radio scanner and does
NOT make any decision as to which antenna has the best signal strength
of S/N ratio. There also a mess of other algorithms and variations.
Some AP's scan between antennas when there's no traffic. Also,
versions of 802.11n (and Pre-N) are completely different and have
seperate receivers on each antenna to try to reconstruct the data from
multiple antennas.
>BTW I just looked at the pictures on your web site. It bears an uncanny
>resemblance to mine i.e. radio and computer junk everywhere. Guess you
>must be a ham too?
See the call sign in the signature. Yeah, I have the ham radio
disease and suffer accordingly. Most of those I know that are
involved in wireless design and development have ham licenses. Most
of the sane ones are inactive. The big difference between ham radio
and product design is that the typical ham can make *ONE* of anything
work. I have to make my stuff produceable. (Now back to doing battle
programming a Motorola GTX-900...).
--
Jeff Liebermann
(E-Mail Removed)
150 Felker St #D
http://www.LearnByDestroying.com
Santa Cruz CA 95060 AE6KS 831-336-2558
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