Generally speaking you should have an unobstructed line of site to the
other end of the link. Think of WiFi as being a light beam.

Work out the path loss based on the basic distance formula and plug your
power, losses and RX sensitivity in to determine your margin and thus
max range. Radiomobile and/or UKWtools can be used for this and some
allowance for terrain as well.

Cheers Bob VK2YQA

wrote:

is.. How high should my tower be for my vertical sector wifi antenna?

Also, I heard 802.11g sucks outdoors. This true? And would you guys say
my 400mw radio is overkill for 4-10mile shot?

On 15 Jan 2006 08:38:41 -0800, "Tekmanx" wrote:

Also, I heard 802.11g sucks outdoors. This true? And would you guys say
my 400mw radio is overkill for 4-10mile shot?

Hi OM,

It, or any link, only sucks as a function of what is called multipath.
This means that reflections combine at the receiver to blur the
signal. For conventional modes this is at worst obnoxious. For
digital it can mean total bit loss. In all practicality it translates
to high BER (bit error rate) and low information bandwidth due to
repeated packets being needed. The solution is not more power because
the problem will still be the same, only louder (so to speak).

Instead, the receiver antenna should have the gain so as to exclude
the signals coming from other directions. This exclusion is a
property of antenna gain, it is like cupping your ear to hear better,
there is not more signal to be had, you are merely excluding
distractions and focusing what is available.

400 mW in the clear and visible to a receiver is more than enough.
Your second problem is that you may not have 400 mW at the end of the
transmission line, at the antenna, because of the enormous loss in the
line if it is very long. That has already been discussed by Bob².

73's
Richard Clark, KB7QHC

So you're saying that anyting less than 400mw on the other end will be
useless?

On 15 Jan 2006 11:20:53 -0800, "Tekmanx" wrote:
So you're saying that anyting less than 400mw on the other end will be
useless?
No, it only takes microwatts at the receiver to do the job. Start at
the receiver, not the transmitter. Ham radios with only a Watt or two
talk to the Space Shuttle (hundreds of miles) without too much
trouble.

73's
Richard Clark, KB7QHC

So what is it you would say determines wither or not my signal will be
received on the other end? I mean with just a regular soho wifi access
point in open space you can only communicate within a couple hundred
feet (That's open space). If gain/wattage isn't so important when we're
talking distance.. what is? Line of site? Are you saying that that I
can shoot my 30mw signal from my soho access point couple of miles?

On 15 Jan 2006 11:56:12 -0800, "Tekmanx" wrote:

So what is it you would say determines wither or not my signal will be
received on the other end? I mean with just a regular soho wifi access
point in open space you can only communicate within a couple hundred
feet (That's open space). If gain/wattage isn't so important when we're
talking distance.. what is? Line of site? Are you saying that that I
can shoot my 30mw signal from my soho access point couple of miles?

Ah!

Only 30 mW? So you were expecting the antenna to boost it to 400
without any loss of the 30 getting to the antenna?

Why it seems limited is in exactly the problem described as multipath.
All those echoes are roughly the same strength because you are sitting
down low near many reflecting surfaces. "Open space," is not always
so open unless you are sitting in a pasture. Simply because there are
no obstructions between you and your destination does not mean the
signal is not traveling by many, many different paths - in fact, it is
guaranteed.

As I said, this is more a receive problem.

73's
Richard Clark, KB7QHC

So what is it you would say determines wither or not my signal will be
received on the other end? I mean with just a regular soho wifi access
point in open space you can only communicate within a couple hundred
feet (That's open space). If gain/wattage isn't so important when we're
talking distance.. what is? Line of site? Are you saying that that I
can shoot my 30mw signal from my soho access point couple of miles?

I've spoken with a guy who has set up a number of reliable
point-to-point links in the Sacramento valley, using standard
unamplified off-the-shelf SOHO-type access points and/or PCI cards or
USB dongles. He said he achieves reliable performance, with a good
margin of signal strength to handle rain fade, etc., with no
amplifiers, over distances of as much as 5 miles.

The key to doing this are a clear line of sight, an antenna with high
directional gain at each end of the link, and careful aiming. Getting
the radio right up at the antenna (rather than at the end of a length
of coax) is also beneficial.

The carefully-aimed highly-directional antennas give you several
advantages, over a standard SOHO omni antennas. The directionality
increases the effective radiated power of the transmitter (50
milliwatts through a 20 dBi antenna is equivalent to 5 watts
isotropic), it increases the receiver's effective sensitivity by the
same degree, and it makes the receiver _less_ sensitive to
interference arriving from other angles (e.g. competing transmitters).

Also, with proper choice of antenna, you can select the signal's
polarization angle. Since most home and business access points seem
to use vertically-oriented antennas (and thus a vertically polarized
signal) you can reduce interference problems by using point-to-
point antennas which are horizontally polarized.

The guy I spoke with was not complementary about the idea of trying to
"blast" signals through by using high-power transmitters or
amplifiers, and blanketing a large area with the signal.

You can buy wire-dish parabolic antennas for the 2.4-gig ISM radio
band quite easily. I think I've seen 'em advertised as having 15 to
19 dBi of gain. One of these at each end of the link would be a good
place to start.

--
Dave Platt AE6EO
Hosting the Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!

In addition to Richards comments

As a guide I use to run two spread spectrum links. One on 2.4GHz over
about 10km and another on 5.6Ghz over 8km.

The 5.6GHz link was one of those Cisco patch panel things with 30dBm
EIRP. The RF power was about 8dBm. We had 56MB/sec about 90% of the
time. (Including the BER) (keeping in mind that this is aggregate)

The 2.4GHz link was initially setup wrongly. There is a parameter one
has to set that defines the max distance of the link I think to reduce
packet retries and collisions. When it wasnt set the rate was a real bad
and flakey 1MB/sec but when fixed 11Mb/sec was good about 80% of the
time (incl BER)

What eventually killed the 2.4GHz link was mainly other users on the
same freq. The radio design didnt seem to allow it to hop away from
interfering signals. A cold power boot often resolved the issue as it
chose another clearer freq. We eventually dropped it to 2MB/sec with
about 50% reliability. We didnt really have any major multipath problems
that were noted in the design phase. We did however have a building go
up in the path and for a while were firing between two concrete floors!
(We moved one end later)

We used a 2 metre gridpack horiz polarization at each end (to avoid some
user interference). One end had a 16m run of LMR400, the other about a
12m run. I dont remember the calcs/margin we did off hand, sorry. We
didnt however exceed the 30dBm EIRP legal limit. (The company had a very
good standing with the ACA/ACMA so we were kind of pedantic about doing
it right)

Both links were kind of high point to high point accross Sydney. ie
There was a large series of valleys between each site.

Hope you find this helpful

Cheers Bob

Tekmanx wrote:

Also, I heard 802.11g sucks outdoors. This true? And would you guys say
my 400mw radio is overkill for 4-10mile shot?

Tekmanx wrote:
Also, I heard 802.11g sucks outdoors. This true? And would you guys say
my 400mw radio is overkill for 4-10mile shot?


You seem to be getting too much information and not answers to your
questions.

400 milliwatts is plenty of power. The performance of your system will
be governed by two simple factors: how high are the antennas; and, how
sensitive is the receiver?

For ten miles your antenna height above average ground should be close
to 100 feet. For seventeen miles you need an antenna height of close to
300 feet. I suspect both heights are excessive for your application.
Also, the cables connecting your transmitter to the antenna have losses.
So, mounting the transmitter at the top of the antenna would be
preferred. An alternative to the 100 foot antenna would be 70 feet
antennas, one at the transmitter and one at the receiver.

Receiver sensitivity is unknown. I regularly communicate 20+ miles with
500 milliwatts from a ham radio walkie talkie. [the receiver is located
on top of a mountain] Most communiation grade radios can receive a
signal as small as 0.000000000000002 watts. [one millionth of a volt].
So, you can see why I say 400 milliwatts is plenty. So, the receiver you
use should have a moderately good sensitivity 5 microvolts or smaller.

Over relatively flat terrain, with very modest antenna [20 feet high] a
four mile circuit should be possible. Longer paths will require spending
$$$.