Hey guys, I don' t have a design ready to post yet, but my question
is.. How high should my tower be for my vertical sector wifi antenna?
I'm currently still undecided on which antenna to get, but my radio is
gonna be a 400mw mini-pci nic and the antenna is going to be a 17-24db
vertical sector antenna. I'm looking to cover distances of 4-15miles.

wrote:
Hey guys, I don' t have a design ready to post yet, but my question
is.. How high should my tower be for my vertical sector wifi antenna?

What you see is what you cover


I'm currently still undecided on which antenna to get

It depends on lot of factors. As a (over) simplistic method use LoS
equation, add some margin for fading and calculate link balance to get
ERP figures. From ERP you can find out the antenna you need for your
feeder losses.

GL,

zoli ha1ag

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

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?

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