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

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

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
$$$.

Thank you so much Amos! I was aiming for a 40feet-ish tower. Tripod
base with an adjustable inner tubing which is where the antenna will be
placed on at the top. So basically I was thinking 20ft tall tripod with
cup-like guides allowing me to adjust the inner 30foot long inner
tubing/pole up to 20feet on top of the tripod giving a total high of
40ft(10feet of the inner pole will sit inside the tripod). I'm
currently in the process of drawing up a diagram to show a local metal
welder, but that's my basic idea right now.