Homemade Antenna Tower
 

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.

Homemade Antenna Tower
 

Homemade Antenna Tower
 

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?

Homemade Antenna Tower
 

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

Homemade Antenna Tower
 

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

Homemade Antenna Tower
 

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?

Homemade Antenna Tower
 

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

Homemade Antenna Tower
 

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

Homemade Antenna Tower
 

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?

Homemade Antenna Tower
 

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

Homemade Antenna Tower
 

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!

Homemade Antenna Tower
 

Ok, I 'think' I understand now :/

Homemade Antenna Tower
 

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
$$$.

Homemade Antenna Tower
 

Dave Platt wrote:
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.

It's important to point out that using these extreme high gain antennas
with out a license is illegal in the U.S. The guy that invented the
"pringles can" antenna was an FBI agent so he was not prosecuted, but
if he had been an average citizen the FCC would have come after him.

Then the question becomes which if any of the 14 WiFi channels is
actually in the 2.4gHz ham band.

Here in Israel it's even worse. WiFi and terrestrial 2.4gHz ham activity
is limited to 100mw EIRP. If you use a gain antenna, you must reduce
the transmitter power proportionaly.

Geoff.
--
Geoffrey S. Mendelson, Jerusalem, Israel N3OWJ/4X1GM
IL Voice: (07)-7424-1667 IL Fax: 972-2-648-1443 U.S. Voice: 1-215-821-1838
The trouble with being a futurist is that when people get around to believing
you, it's too late. We lost. Google 2,000,000:Hams 0.

Homemade Antenna Tower
 

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.

Homemade Antenna Tower
 

Here's my first draft. - http://tekmanx.serveftp.com/~tekmanx/pole.gif

Homemade Antenna Tower
 

"Tekmanx" wrote in message
ups.com...
Here's my first draft. - http://tekmanx.serveftp.com/~tekmanx/pole.gif


Hi Tek

Your ambition impresses me. 40 feet seems awful high unless those
support legs are anchored well.
I have a little WiFi site that uses surplus satellite TV dishes with
BiQuad illuminators. I have chosen to locate the Access Point at the
transmitting antenna in an effort to minimize the loss to the coax to the
computer. I have a Bridge at the receiving computer end. CAT-5 cable
takes the signals to the computers.

Jerry

Homemade Antenna Tower
 

Hi Amos

From memory the Cisco 5.6GHz box had a path prediction sensitivy number
of about -87dBm. ie full speed with that input to the RX. I suspect that
at a lower data rates and extra 10dB might be a good number to use. I
have seen that larger microwave system manufacturers do publish their
specs so look it up for the equipment that is going to be used.

FM rcvs on 2M can often go to about -117dBm. An SSB rcvr on 2m with a
preamp hits thermal noise at about -141dBm. This also needs a good human
ear.

Cheers Bob VK2YQA

Amos Keag wrote:
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.

Homemade Antenna Tower
 

Well I plan to have a 5-6' squared concrete slab inwhich the tripod
legs will be bolted down to. The access point will sit in a outdoor
enclosure prolly 2-3feet away from the antenna on the pole. The inner
tubing will come down for easy access making the total height of the
tower 30ft. There's just soo much variables involved that determine if
this thing works properly.. so I'm planning to overshoot in hopes that
I will get it just right.

Here's the access point :
http://www.wisp-router.com/product_i...02459103e1168b

And the radio:
http://www.wisp-router.com/product_i...roducts_id=399

Plan to use a based OS on this setup.

Homemade Antenna Tower
 

In article ,
Geoffrey S. Mendelson wrote:

Dave Platt wrote:
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.

It's important to point out that using these extreme high gain antennas
with out a license is illegal in the U.S. The guy that invented the
"pringles can" antenna was an FBI agent so he was not prosecuted, but
if he had been an average citizen the FCC would have come after him.

Not strictly true, although your point is well taken.

It's true that WiFi equipment must be tested and certificated as a
system. Using an off-the-shelf gain antenna, cabled to an
off-the-shelf WiFi AP/card, will almost certainly void the
certification, and then using the device becomes technically illegal
under Part 15, and unless you have a license for another service which
allows it (e.g. Part 97 ham license) you could be cited for it.

There are commercial WiFi radios whose manufacturers have tested and
certificated them with such high-gain antenna systems, specifically
for point-to-point connections. If you buy one of their radios and
one of their antennas, you can use 'em within the Part 15 rules, and
you'll be fully legal.

It's not so much the antenna itself... it's the certification status
of the antenna/radio system, as well as the EIRP.

Then the question becomes which if any of the 14 WiFi channels is
actually in the 2.4gHz ham band.

Check the ARRL's "multi-media wireless" interest group pages for
details on this. My recollection is that there are one or two 802.11b
channels whose power spectra fall within the ham-band 2.4-gig
allocation and also fall outside of the ham 2.4-gig "weak signal"
bandplan segments.

Here in Israel it's even worse. WiFi and terrestrial 2.4gHz ham activity
is limited to 100mw EIRP. If you use a gain antenna, you must reduce
the transmitter power proportionaly.

There's a similar proviso here in the U.S., but for point-to-point
links it's not as severe as that. If I recall properly, for a
point-to-point link, once you exceed 1 watt EIRP, you have to subtract
one dB of transmitter power for each additional 3 dB of antenna gain
you throw into the equation.

--
pDave 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!

Homemade Antenna Tower
 

Uh.. I'm not in the US. I'm in the Bahamas. Totally different when it
comes to this stuff :D

Homemade Antenna Tower
 

Techmanx wrote:
"How high should my tower be for my vertical sector with antenna?"

Assuming line-of-sight is needed, the distance in miles to the horizon
over smooth earth is: the square root of twice the antenna slevation in
feet. You may want to add to the calculated height to allow first
Fresnel zone clearance at the path grazing point.

Best regards, Richard Harrison, KB5WZI

Homemade Antenna Tower
 

Bob Bob wrote:
"Work out the path loss based on the basic distance formula and plug in
your power losses, and RX sensitivity to determine your margin and max
range."

Add in the antenna gains. You`ll need them to offset path and
transmission losses.

Margins involve choices. Performance and reliability depend on those
choices. I often choose to imagine the earth with only 2/3 its actual
diameter for my path profile. Then I use 0.6 of the first Fresnel zone
for added clearance at the path grazing point. You need 30 or 40 dB
excess signal into the receiver under normal conditions to make the
receiver very quiet and to allow margin for path fades if you need high
performance and freedom from fades.

Path loss is computed from frequency squared times distance squared with
constants as needed for the system you are working with. Someone has
usually worked up a chart of path loss versus distance for the frequency
you are working with. Every time distance doubles, loss increases about
6 dB.

Best regards, Richard Harrison. KB5WZI

Homemade Antenna Tower
 

Telemax wrote:
"And would you guys say my 400 mw radio is overkill for 4 - 10 mile
shot?"

Depends on the whole system. The gains and losses must be totalled to
check for satisfaction.

Industrial microwave I put in almost 50 years ago was limited by the
klystrons available at reasonable cost at the time. As I recall, the
power was 1-watt or +30 dBm. At 6 GHz, the wavelength is only about 1
inch, so plenty of antenna gain was readily available. 6-ft parabolas
were common then and may have been a good cost to benefit trade off.
Their gain was near 38 dB at 6 GHz. We tried to limit our path to about
25 miles for reliability and the cost of radio towers. Here is how our
path might add up:

Free space loss: -140 dB
Antenna Gain (2ea): + 77 dB
Transmit power: + 30 dBm
Misc. loss (2 ea): - 5 dB

Received carrier pwr: - 38 dBm

A receiver sensitivity
for a multichannel RCVR: -80 dBm
would give us a fade margin of: 42 dB.

How would your system add up?

Best regards, Richard Harrison, KB5WZI

Homemade Antenna Tower
 

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

Wasn't an article about long-range ham-band radio via 802.11 in the
Tennessee (?) Valley printed in QST a few months ago?

--
--Myron A. Calhoun.
Five boxes preserve our freedoms: soap, ballot, witness, jury, and cartridge
PhD EE (retired). "Barbershop" tenor. CDL(PTXS). W0PBV. (785) 539-4448
NRA Life Member and Certified Instructor (Home Firearm Safety, Rifle, Pistol)

Homemade Antenna Tower
 

Tekmanx wrote:
Here's my first draft. - http://tekmanx.serveftp.com/~tekmanx/pole.gif


40 feet should be reliable line of sight for 6 +/- miles assuming ground
base antenna [ 6 +/- feet] at receiving end.