1.3 Miles PtP Link - Seeking suggestions.

I've got a 1.3mi distance I'd like to do a PtP link with 2 WRT54GL's.
Its not exactly a pure LOS between the 2 locations, but the obstacles
are minimal.

Using a 2400MHz Tx frequency, the 1.31mi distance, along with a 200mW
Tx power, -80dBm Rx sensitivity, 1dB loss for connectors and cable,
and 14dB antenna gain on the Tx and Rx, I get a Theoretical System
Operating Margin of 23.5 dB.

This is using a helical antenna, chosen because it is of a circular
polarization and according to my research, the best for getting through
and around obstructions.

Does this configuration sound about right?

I travel and connect over those distances routinely
with nothing special, except effort and experience.
Easily doable..With higher gain antenna.."piece-of-
cake." Luck

"SM" <(E-Mail Removed)> hath wroth:

>I've got a 1.3mi distance I'd like to do a PtP link with 2 WRT54GL's.
>Its not exactly a pure LOS between the 2 locations, but the obstacles
>are minimal.
>
>Using a 2400MHz Tx frequency, the 1.31mi distance, along with a 200mW
>Tx power, -80dBm Rx sensitivity, 1dB loss for connectors and cable,
>and 14dB antenna gain on the Tx and Rx, I get a Theoretical System
>Operating Margin of 23.5 dB.
>
>This is using a helical antenna, chosen because it is of a circular
>polarization and according to my research, the best for getting through
>and around obstructions.
>
>Does this configuration sound about right?

Assuming your numbers are correct, a 23dB fade margin should work. You
didn't specify the connection speed that you were getting the -80dBm
Rx sensitivity so I'll guess you're going for about a 24Mbits/sec
connection. That might be a bit optimistic in the presense of any
interference. 1dB for connectors and cables seems a bit low unless
the WRT54GL is mounted directly on the back of the antenna. 200mw
will cook the WRT54GL eventually. I wouldn't go much over 100mw
(+20dBm) tx power. Your "minimal obstacles" are still obstacles and
undoubtably infringe on the Fresnel Zone. However, without knowing
their composition, I can't assign a loss value estimate.

I've had some experience with helicals and do not agree with your
assessment. They're great for eliminating odd number reflections and
multipath. They're also good for dealing with odd mounting and
pointing arrangements. However, they are not useful for "drilling"
through obstructions. Details please?

I'm too busy right now to run the numbers. See:
http://wireless.wikia.com/wiki/Wi-Fi#Link_Calculations
I'll run them later...

--
Jeff Liebermann (E-Mail Removed)
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558

Jeff Liebermann <(E-Mail Removed)> hath wroth:

>"SM" <(E-Mail Removed)> hath wroth:
>
>>I've got a 1.3mi distance I'd like to do a PtP link with 2 WRT54GL's.
>>Its not exactly a pure LOS between the 2 locations, but the obstacles
>>are minimal.
>>
>>Using a 2400MHz Tx frequency, the 1.31mi distance, along with a 200mW
>>Tx power, -80dBm Rx sensitivity, 1dB loss for connectors and cable,
>>and 14dB antenna gain on the Tx and Rx, I get a Theoretical System
>>Operating Margin of 23.5 dB.
>>
>>This is using a helical antenna, chosen because it is of a circular
>>polarization and according to my research, the best for getting through
>>and around obstructions.
>>
>>Does this configuration sound about right?
>
>Assuming your numbers are correct, a 23dB fade margin should work. You
>didn't specify the connection speed that you were getting the -80dBm
>Rx sensitivity so I'll guess you're going for about a 24Mbits/sec
>connection. That might be a bit optimistic in the presense of any
>interference. 1dB for connectors and cables seems a bit low unless
>the WRT54GL is mounted directly on the back of the antenna. 200mw
>will cook the WRT54GL eventually. I wouldn't go much over 100mw
>(+20dBm) tx power. Your "minimal obstacles" are still obstacles and
>undoubtably infringe on the Fresnel Zone. However, without knowing
>their composition, I can't assign a loss value estimate.
>
>I've had some experience with helicals and do not agree with your
>assessment. They're great for eliminating odd number reflections and
>multipath. They're also good for dealing with odd mounting and
>pointing arrangements. However, they are not useful for "drilling"
>through obstructions. Details please?
>
>I'm too busy right now to run the numbers. See:
> http://wireless.wikia.com/wiki/Wi-Fi#Link_Calculations
>I'll run them later...


TX power = +20dBm (100mw)
TX coax loss = -2dB
TX ant gain = +14dBi
Distance = 1.3 miles
RX ant gain = +14dBi
RX coax loss = -2dB
RX sens = -82dBm (at 18Mbits/sec)
Fade margin = unknown

Plugging into:
<http://www.terabeam.com/support/calculations/som.php>
yields a fade margin of 19.5dB. That's close enough to 20dB to be
workable.

The catch is that this is the *BEST* case estimate. It just doesn't
get any better than the above numbers, but always ends up worse.
Antennas don't quite have the advertised gain. Radios don't quite put
out the rated power or have the rated sensitivity. Interference
always causes problems. Of course, I added no consideration for
whatever obstructions you have in the line of sight.

Who's helical antennas were you considering?

--
Jeff Liebermann (E-Mail Removed)
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558

Jeff Liebermann wrote:
> Jeff Liebermann <(E-Mail Removed)> hath wroth:
>
> >"SM" <(E-Mail Removed)> hath wroth:
> >
> >>I've got a 1.3mi distance I'd like to do a PtP link with 2 WRT54GL's.
> >>Its not exactly a pure LOS between the 2 locations, but the obstacles
> >>are minimal.
> >>
> >>Using a 2400MHz Tx frequency, the 1.31mi distance, along with a 200mW
> >>Tx power, -80dBm Rx sensitivity, 1dB loss for connectors and cable,
> >>and 14dB antenna gain on the Tx and Rx, I get a Theoretical System
> >>Operating Margin of 23.5 dB.
> >>
> >>This is using a helical antenna, chosen because it is of a circular
> >>polarization and according to my research, the best for getting through
> >>and around obstructions.
> >>
> >>Does this configuration sound about right?
> >
> >Assuming your numbers are correct, a 23dB fade margin should work. You
> >didn't specify the connection speed that you were getting the -80dBm
> >Rx sensitivity so I'll guess you're going for about a 24Mbits/sec
> >connection. That might be a bit optimistic in the presense of any
> >interference. 1dB for connectors and cables seems a bit low unless
> >the WRT54GL is mounted directly on the back of the antenna. 200mw
> >will cook the WRT54GL eventually. I wouldn't go much over 100mw
> >(+20dBm) tx power. Your "minimal obstacles" are still obstacles and
> >undoubtably infringe on the Fresnel Zone. However, without knowing
> >their composition, I can't assign a loss value estimate.
> >
> >I've had some experience with helicals and do not agree with your
> >assessment. They're great for eliminating odd number reflections and
> >multipath. They're also good for dealing with odd mounting and
> >pointing arrangements. However, they are not useful for "drilling"
> >through obstructions. Details please?
> >
> >I'm too busy right now to run the numbers. See:
> > http://wireless.wikia.com/wiki/Wi-Fi#Link_Calculations
> >I'll run them later...
>
>
> TX power = +20dBm (100mw)
> TX coax loss = -2dB
> TX ant gain = +14dBi
> Distance = 1.3 miles
> RX ant gain = +14dBi
> RX coax loss = -2dB
> RX sens = -82dBm (at 18Mbits/sec)
> Fade margin = unknown
>
> Plugging into:
> <http://www.terabeam.com/support/calculations/som.php>
> yields a fade margin of 19.5dB. That's close enough to 20dB to be
> workable.
>
> The catch is that this is the *BEST* case estimate. It just doesn't
> get any better than the above numbers, but always ends up worse.
> Antennas don't quite have the advertised gain. Radios don't quite put
> out the rated power or have the rated sensitivity. Interference
> always causes problems. Of course, I added no consideration for
> whatever obstructions you have in the line of sight.
>
> Who's helical antennas were you considering?

The intent is to setup for 802.11b, shooting for 11 Mbps. I couldn't
find a manufacturer's published spec for Rx sensitivity, but research
at
http://www.seattlewireless.net/LinksysWrt54g found that the WRT54GL had
an Rx sensitivity of -80db in 802.11b mode (-65db in 802.11g)

Next intent was to rig the WRT54GL for outdoor habitation - use POE -
and have it live at the antenna height thereby reducing many feet of
cable line-loss.

All obstacles from point "A" to point "B" are trees and residential
houses of roughly the same size and shape. (See attached photo.) And
this is why I 'm asking for input. As point "B" is about 30' higher
than point "A" with no rise in-between, I hoped using a 14db helical
antenna from 50' mast at Point "A" to roof-top at point "B" would get
me
past all the odd shapes.

Or should I just use a 23db parabolic antennas with vertical
polarization (this would give me a 37.5db fade margin)? Would this
amount of fade margin get me past the various trees and roofs, or must
I
install masts at both ends?

Will an increased fade margin help me with my obstacles, or will the
only thing that helps me is height?

If an increased fade margin does help, I'm thinking of going with
parabolics of 23db gain.

The antennas are made by Wade here in Canada. See below to a link to
the specs.

http://www.wade-antenna.com/Wade/Helical.PDF

http://www.wade-antenna.com/Wade/Parabolic.PDF

Thanks.

SM wrote:
> Jeff Liebermann wrote:
> > Jeff Liebermann <(E-Mail Removed)> hath wroth:
> >
> > >"SM" <(E-Mail Removed)> hath wroth:
> > >
> > >>I've got a 1.3mi distance I'd like to do a PtP link with 2 WRT54GL's.
> > >>Its not exactly a pure LOS between the 2 locations, but the obstacles
> > >>are minimal.
> > >>
> > >>Using a 2400MHz Tx frequency, the 1.31mi distance, along with a 200mW
> > >>Tx power, -80dBm Rx sensitivity, 1dB loss for connectors and cable,
> > >>and 14dB antenna gain on the Tx and Rx, I get a Theoretical System
> > >>Operating Margin of 23.5 dB.
> > >>
> > >>This is using a helical antenna, chosen because it is of a circular
> > >>polarization and according to my research, the best for getting through
> > >>and around obstructions.
> > >>
> > >>Does this configuration sound about right?
> > >
> > >Assuming your numbers are correct, a 23dB fade margin should work. You
> > >didn't specify the connection speed that you were getting the -80dBm
> > >Rx sensitivity so I'll guess you're going for about a 24Mbits/sec
> > >connection. That might be a bit optimistic in the presense of any
> > >interference. 1dB for connectors and cables seems a bit low unless
> > >the WRT54GL is mounted directly on the back of the antenna. 200mw
> > >will cook the WRT54GL eventually. I wouldn't go much over 100mw
> > >(+20dBm) tx power. Your "minimal obstacles" are still obstacles and
> > >undoubtably infringe on the Fresnel Zone. However, without knowing
> > >their composition, I can't assign a loss value estimate.
> > >
> > >I've had some experience with helicals and do not agree with your
> > >assessment. They're great for eliminating odd number reflections and
> > >multipath. They're also good for dealing with odd mounting and
> > >pointing arrangements. However, they are not useful for "drilling"
> > >through obstructions. Details please?
> > >
> > >I'm too busy right now to run the numbers. See:
> > > http://wireless.wikia.com/wiki/Wi-Fi#Link_Calculations
> > >I'll run them later...
> >
> >
> > TX power = +20dBm (100mw)
> > TX coax loss = -2dB
> > TX ant gain = +14dBi
> > Distance = 1.3 miles
> > RX ant gain = +14dBi
> > RX coax loss = -2dB
> > RX sens = -82dBm (at 18Mbits/sec)
> > Fade margin = unknown
> >
> > Plugging into:
> > <http://www.terabeam.com/support/calculations/som.php>
> > yields a fade margin of 19.5dB. That's close enough to 20dB to be
> > workable.
> >
> > The catch is that this is the *BEST* case estimate. It just doesn't
> > get any better than the above numbers, but always ends up worse.
> > Antennas don't quite have the advertised gain. Radios don't quite put
> > out the rated power or have the rated sensitivity. Interference
> > always causes problems. Of course, I added no consideration for
> > whatever obstructions you have in the line of sight.
> >
> > Who's helical antennas were you considering?
>
> The intent is to setup for 802.11b, shooting for 11 Mbps. I couldn't
> find a manufacturer's published spec for Rx sensitivity, but research
> at
> http://www.seattlewireless.net/LinksysWrt54g found that the WRT54GL had
> an Rx sensitivity of -80db in 802.11b mode (-65db in 802.11g)
>
> Next intent was to rig the WRT54GL for outdoor habitation - use POE -
> and have it live at the antenna height thereby reducing many feet of
> cable line-loss.
>
> All obstacles from point "A" to point "B" are trees and residential
> houses of roughly the same size and shape. (See attached photo.) And
> this is why I 'm asking for input. As point "B" is about 30' higher
> than point "A" with no rise in-between, I hoped using a 14db helical
> antenna from 50' mast at Point "A" to roof-top at point "B" would get
> me
> past all the odd shapes.
>
> Or should I just use a 23db parabolic antennas with vertical
> polarization (this would give me a 37.5db fade margin)? Would this
> amount of fade margin get me past the various trees and roofs, or must
> I
> install masts at both ends?
>
> Will an increased fade margin help me with my obstacles, or will the
> only thing that helps me is height?
>
> If an increased fade margin does help, I'm thinking of going with
> parabolics of 23db gain.
>
> The antennas are made by Wade here in Canada. See below to a link to
> the specs.
>
> http://www.wade-antenna.com/Wade/Helical.PDF
>
> http://www.wade-antenna.com/Wade/Parabolic.PDF
>
> Thanks.

Forgot the link to the picture between location A and B.

Its here:
http://i40.photobucket.com/albums/e2...reless/A-B.jpg

Thanks.

SM wrote:
> Will an increased fade margin help me with my obstacles, or will the
> only thing that helps me is height?

That depends how you phrase it.

Increasing you acceptable fade margin means you are setting your
expectations for reliability higher.

If you design you system with a higher allowable fade margin, it will work
under worsening conditions. If you allow for only a slight fade margin, it
may fall under adverse conditions.

Increasing the antenna heights will help you clear the obstacles better,
but the actually hight may have to be varied at one end for optimum signal
as you will find peaks and nulls up and down the mast.

[snipped from a previous post of mine]

decaturtxcowboy wrote:
> Jeff Liebermann wrote:
>> On Sat, 18 Nov 2006 01:01:17 GMT, decaturtxcowboy
>> <nope_none_@nowayspam.com> wrote:
>>
>>> lk wrote:
>>>> For distances like that you will need point to point Microwave
>>>> transcievers, not 802.11 access point hardware.
>>> For 2 KM? That's only just over a mile. Off the shelf WiFi gear
>>> will work just fine as long as you don't have obstructions. Two
>>> antennas at 20 feet should give you .3 Fresnel factor (70%
>>> encroachment of the path midway) with only about five foot
>>> variations in the ground elevations. Around 25-30 dB link budget,
>>> so with a 15 dB fade margin, you'll have room to play with.
>>> Most likely you d want to run 802.11g 54 Mbps gear for the
>>> OFDM and turn it down to only 6 Mbps.
>>
>> You left out the antenna gains.
>>
>> Let's do the numbers my way. See:
>> | http://wireless.wikia.com/wiki/Wi-Fi#Link_Calculations
>>
>> At typical 802.11g system, with radios mounted near the antennas and
>> using panel or dish antennas:
>> TX power = +15dBm
>> TX coax loss = 2dB (some LMR240 plus connectors)
>> TX ant gain = +17dBi (typical panel or small dish)
>> Distance = 2km (1.25 miles)
>> RX ant gain = +17dBi (typical panel or small dish)
>> RX coax loss = 2dB (some LMR240 plus connectors)
>> RX sens = -84dBm (at 12Mbits/sec OFDM)
>> Fade margin = unknown
>
> Here's my empirical (real life, non-calculated) measurements.
> I have two 60 ft. towers 2.12 KM (1.32 miles) apart. Test gear
> can be located at any height on the tower.
>
> TX power = +17dBm
> TX coax loss = 1.2dB (measured with wattmeter)
> TX ant gain = +15dBi (omni)
> Distance = 2.1 km (1.3 miles)
> RX ant gain = +18dBi (patch antenna)
> RX coax loss = 0dB (built in antenna)
> RX sens = -88dBm (at 6Mbits/sec OFDM)
> Fade margin = -15dB
>
>> As for Fresnel Zone, it needs 4.8 meters clearance at midpoint, which
>> sets the minimum mounting height of the antennas.
>
> AP SU RX sig. 1st Fresnel zone
> level Clearance
> 10' 10' 22.3dB .4 (60% encroachment)
> 12' 12' 24.9 .4
> 14' 14' 27.2 .5
> 16' 16' 29.4 .6
> 18' 18' 31.5 .7
> 20' 20' 32.7 .7
> 22' 22' 32.3 .8
> 24' 34' 32.4 .9
> 26' 26' 32.5 1.0
>
>
> AP SU RX sig. 1st Fresnel zone
> level Clearance
> 26' 10' 30.7dB .6 (40% encroachment)
> 26' 12' 31.4 .7
> 26' 14' 31.8 .7
> 26' 16' 32.1 .8
> 26' 18' 32.2 .8
> 26' 20' 32.4 .9
> 26' 22' 32.4 .9
> 26' 24' 32.5 .9
> 26' 26' 32.5 1.0
>
> Further more, just because you have clear LOS and
> clear the first Fresnel zone, you will hit null spots
> at certain SU antenna heights. Notice the drop in
> signal at 28 feet and the peaks at 10-20 feet and
> again at 46'.
>
> AP SU RX sig. 1st Fresnel zone
> level Clearance
> 50' 10' 32.3dB .8 (40% encroachment)
> 50' 12' 32.5 .9
> 50' 14' 32.5 1.0
> 50' 16' 32.1 1.1
> 50' 18' 32.4 1.1
> 50' 20' 32.1 1.2
> 50' 22' 31.5 1.2
> 50' 24' 29.5 1.3
> 50' 26' 24.4 1.4
> 50' 28' 7.9 1.4
> 50' 30' 21.4 1.5
> 50' 32' 28.1 1.5
> 50' 34' 31.6 1.6
> 50' 36' 32.0 1.6
> 50' 38' 32.3 1.6
> 50' 40' 32.5 1.7
> 50' 42' 32.5 1.7
> 50' 44' 32.5 1.7
> 50' 46' 34.4 1.8
> 50' 48' 32.1 1.8
> 50' 50' 31.6 1.9
>
> I can place two SU units almost the exact same distance
> from the AP tower, and both at same elevation and only
> a thousand feet apart, but with slightly different
> interfering terrain figures. One SU will work great at
> 40 feet and nearly dead at 32 feet, but gets better at
> 26 feet. The other SU was optimum at 22 feet.
>
> Information gathered from several SUs around -
> * Seasonal variations and time of day can vary signal as
> much as 10 dB from what I have noticed. Possible factors
> are morning dew on the leaves, temperature inversions
> from sun warmed earth on a cool morning, warm sundown
> evening with suddenly cooling in low lying areas.
>
> * Some locations never seem to have any significant fading
> under the same variations.
>
> * ODFM appears more robust and works with marginal paths where
> DQPSK and DBPSK were unusable.

"SM" <(E-Mail Removed)> hath wroth:

>The intent is to setup for 802.11b, shooting for 11 Mbps. I couldn't
>find a manufacturer's published spec for Rx sensitivity, but research
>at
>http://www.seattlewireless.net/LinksysWrt54g found that the WRT54GL had
>an Rx sensitivity of -80db in 802.11b mode (-65db in 802.11g)

Those numbers are garbage. Sensitivity varies with connection speed.
See table at:
<http://wireless.wikia.com/wiki/Wi-Fi#Link_Calculations>
These are for a DLink DI-624, but are close enough for ballpark
calculations. At 11Mbits/sec, use -82dBm.

However, I wouldn't use 802.11b modes. 802.11g is more effective in
the presense of reflections and interference. It's also more
sensitive for the same speeds. Try a 12Mbits/sec OFDM connection for
a sensitivity of -84dBm.

>Next intent was to rig the WRT54GL for outdoor habitation - use POE -
>and have it live at the antenna height thereby reducing many feet of
>cable line-loss.

Good idea, but there are some serious environmental concerns with
outdoor repackaging. Keeping the moisture off the circuit board is
always a problem. Lots of RF connectors. However, it's been done
hundreds of times, so it will work.

>All obstacles from point "A" to point "B" are trees and residential
>houses of roughly the same size and shape. (See attached photo.) And
>this is why I 'm asking for input. As point "B" is about 30' higher
>than point "A" with no rise in-between, I hoped using a 14db helical
>antenna from 50' mast at Point "A" to roof-top at point "B" would get
>me
>past all the odd shapes.

Ummmm.... I can see you've never tried to install a 50ft mast. It's
not easy. The common Radio Shack 30ft push up mast is also a
challenge. Finding points to anchor the guy wires is the major
headache. Aligning the antenna 50ft in the air is VERY difficult. A
24dBi dish has a -3dB beamwidth of about 10 degrees. That means you
have to align the antenna to within about +/- 4 degrees. Mark a 10
degree angle on a piece of paper and visually see how well you can
align it. The pole has to be adjusted and aligned vertically to the
same accuracy as the antenna.

>Or should I just use a 23db parabolic antennas with vertical
>polarization (this would give me a 37.5db fade margin)? Would this
>amount of fade margin get me past the various trees and roofs, or must
>I
>install masts at both ends?

Basically, you're asking if it's better to use high gain at the
rooftop level and try to drill through the obstructions, or if it's
better to use a 50ft pole and a lower gain antenna. The 50ft pole
with a lower gain (10 to 12dBi) antenna will probably work best.
However, I question your ability to actually install a 50ft antenna
mast.

Unfortunately, the picture you posted is useless:
<http://i40.photobucket.com/albums/e231/905shmick/wireless/A-B.jpg>
It doesn't show any of the obstructions or Fresnel Zone clearances.
What's needed is a side view picture, from ground level, showing the
endpoints of the link. That's often difficult to do if there's no
convenient viewing point. If you can't do that, two photos, along the
line of sight, with the target marked, will suffice.

Looking at the photo and from your description, it appears that you're
barely skimming across the rooftops with several large trees in the
way. First, the Fresnel Zone needs to be considered. At 1.3 miles,
that's about 22ft at 80% clearance at midpoint. I don't think you
have 22ft of ground clearance, much less building clearance. This is
not going to work too well without clearance.

>Will an increased fade margin help me with my obstacles, or will the
>only thing that helps me is height?

Obstacles create both attenuation and reflections. An increase in
fade margin will certainly help with attenuation. However,
reflections are a bigger problem. Position the antennas in a null,
and the signal will completely disappear. Worse, it will not stay
put. If something moves along the path, then the reflections will
change and the null might get better, but more probably worse. It's
really a difficult call. For links without spacial diversity, I move
the antenna up and down the tower until I find a location that doesn't
seem to be anywhere near a null and hope for the best. That's not a
great way to do it, but I usually don't have much of a choice. In
your case, I don't know what to suggest.

>If an increased fade margin does help, I'm thinking of going with
>parabolics of 23db gain.
>
>The antennas are made by Wade here in Canada. See below to a link to
>the specs.
>http://www.wade-antenna.com/Wade/Helical.PDF

I wonder how well it works with a bird sitting on the helical? 14dBi
gain with 36 degree beamwidth. That should be fairly easy to aim.

>http://www.wade-antenna.com/Wade/Parabolic.PDF

Dish 23dBi with 11 degree beamwidth. 3 times as difficult to aim but
not impossible. I don't think you can aim it when mounted on top of a
50ft pole. A climbable tower would be no problem, but not a pole.
Incidentally, we have a local CATV cooperative that has a 2.4GHz 24dBi
dish on top of what I guess is a 60ft pipe. It's fun to watch it
swing around in the wind. It works only because they have 40dB of
fade margin.

I'm not sure what to suggest. If you can borrow some directional
antennas and actually try the path, it might give you a clue if you're
close. I use a 25ft fiberglass window washing pole for "site surveys"
which really is the same "try it first" type of test. Throw something
together temporarily and see if there's hope. If it looks good, build
something permanent. If not, try something different.

Incidentally, a big problem with high gain antennas is picking up
interference from other systems along the line of sight. I don't mean
just along the 1.3 mile path, but also well beyond the intended
target. The high gain antennas will make any interference much worse.
They do eliminate interference sources to the sides of the antenna
pattern, but greatly increase interference inside the gain pattern.
When you're testing for a useable path, also use a sniffer (Kismet) to
test for other Wi-Fi systems along the path.

--
Jeff Liebermann (E-Mail Removed)
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558

Jeff Liebermann wrote:
> "SM" <(E-Mail Removed)> hath wroth:
>
> >The intent is to setup for 802.11b, shooting for 11 Mbps. I couldn't
> >find a manufacturer's published spec for Rx sensitivity, but research
> >at
> >http://www.seattlewireless.net/LinksysWrt54g found that the WRT54GL had
> >an Rx sensitivity of -80db in 802.11b mode (-65db in 802.11g)
>
> Those numbers are garbage. Sensitivity varies with connection speed.
> See table at:
> <http://wireless.wikia.com/wiki/Wi-Fi#Link_Calculations>
> These are for a DLink DI-624, but are close enough for ballpark
> calculations. At 11Mbits/sec, use -82dBm.
>
> However, I wouldn't use 802.11b modes. 802.11g is more effective in
> the presense of reflections and interference. It's also more
> sensitive for the same speeds. Try a 12Mbits/sec OFDM connection for
> a sensitivity of -84dBm.
>
> >Next intent was to rig the WRT54GL for outdoor habitation - use POE -
> >and have it live at the antenna height thereby reducing many feet of
> >cable line-loss.
>
> Good idea, but there are some serious environmental concerns with
> outdoor repackaging. Keeping the moisture off the circuit board is
> always a problem. Lots of RF connectors. However, it's been done
> hundreds of times, so it will work.
>
> >All obstacles from point "A" to point "B" are trees and residential
> >houses of roughly the same size and shape. (See attached photo.) And
> >this is why I 'm asking for input. As point "B" is about 30' higher
> >than point "A" with no rise in-between, I hoped using a 14db helical
> >antenna from 50' mast at Point "A" to roof-top at point "B" would get
> >me
> >past all the odd shapes.
>
> Ummmm.... I can see you've never tried to install a 50ft mast. It's
> not easy. The common Radio Shack 30ft push up mast is also a
> challenge. Finding points to anchor the guy wires is the major
> headache. Aligning the antenna 50ft in the air is VERY difficult. A
> 24dBi dish has a -3dB beamwidth of about 10 degrees. That means you
> have to align the antenna to within about +/- 4 degrees. Mark a 10
> degree angle on a piece of paper and visually see how well you can
> align it. The pole has to be adjusted and aligned vertically to the
> same accuracy as the antenna.
>
> >Or should I just use a 23db parabolic antennas with vertical
> >polarization (this would give me a 37.5db fade margin)? Would this
> >amount of fade margin get me past the various trees and roofs, or must
> >I
> >install masts at both ends?
>
> Basically, you're asking if it's better to use high gain at the
> rooftop level and try to drill through the obstructions, or if it's
> better to use a 50ft pole and a lower gain antenna. The 50ft pole
> with a lower gain (10 to 12dBi) antenna will probably work best.
> However, I question your ability to actually install a 50ft antenna
> mast.
>
> Unfortunately, the picture you posted is useless:
> <http://i40.photobucket.com/albums/e231/905shmick/wireless/A-B.jpg>
> It doesn't show any of the obstructions or Fresnel Zone clearances.
> What's needed is a side view picture, from ground level, showing the
> endpoints of the link. That's often difficult to do if there's no
> convenient viewing point. If you can't do that, two photos, along the
> line of sight, with the target marked, will suffice.
>
> Looking at the photo and from your description, it appears that you're
> barely skimming across the rooftops with several large trees in the
> way. First, the Fresnel Zone needs to be considered. At 1.3 miles,
> that's about 22ft at 80% clearance at midpoint. I don't think you
> have 22ft of ground clearance, much less building clearance. This is
> not going to work too well without clearance.
>
> >Will an increased fade margin help me with my obstacles, or will the
> >only thing that helps me is height?
>
> Obstacles create both attenuation and reflections. An increase in
> fade margin will certainly help with attenuation. However,
> reflections are a bigger problem. Position the antennas in a null,
> and the signal will completely disappear. Worse, it will not stay
> put. If something moves along the path, then the reflections will
> change and the null might get better, but more probably worse. It's
> really a difficult call. For links without spacial diversity, I move
> the antenna up and down the tower until I find a location that doesn't
> seem to be anywhere near a null and hope for the best. That's not a
> great way to do it, but I usually don't have much of a choice. In
> your case, I don't know what to suggest.
>
> >If an increased fade margin does help, I'm thinking of going with
> >parabolics of 23db gain.
> >
> >The antennas are made by Wade here in Canada. See below to a link to
> >the specs.
> >http://www.wade-antenna.com/Wade/Helical.PDF
>
> I wonder how well it works with a bird sitting on the helical? 14dBi
> gain with 36 degree beamwidth. That should be fairly easy to aim.
>
> >http://www.wade-antenna.com/Wade/Parabolic.PDF
>
> Dish 23dBi with 11 degree beamwidth. 3 times as difficult to aim but
> not impossible. I don't think you can aim it when mounted on top of a
> 50ft pole. A climbable tower would be no problem, but not a pole.
> Incidentally, we have a local CATV cooperative that has a 2.4GHz 24dBi
> dish on top of what I guess is a 60ft pipe. It's fun to watch it
> swing around in the wind. It works only because they have 40dB of
> fade margin.
>
> I'm not sure what to suggest. If you can borrow some directional
> antennas and actually try the path, it might give you a clue if you're
> close. I use a 25ft fiberglass window washing pole for "site surveys"
> which really is the same "try it first" type of test. Throw something
> together temporarily and see if there's hope. If it looks good, build
> something permanent. If not, try something different.
>
> Incidentally, a big problem with high gain antennas is picking up
> interference from other systems along the line of sight. I don't mean
> just along the 1.3 mile path, but also well beyond the intended
> target. The high gain antennas will make any interference much worse.
> They do eliminate interference sources to the sides of the antenna
> pattern, but greatly increase interference inside the gain pattern.
> When you're testing for a useable path, also use a sniffer (Kismet) to
> test for other Wi-Fi systems along the path.
>

Jeff,

Thanks for the info.

Would setting up for horizontal polarization with the big 23db
parabolic dish (presuming we can aim them properly) help at all with
the long-distance interference?

On 14 Dec 2006 13:04:57 -0800, "SM" <(E-Mail Removed)> wrote:

>Would setting up for horizontal polarization with the big 23db
>parabolic dish (presuming we can aim them properly) help at all with
>the long-distance interference?

That's been the topic of many a technical arguement. In theory, most
of the access points are vertically polarized. So, running your link
with horizontal polarization should see a considerable reduction in
interference. Unfortunately, reality doesn't follow theory. The
typical laptop is actually horizontally polarized. One a signal
bounces around a room, it can almost any polarization. Long ago, I
did a study which measure the polarization of 2.4GHz signals in
varying conditions. Close in, the source polarization was maintained.
Everything else was almost random. At this point, I would say that it
doesn't really matter, but it's probably a good idea to do it anyway.

Incidentally, one of the benifits of a helical antenna is that it is
insensitive to linear polarization (vert or horiz) with only a 3dB
loss in gain. That's handy when you're trying to track a moving
vehicle or deal with a reflection infested environment. However, it's
useless when you want to use polarization to minimize interference
pickup.

Incidentally, if you need some hint as to what it's like to aim a
dish, just grab a DBS satellite pizza dish and try to aim it at the
satellite. It doesn't need to be activated for this exercise. The
birds are 9 degrees apart in the sky and you have to be within about
+/- 2 degrees in order for it to work. It's fairly easy with an
inline signal strength meter ($10 almost anywhere). Once you get a
feel for what I mean as critical, then put the antenna on the end of a
10ft pole and try again to aim it. You'll find that it's not very
easy. If you have a 25ft fiberglass collapsable window washer pole,
try it again with a longer pole. The 24dBi dish is considerably
heavier and more cantilevered, but about half as critical as the pizza
dish. Still, it's quite a challenge to aim.

--
# Jeff Liebermann 150 Felker St #D Santa Cruz CA 95060
# 831-336-2558 (E-Mail Removed)
# http://802.11junk.com (E-Mail Removed)
# http://www.LearnByDestroying.com AE6KS