Re: rooftop antenna range


Jeff Liebermann <jeffl@xxxxxxxxxxxxxxxxxxxxxx> writes:
On Tue, 30 May 2006 13:07:03 -0700, "Wolfgang S. Rupprecht"
<wolfgang+gnus20060530T123822@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxx> wrote:
Given two vanilla wifi radios

I prefer my radios chocolate flavored. If you insist on vanilla, at
least try chocolate chip flavored.

Me too. I was just going to talk about the vanilla though and
secretly add the chocolate when nobody was looking. ;-)

That's easy but depends on what speed you're expecting.

Ah, I should have mentioned. I'm not going for any speed record.
1Mbs is fine. I'm just curious if I can expect to hit a rooftop omni
from Mission Peak 4.3 miles away or get at it from a Fremont's Central
Park 1.5 miles away.

http://wireless.wikia.com/wiki/Wi-Fi#Link_Calculations

Great link. Thanks for all the information!

Ok, it looks like 6Mbps OFDM is roughly the same as 1Mbps BPSK.
Cranking it down to 1Mbps seems to buy me very little with that radio.

The trick is to get 20dB of fade margin. Grinding the numbers for a
pair of vanilla flavored wireless bridge radios.

TX power = +15dBm
TX coax loss = 2dB (1ft LMR-240 plus a mess of connectors)
TX ant gain = +15dBi
Distance = unknown
RX ant gain = +15dBi
RX coax loss = 2dB (same at other end)
RX sens = -84dBm (at 12Mbits/sec)
Fade margin = 20dB

Plugging into:
http://www.terabeam.com/support/calculations/som.php
I conveniently get 1 mile range. You can go somewhat farther with
bigger antennas or slower speed. For example, if you go to 24dBi dish
antennas as both ends, the calcs show that you can go 8 miles with
everything else being the same. As a rule of thumb, 6dB gain is worth
double the range. 12dB gain is 4 times the range. Etc.

I wouldn't have guessed that one wants to shoot for a 20db margin.

The cards I was really lusting after were these:

http://www.ubnt.com/super_range_cardbus.php4

Receive Sensitivity

Rate Level
1 Mbps -96 dBm, +/- 2dB
2 Mbps -95 dBm, +/- 2dB
5.5 Mbps -91 dBm, +/- 2dB
11 Mbps -91 dBm, +/- 2dB
6 Mbps -93 dBm, +/- 2dB
9 Mbps -92 dBm, +/- 2dB
11 Mbps -91 dBm, +/- 2dB
12 Mbps -90 dBm, +/- 2dB
18 Mbps -89 dBm, +/- 2dB
24 Mbps -85 dBm, +/- 2dB
36 Mbps -82 dBm, +/- 2dB
48 Mbps -76 dBm, +/- 2dB
54 Mbps -73 dBm, +/- 2dB

Transmit Power 802.11b/g

Rate Level
1-24 Mbps 24 dBm, +/- 1dB
36 Mbps 23 dBm, +/- 1dB
48 Mbps 22 dBm, +/- 1dB
54 Mbps 20 dBm, +/- 1dB

It looks like I could buy myself another 12db on the RX side (-96dbm
vs -84dbm) and another 9db on the tx side (24db vs 15db). That 21 db
should be good for 3.5 doublings in distance - call it

(* 2.0 2.0 2.0 1.414) 11.312 ; ~11x further

That's obstructed. You need *MORE* than line of sight at RF
frequencies. You need at least 0.8 times the Fresnel Zone diameter at
midpoint. See:
http://www.terabeam.com/support/calculations/fresnel-zone.php
For 1 mile, you need a radius of 19ft around the line of sight at
midpoint. For 8 miles, you need 53ft radius. Skimming the rooftops
might work if the roofs are wood or shingle. However, if there are
trees, masonry construction, or steel buildings, forget it.

The houses are mostly stucco over chicken-wire and concrete roof tiles
painted to look like clay.

So it looks like the lower half-cylinder of the fresnel zone will get
clobbered. Is this equivalent to half the signal getting lost or
would it be worse? If it is only half the signal getting lost, what's
another 3 db among friends?

Does anyone have any words of advice about rooftop wifi antenna
placement, anchoring, grounding etc?

Yes. However, it's difficult to give general advice. The basic
question is whether you can mount it on the building, on a bracket of
sorts, on a tubular pipe, or on a tower. Give me some idea of the
roof constuction and height involved and I can offer some hints.

Concrete roof tiles, stucco building, wide eves with a 2x12 beam
running along the end of the roof.

I've already got a weather station mounted on an steel antenna pole
thats lag-bolted to the 2x12 near the apex using 2 of those bent steel
legs. I think it was the same RCA badged tv-antenna mounting brackets
as in the following kit.

http://www.sjgreatdeals.com/rcavh120x.html

Because of the way the legs are bent, there is considerable flex. I
have no doubt that I could mount it within a few degrees of correct
and then just gently bend it until it was perfect. Of course that is
what worries me a bit too.

Omni's suck for point to point for exactly that reason. The vertical
radiation angle is far too narrow. Use a dish, sector antenna, or
panel.

I was actually going to use a lightweight 15dbi yagi for the laptop
antenna. Its only 18" long and weighs 3oz. I figured the weight and
size were ideal for laptop use.

http://www.mfjenterprises.com/products.php?prodid=MFJ-1800

Those area actually fairly rigid for an omni. The problem is getting
the mounting pipe exactly vertical. There's no easy adjustments once
the bolts go into the side of the house. That leaves shims and pipe
benders.

Sounds like I should probably just use the 4ft pipe attached to my
brackets.

Thanks again for the great info.

-wolfgang
--
Wolfgang S. Rupprecht http://www.wsrcc.com/wolfgang/
.

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