Long Distance Wireless Internet

On 14 Nov 2006 12:03:42 -0800, "BlackJackal" wrote in :

What exactly do you mean by "80% clear line of sight"? That 20% may or may not be a big deal.

Try a Linksys WRT54G and WAP54G (in client mode). If that doesn't work, go with some commercial gear and higher gain antennas.

"BlackJackal" hath wroth:

You don't have line of sight. At 2.4GHz, you need MORE than optical line of sight. You need to clear the Fresnel zone. See:

calcs and explanations. If there's any garbage within 80% of the Fresnel Zone, you will have signal loss and variations as thing move around.

You might want to measure the distance instead of ballpark guessing. At about 600ft, you'll need some outdoor antennas. See the FAQ at:

how to do the link and antenna size calculations. With junk in the Fresnel zone, I would just add some extra antenna gain to take care of any fades. Instead of a 20dB fade margin, try about 30dB instead. Ask if you need help.

I'll assume you want to transparently bridge the two networks so that they look like one big network. You need a "transparent wireless bridge" or just plain "wireless bridge". These are two identical wireless bridges the simply glue your two networks together. However, you need to disclose the number of MAC addresses at each site that will need to be bridged. Most of the cheapo, bottom of the line wireless devices have some real limits on the number of MAC addresses they will pass.

We also need to know what speed you're expecting. I suspect that sharing the T1 (1.544Mbits/sec) is not the only purpose of this link. My guess(tm) is that there will also be some connections between PC's, printers, and servers at the two locations. 1.5Mbits/sec isn't fast enough for that. What speed thruput were expecting for the link?

Maintaining a fresnel zone as computed on this website seems darn near impossible in most situations. Is this independent of polarization?

What frequency range is valid for this website?

snipped-for-privacy@sushi.com hath wroth:

It's mostly independent of polarization. By mostly, I mean that there are some differences. The effect of encroaching on the Fresnel Zone is that the object creates edge diffraction, which redirects the signal out of the line connecting the RF source and receiver. The degree of edge diffraction is polarization sensitive, where horizontal object tend to diffract a verically polarized signal more than horizontal. However, the effect is not huge and for initial calculations, it should be considered polarization independent.

It's NOT impossible for most situations. For example, at a range of

300ft (the alleged maximum range of conventional wireless), the 80% Fresnel Zone radius is only 4.4 ft. That's easily achievable.

Where it gets difficult is for long range links. For example, at 5 miles, the midspan clearance should be 42 ft. In effect, that means that both antennas at the link ends must be at least 42 ft off the ground level or the ground will end up inside the 80% Fresnel Zone. Actually, it's somewhat higher than 42 ft because of the curvature of the earth. Also note that this is only at the midpoint of the link. As you get closer to each end, the required clearance becomes less.

The Fresnel Zone varies with the inverse square of the frequency: Midpoint clearance in ft = 72.1 * sqrt(D/4f) D = distance in miles. f = frequency in GHz. It breaks down at extremely low frequencies due to the inability to find a sufficiently large clearance area. At the high end, it starts to involve optical effects (fringing, scatter, speckle, interference patterns, etc) at about 22GHz. I would guess(tm) that the simple approximation is valid from about 500MHz to 22GHz. It's also a far field calculation that will break down for short distances which involve near field calculations.

This online calculator seems a bit more useful for calculating required clearance for various objects along the path: |

I guess the effects of the fresnel zone are not very significant on simple NFM. I plugged in 440Mhz and came to the conclusion that simplex ham radio was impossible unless I and the persom I am communicating with take human growth hormone.

When I have run Splat!, the lower frequencies had better propagation due to diffraction. That is, radio waves exceed line of sight.

snipped-for-privacy@sushi.com hath wroth:

Apple and oranges. Actually, Fresnel Zone diffraction is very signifigant at proportionately longer ranges on VHF/UHF. The difference is that you can yack away on NBFM VHF/UHF with perhaps a

0dB SNR (signal to noise ratio). Try that with data communications running at megabit rates and nothing will work. High speed data requires more bandwidth and a much better SNR. Much of the common obstructions are fairly transparent at VHF/UHF and opaque at 2.4GHz. Lower frequencies have less path loss. Lots of other differnces making comparisons rather awkward. If you're dealing with long range VHF/UHF links, you most certainly will be dealing with the Fresnel Zone.

Yep. The radio horizon is well beyond the optical horizon. Put crudely, lower frequencies bend more easily. The ultimate form of bending is at the old Loran frequencies (about 100KHz) where the space between the ionosphere and the ground form two sides of a waveguide. You could go around the world several times without bouncing or diffraction.