Parabolic Dish Wi Fi Antenna

[POSTED TO alt.internet.wireless - REPLY ON USENET PLEASE] 24dBi for $80:

Hyperlink has a minimum order of $100, but that doesn't matter, because this antenna is priced over that.

has several parabolic antennas for $30-40.

What are you trying to do? 20dBi is a lot of antenna.

I worked out the numbers for a pizza dish illuminated by a USB dongle in the past. You don't get much gain because most of the RF radiated by the USB dongle goes in directions that will NOT hit the dish.

's also a section in that thread that discusses range calculations. The real problem with using a pizza dish with a hemispherical radiator (laptop antenna or USB dongle) is that the antenna gain will be radically different between transmit and receive. In receive, all of the signal that hits the dish will also hit the laptop antenna. However in transmit, most of the signal from the laptop antenna will go to all kinds of useless directions, with very little hitting the dish. Still, such an arrangement is probably better than just the laptop antenna and might be worth testing.

That really depends on what is at the other end. If the access point is not line of sight, the antenna is crude, and there are obstructions, forget it.

It would be no problem with external antennas and line of sight. My guess (I'm too lazy to run the numbers) is about 12dBi of gain in the antennas at both ends will be sufficient at the slowest 802.11g speeds. You can always trade range for speed, so I need to know your minimum acceptable thruput.

With a 14dbi antenna from fab-corp, I could communicate just about exactly

1km to a netgear wireless router without external antenna (at only about 2Mbps, though). I could probably have improved it a bit - the antenna was hand-held!

I can reliably get the same throughput over 1000' between a Linksys WRT54G and my Dell laptop - with no external antennas - provided I only have two windows in the way. However, as soon as I get a stud wall in the way, reception drops to zero. I suspect 500m to a laptop's internal antenna - using any size of antenna at the other end - is only going to work in exceptional circumstances.

On a related matter, what sort of range could be achieved by using a parabolic antenna of say 2ft diameter, at one end, with a conventional wifi antenna (say inside a laptop) at the other end?

Could 500m be done?

The purpose is to enable running a laptop in a shed about 500m from my house.

If not, could it be done with such an antenna at both ends? I do have some Cisco 350 wifi cards with external antenna (MMCX) sockets.

On Fri, 11 Nov 2005 08:53:03 +0000, Pete Jeff Liebermann wrote

The common "barbeque grill" Pacific Wireless dish antennas come in

15dBi, 19dBi, and 24dBi models with prices from $30-$70. At those prices, it's not worth building one.

The gain of the antenna has nothing to do with line of sight. If you don't have line of sight, no amount of gain will "drill through" the obstruction. In my opinion, line of sight is far more important than antenna gain. However, if the obstruction is porous, such as trees and bushes, antenna gain might be able to penetrate the foliage.

What really happens with antenna gain is that the "fade margin" increases with more gain. This means that the system will tolerate more reflections, multipath, obstructions, deterioration, water in the coax, and interference. 20dB fade margin is a starting number. Larger is better.

Increased directional antenna gain also implies a much narrower antenna -3dB beamwidth. That will keep out any co-channel interference but as you mention, does make antenna alignment an adventure. At 500m it should not be a problem. However at longer distances, alignment becomes critical with the big 24dBi dishes. At

5.6GHz with a 32dBi dish, it's really touchy.

Another problem with "too much gain" is overload. The dynamic range of the typical access point and client radio receivers is not very good. My guess(tm) is any signal over -20dBm will cause a problem. One has to be fairly close to get such a strong signal, but it can be done easily with two high gain antennas. The result is "envelope distortion" which mutilates the AM component of the 802.11b/g signal and causes the error rate to increase. This is why you don't want two

24dBi antennas pointed at each other in close proximity. Running the numbers: TX power: 17dBi TX coax loss: 3dB TX Ant gain: 24dBi Path loss: Unknown RX Ant gain: 24dBi RX coax loss: 3dB Signal at receiver: -20dBm Since bankrupt Proxim bought Terrabeam which bought YDI, my favorite simple online RF path loss calculator is now history, so let's do this one mostly by hand. 17 -3 +24 +24 -3 = +59dBm Therefore, the path loss to get -20dBm at the receiver is: 59 - (-20) = 79dB Plugging by trial and error to get 79dB into: a distance of 0.06 miles or 97 meters. Therefore, any such arrangement of two big dish antennas closer than 97 meters is going to cause overload problems at the receiver.

Note that the -20dBm overload point is my guess and will vary substantially by receiver design and chipset.

Jeff Liebermann wrote

I've seen 20dbi+ antennae so line of sight seems achievable. Presumably the drawback of "too much gain" is that it needs accurate alignment.