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Discussion Starter #1
Hopefully it's okay to ask the question here in this forum. Seems there is a lot of expertise in antennas here.

I want to connect my place - 15 floors up in an apartment to my friends house that is 1.65 miles apart - direct line of sight.

What would be the best antenna to do this, or is this even possible?

--- information that may be relevant --
I'll be using the Atheros CM9 wifi cards -
http://www.mini-box.com/s.nl/it.A/id.387/.f
http://resources.mini-box.com/online/AOC-MPCI-WI-CM9/specs.html

card has two antenna connections if needed. It can do 2.4 or 5.8 Ghz, 802.11/a/b/g

I don't know anything about antennas, I'm a computer programmer than can easily build linux kernels, drivers, etc. etc. I have two left over SBC's (Wrap 1C from PC Engines with these wifi cards) from a project I did some 4-5 years ago using nycwireless pebble to create free wifi access points. I'd like to connect my house to my friends house via wifi.

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Thanks for any help, if you have any other questions, feel free to ask.
 

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Im not sure how those cards connect up, but for an antenna, a bi-quad on an old small satellite dish will give you about 24 - 31 dbi, which should be plenty and is simple to build. Im very happy with my non dish biquad.

http://martybugs.net/wireless/biquad/
http://koti.mbnet.fi/zakifani/biquad/
http://www.trevormarshall.com/biquad.htm

Analysis of the biquad here : http://www.lecad.uni-lj.si/~leon/other/wlan/biquad/

For feeding a dish, you want to stick to the basic biquad, not the double biquad.
The biggest losses you face with wifi is the antenna coax cable, which can be horrific. The cable must be kept very short, about a couple of meters max.
2 x UFL Ultra-miniature coaxial connectors
This sounds very bad. Skinny coax has the highest losses.

Another option is to scrap those wifi cards and put a USB wifi adapter at the focal point of a dish. Then you can use slightly longer USB cabling.

WiFi distance record: 237 miles : http://www.engadget.com/2007/06/19/venezuelans-set-new-wifi-distance-record-237-miles/
 

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Discussion Starter #3
hmmm, no sense in using these boards if I have signal loss from the connector.

Here is a pic of single board computer with two wifi cards.



Each card has two ultra miniature connectors. The main antenna connector is in the corner if you are using is the one coax connection. I have mini cables that are about 11.5 inches long that break out the ultra miniature connector to an N-connector. If there is significant loss using this method then I'll have to look into a different wifi card - like the USB you mention that doesn't have these type of connectors.

Also, if I decide to go 802.11a at 5.8Ghz does the antenna scale linearly (can I change the dimensions to a fraction of 2.4/5.8 the lengths) or does the physical properties of the antenna change because of the higher frequency and at this point some sort of wave guide is better.
 

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Yeah, with those skinny cables, I wouldnt be surprised youre losing 2 dbi between the board and the connector pictured above. Is there a number on that cable ? Then we could look up the specs.

Also, if I decide to go 802.11a at 5.8Ghz does the antenna scale linearly (can I change the dimensions to a fraction of 2.4/5.8 the lengths) or does the physical properties of the antenna change because of the higher frequency and at this point some sort of wave guide is better.
The biquad does seem to scale nicely, but it would have to be modeled to make sure. Also, at 2.4 Ghz its a small antenna to start with. At 5.8 Ghz it would be about half as small, so harder to build and adjust in some ways. Plus the analysis shows its sensitive to wire size. At 5.8 Ghz, that would be very skinny wire and make it very delicate.

like the USB you mention that doesn't have these type of connectors.
Also, USB ones arent that pricey. Ive seen them at newegg recently for $10 - $12 on sale with free shipping. 7 ft of wifi cable with connector is that much,(and thats dirt cheap for it, heh.) 15 ft usb 2.0 extention cable is cheaper.
 

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Discussion Starter #5
Unfortunately no markings or numbers on the cables. I would assume if the cable isn't even marked it can't be that great in quality. I'm going to source a better wifi radio.

However, those plans look great for turning these single board computers into simple wireless access points for shorter distances. I can use the sheet metal the board is attached to as a reflector. If I can get better wifi coverage in my apartment that would be great too.
 

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I wouldnt be surprised youre losing 2 dbi
"dBi" refers to antenna gain relative to a theoretical single point antenna. When you're discussing gain & loss in amplifiers, cables, etc. you just use "dB", which expresses a ratio of output/input. Please note it is an upper case "B".
 

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However, those plans look great for turning these single board computers into simple wireless access points for shorter distances.
Yep. My little biquad with the 123mm reflector, 29mm lips, gets me solid, all bars connection. My garage computer, 200 ft away, downloads/uploads exactly as fast as my indoor computer which is connected to the router via cat 6 ethernet cable. Test download/upload speeds here : http://speedtest.net/
I can also pick up the neighbors wifi at over 400 ft with the signal going thru a foil wrapped house.

Cant ask for any better, heh. (and 10X better than the (freebie with router) 6dBi Dlink antenna that I sent back )
 

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suggesting 24dBi parabolic dish

You can get 24dBi parabolic dish antenna for 2.4GHz for a very cheap price, relatively speaking (of the gain). They look like a BBQ grill except it is curve in a parabolic shape, and has a dipole antenna sticking out. About $60. each, but sometimes you get bulk discount.

You should keep the thin cable, because you cannot attach thick cable to the card without damaging things. Just use as short as possible on the thin cable, and then connect to thick cable, low loss ones (like Times Microwave LMR-400), depending how long a run you need. The cable could cost more than the antenna, if it is a long run, plus the connector. Keep that in mind.

The advantage for a parabolic dish antenna is not only the gain, but the rejection of other signals from other locations not in the direct gain path (straight ahead) is much better than other antenna types. If the dish is too bulky for your installation, then try to use a multi-element yagi design, which is also directional.

The problem with WiFi is not just about the path loss, but there are many other signals on the same channel(s), causing interference, as there are only 11 channels used in North America, and those channels also overlapping each other (mutually exclusive only 1, 6, 11). So you want to have the least interference as possible, and a dish is the better method.

I bought mine from L-Com (actually Hyperlink which got bought out by L-Com) but there are many makers.
 

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Discussion Starter #10
That's why I'm considering going 802.11a which is at 5Ghz, to get around the 2.4GHz clutter.

Looks like I can get minipci wifi radios with MMCX connectors, the pigtails seem to be rated at 0.7db loss. Don't know if this is good or better.

Is there a legal limit to transmit power? I can get higher power radios rated between 500mw to 1000mw.

hmm... there is a measurement for transmit in dBm. One card that looks good is rated 26 dBm for 24Mbps.

Well, this is going to be a learning experience figuring out the right radio. Seems a parabolic antenna is the way to go for a tight beam between the two locations.
 

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Had 2.4 wireless Internet service for a few years here and the curved dish(barbecue look referenced above) We communicated across a lake at more than 4 miles and it was great service. I would use those antennas. The regular coax from the antenna to the PC was over 50 feet.
 

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That 50' would cost you a significant amount of signal, depending on cable type. With microwave it's all about path gain/loss. Modern industrial microwave links place the radio right at the dish and run ethernet down to the network.
 

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Sounds like you're technically inclined enough to design your own equipment, but for those of us who aren't, I've solved my range problems with these products: http://www.engeniustech.com/datacom/products/Category.aspx?id=17 from EnGenius. I use an outside omni-directional WAP to send the signal around our resort. It covers 2.5 Ha nicely, but they also make directional transmitters and receivers for your application.
 

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You can get 24dBi parabolic dish antenna for 2.4GHz for a very cheap price, relatively speaking (of the gain). They look like a BBQ grill except it is curve in a parabolic shape, and has a dipole antenna sticking out. About $60. each, but sometimes you get bulk discount.
Old Dish Network or PrimeStar etc dishes found along side the road or at flea markets are way cheaper, heh. LMR-400 is pricey stuff.
 

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Better than a USB-WiFi adapter is an Ethernet-Wi-Fi bridge. You can run ethernet as long as you need, and make a POE adapter to send power to the unit on the Ethernet cable. You can get such units made for outdoor use.
 

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An "ethernet-WiFi bridge" is precisely what many short haul microwave links are. Some even use unlicensed spectrum, just like WiFi. They also use POE.
 

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Discussion Starter #19
Understanding the math of tx/rx

Okay, so I've done some reading and I think I understand the math/terminology when it comes to radios/antenna. Here is what I think I understand.

Line of sight loss over the air

This is a basic formula that takes frequency and distance and returns a db value of the loss of power in an ideal environment. Doesn't account for any other losses or reflections.

http://en.wikipedia.org/wiki/Free-space_path_loss

Simple formula is

loss = 20 x log10 (1.65 miles) + 20 x log10 (2400Mhz) + 36.58

So my loss in transmitting wifi over this distance would be -108.53 db

Antenna gain/cable losses in transmitter/receiver

If I take my Antenna gain and subtract any losses from the cabling to get the total gain out of the antenna/cabling, I would add this to the line of sight loss. Assuming I can get 12db at the receiver and transmitter for a total of 24db, my loss now would be

-108.53db + 12db + 12db = -84.54db

Transmitter power

Radios have a transmit power measured in dBm.

http://en.wikipedia.org/wiki/DBm

So a 100mW radio would be 20 dBm, so I would add this to the loss

-84.54db + 20db = -64.54db

Receiver sensitivity

Receivers have a sensitivity measured in dBm as well. A typical wifi receiver is -60dBm to -80dBm. So technically a wifi receiver should be able to pick up the signal using this ideal scenario.

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Is my understanding basically correct? I know there should be some leeway to account for less than ideal conditions.

As well, wifi receivers report RSSI. I'm assuming this value is in dBm so I can get an idea of how well my gains/losses are (or how close I'm to getting no signal).

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Last question, how does signal to noise factor in all of this?
 

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Signal to noise ratio determines whether you'll receive the signal or not. If there too much noise, the signal will not be detected properly. In analog TV the result was snow. With digital, it means loss of signal. Please note it's the ratio that's important. It's possible to have a very strong signal, but if the noise is stronger your reception will still suffer.
 
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