Yagi question?

Yagi question?

Postby Voltron » Wed Oct 02, 2002 11:17 pm

What is the best way to position a yagi for driving? I can't find anything really on how there signal gain pattern is.
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Re: Yagi question?

Postby blackwave » Wed Oct 02, 2002 11:24 pm

Originally posted by Voltron
What is the best way to position a yagi for driving? I can't find anything really on how there signal gain pattern is.


Which directional do you have ? If it is a tube like antenna (for example a pringles can) you may prove to have best results with it pointing perpendicular to the vehicle in motion.


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Re: Yagi question?

Postby Thorn » Thu Oct 03, 2002 6:18 am

Originally posted by Voltron
What is the best way to position a yagi for driving? I can't find anything really on how there signal gain pattern is.


A yagi, due to the directional nature, isn't the best for driving. In fact, unless you are attempting to locate a specific AP, it's about the worst choice.

That said, most antenna manufacuters will specify the yagi's beamwidth in degrees. Also, most will actually have diagrams of the E-Plane and H-Plane radiation patterns. Look up the manufacturer's web site of your antenna, and see what is listed for that model. The diagrams should help show you how to aim it.

Look for something like this:
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Postby bigezy » Thu Oct 03, 2002 9:01 am

Hmmm. I don't want to start a Holy war, but the other night with this setup:

Laptop 1 NS 3.3, 5 dbi Omni, Orinoco Gold

Laptop2 NS 3.3, 15.5 db Yagi, Orinoco Gold


Netstumbler Results:

Laptop 1 : 1350 AP's
Laptop 2 : 1605 AP's

This would seem to be a contradiction to the presvious message, but, You do have to remember the Yagi is directional. The omni did pick up about 20 networks the Yagi did not find according to wigle.

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Postby Thorn » Thu Oct 03, 2002 9:29 am

Originally posted by bigezy
Hmmm. I don't want to start a Holy war, but the other night with this setup:

Laptop 1 NS 3.3, 5 dbi Omni, Orinoco Gold

Laptop2 NS 3.3, 15.5 db Yagi, Orinoco Gold


Netstumbler Results:

Laptop 1 : 1350 AP's
Laptop 2 : 1605 AP's

This would seem to be a contradiction to the presvious message, but, You do have to remember the Yagi is directional. The omni did pick up about 20 networks the Yagi did not find according to wigle.

bigezy



No holy war here. But I do know a bit about antennae. :)


The APs picked up by the omni will be much closer to the true location, than those that are seen by the Yagi. Those 20 seen by the omni, but not by the yagi were probably 180 degrees from the yagi's front.

Think of a yagi like a telescope: You'll see a narrow view of stars far away. You won't see the closer stuff off to the sides.
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Postby Thorn » Thu Oct 03, 2002 10:38 am

Maybe this will help, and be slightly clearer.

Here are two views of antenna patterns, a 5dB omni and a 15dB directional (yagi), using the same theoretical tx/rx point.

The top diagram shows the patterns from above, while the lower diagram shows the pattern from the side.

The black is the 5dB pattern, the blue is the 15dB pattern, and the red are APs that someone is trying to find.

AP1. Not detected. Within range of 15dB, but below signal area. Not detected by the 15dB, because it is almost 180 degrees opposite the directional's pattern. Out of range for the 5dB.
AP2. Detected by 5db. Within range of of both; but only within the the signal area of 5dB. The 15dB would not have detected it.
AP3. Not detected. Within range of 15dB, but above signal area. Out of the range of the 5dB.
AP4. Detected by 15dB. Barely within range.
AP5. Detected by 5dB. Not detected by the 15dB, because it is almost 180 degrees opposite the directional's pattern.
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Postby Voltron » Thu Oct 03, 2002 11:02 am

Thanks Thorn your images are very helpful. I may have to switch to the fab corp omni mag mount. Only thing I found good with my yagi was a orinoco AP with no wep using the default ssid and I stayed connected for a little over 2min. while driving.
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Postby Thorn » Thu Oct 03, 2002 11:14 am

You're welcome, Voltron.

And I hope you mean "detected" rather than "connected." Let's keep it legal. :)
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Postby Voltron » Thu Oct 03, 2002 11:20 am

Originally posted by Thorn
You're welcome, Voltron.

And I hope you mean "detected" rather than "connected." Let's keep it legal. :)


That I do. I was driving so I couldn't do anything. NS had it as an active AP for a little over 2min. I drive and check what I got when I get home.
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Postby astcell » Thu Oct 03, 2002 12:58 pm

So a spinning yagi would be best. Would it detect all the time while spinning or just when the light on the orinoco flashes?

And how would I spin the yagi and not end up with a bunch of coiled wires between me and the antenna?
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Postby blackwave » Thu Oct 03, 2002 1:01 pm

Originally posted by astcell
And how would I spin the yagi and not end up with a bunch of coiled wires between me and the antenna?


Hell, the military figured out how to do it with their radar systems... it can't be that hard.

Actually it would probably be best to get a high gain omni unless you just want to be selective in locating aps. The only bad thing is that you will end up having something like a 70 inch whip sticking out of the top of your car... hopefully you don't have an echo, or focus.. ;) otherwise you may as well put a big shiny penny on the back.
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Postby Thorn » Thu Oct 03, 2002 1:08 pm

Yeah, a radar set is essentially a spinning yadi. Microwave, even!

astcell, before you run over to the airport and start figuring out how to mount that 50' rotating dish on your car, search on "fox", "hound" or "foxhunt". Hams do this with some pretty neat directional gear. It's been discussed here before. It might give you some ideas.

As to a high-powered omni, don't forget the pattern gets increasingly flat, as the dB increases.
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Postby Voltron » Thu Oct 03, 2002 7:57 pm

Originally posted by astcell

And how would I spin the yagi and not end up with a bunch of coiled wires between me and the antenna?


If you figure that one out let me know also. Id like to see how that works in my car.
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Postby Barry » Wed Jan 01, 2003 1:12 pm

you could use one of the inclosed ones they use on boats, then it wouldn't be too conspicous. Just my completely uninformed 2 cents. :D
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Postby razta » Wed Jan 01, 2003 2:48 pm

Barry,
A simular setup would work great if the antenna was tuned for the 2.4GHz spectrum.

My 2 cents on the yagi vs omni is that most yagi's are not great directional antennas (IMO) due to thier transmitting in all directions. As you can see in Thron's picture, the yagi still has half its power out ~40 degrees, and about 1/4 power at 90 degree.

So you could see how it is possible for a yagi to pick up more due to its higher gain and wide beam width. IMO missing 20 networks compared to a omni is not much.

My point is, a yagi is not much better than a Omni except that an omni has more consitant output at 360 degrees. =)
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