Yagi question?

Postby Thorn » Wed Jan 01, 2003 7:01 pm

Originally posted by razta
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. =)

razta,
A yagi is a directional. It is just the name for a particular kind of directional. They transmit and receive in one direction only. Omnis transmit and receive in 360 degrees.

Yagis are great directional antennae.

Please read through the text with the diagram again. It explains the two patterns.
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Postby razta » Wed Jan 01, 2003 7:17 pm

Not to start a bitch match, but a spectrum anaylzer can prove different.
I have had yagi's connected to Western Multiplex Lynx radios on a Point-to-point link wipe out two of my 802.11 towers. This PTP link was over a mile away and pointed aprox 60 degrees away from my towers.
My response is just based on my experience. I know Yagi's are designed as directional antenna, but they do bleed the other directions. It also depends on the quality of the antenna build. IMO, you usually get what you pay for. :)
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Postby db8tr » Wed Jan 01, 2003 9:53 pm

Originally posted by razta
I know Yagi's are designed as directional antenna, but they do bleed the other directions. It also depends on the quality of the antenna build. IMO, you usually get what you pay for. :)


I realize that there is some "bleeding", but as an experienced foxhunter (ham radio direction finding) I know that a properly constructed yagi is definately a directional antenna. However, due to the nature of the design, coupled with math I readily admit to not completely understanding, the dimensions of the reflector, driven element, and directors are never the same for both maximum front / back ratio and maximum forward gain. In other words there is not a yagi with a certain number of elements where there is a "perfect" length and spacing for the elements. One has to decide which quality they want. I would expect that for marketing purposes 2.4GHz antenna manufacturers want to have the maximum forward gain possible for a certain number of elements. Thus they compromise on the directionality of the design. However, there is still going to be a dramatic dropoff of signal anywhere away from the driven (designed) direction of the antenna.

What it comes down to is, I would be extremly surprised if, long term, a person using a yagi to netstumble found more APs than a person with an omni. The only situation where this likely would not be the case is when surveying a tall building. In that case the relatively flat pattern of the omni might not reach to the upper floors whereas a directional antenna, such as a yagi, could be pointed up towards the building.

Just throwing my $.02 into the fray...
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Postby razta » Thu Jan 02, 2003 5:19 am

What it comes down to is, I would be extremly surprised if, long term, a person using a yagi to netstumble found more APs than a person with an omni. The only situation where this likely would not be the case is when surveying a tall building. In that case the relatively flat pattern of the omni might not reach to the upper floors whereas a directional antenna, such as a yagi, could be pointed up towards the building.


I agree. =)
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Postby Thorn » Thu Jan 02, 2003 5:39 am

Originally posted by razta
Not to start a bitch match, but a spectrum anaylzer can prove different.
I have had yagi's connected to Western Multiplex Lynx radios on a Point-to-point link wipe out two of my 802.11 towers. This PTP link was over a mile away and pointed aprox 60 degrees away from my towers.
My response is just based on my experience. I know Yagi's are designed as directional antenna, but they do bleed the other directions. It also depends on the quality of the antenna build. IMO, you usually get what you pay for. :)

No bitchin' here. I was just afraid that the only exposure you had to yagis was my diagrams above. Obviously, if you can use a spectum analyzer, you're ahead of about 75% of the people here. I've never seen an antenna that didn't have some bleed to the back, but considering that this is the first time many of the people on these forums have ever even thought about RF stuff, I figure that discussing F/B ratios, etc. is a bit over their heads. Getting some to even look at E and H pattern diagrams has been a chore at times. They have to take baby steps first. :)
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Postby Slashhax0r » Sat Jan 04, 2003 7:25 pm

Originally posted by razta
Not to start a bitch match, but a spectrum anaylzer can prove different.
I have had yagi's connected to Western Multiplex Lynx radios on a Point-to-point link wipe out two of my 802.11 towers. This PTP link was over a mile away and pointed aprox 60 degrees away from my towers.
My response is just based on my experience. I know Yagi's are designed as directional antenna, but they do bleed the other directions. It also depends on the quality of the antenna build. IMO, you usually get what you pay for. :)


It depends on the yagi's build, you see.. the way a yagi is fed makes quite the difference, the yagi's i've built for 440 mhz actually work better with a folded dipole driven element. The beam width is liek 15 degrees, the only caveat is that the yagi doesn't really "fire" exactly along its axis, it's off slightly.

Using a gamma match, you get like a 30 degree beam width.. :)

Also, the best way to get straight forward (not along the sides) type of direction is build/get a dish. some of them are like 40db gain all pointing in one direction.


Hmm, come to think of it, for stumbling, I feel that just a couple whip antennas (one vert, one horizontal), would be fine, at least that way you are getting a realistic location of where the AP's are.


Is't Spread spectrum hard to see on a spectrum analyser?

Regards,

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Postby razta » Sat Jan 04, 2003 9:52 pm

Is't Spread spectrum hard to see on a spectrum analyser?


No, it is very easy to identify if you can pick up a signal that is constant like a large file download. Also if your SA has a max hold option the graph will shape up to show spread spectrum.
Image
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Postby Slashhax0r » Sun Jan 05, 2003 9:20 am

Originally posted by razta
No, it is very easy to identify if you can pick up a signal that is constant like a large file download. Also if your SA has a max hold option the graph will shape up to show spread spectrum.
Image


Very nice.. I need to have a little chat with a friend of mine, we were troubleshooting a long distance 802.11 link, he was showing me the spectrum analyser and it barely saw anything..

Thanks for the info... I take it you do this for a living?
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Postby razta » Sun Jan 05, 2003 11:51 am

I used too, now its a side thing while I work as a IT person for a small city in Arizona. I'm trying to keep my skills up to date. =)

(Sorry for helping this thread go off topic)
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