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Can anyone help me understand how to interpret polar charts? I am having some trouble grasping the concept here from the study guide. I understand the rule of 6 dB and understand that a 10dB decrease of power decreases the distance the signal travels 70%. I just don't get how to read the patterns against the lines of the chart and how they appear to slice up the pattern with a dash line?

Could use the help with this I am planning on taking the CWNA test this Saturday.

Search on antenna azimuth and elevation charts or RF Antenna Patterns!

All of these pages have something good on them, but don't waste time reading it all right now before your test - stick to the patterns for now.


http://www.connect802.com/antennas.htm

http://www.connect802.com/wcu/2005/newsletter_050601.htm

This one from Cisco has lots of info specific to their Antennas:

http://www.cisco.com/en/US/prod/collateral/wireless/ps7183/ps469/product_data_sheet09186a008008883b.html

http://www.antennex.com/shack/Mar06/nc.html

One thing to remember is that increasing the power to the antenna does NOT change the shape of the pattern.

Best of luck !

Oh, crap... just realized this was already posted. Moderators... geesh.

As for understanding how to use an azimuth and elevation chart, see here, about halfway down the page they have a helpful discussion.

http://www.connect802.com/wcu/2005/newsletter_050601.htm

Thanks very much for the links. I hope I pass!

One of the strangest [ and least understood by many ] aspects of a polar chart concerns the physical shape of the pattern.

Let's consider a yagi antenna and it's pattern in azimuth [ looking down from the top ]

Firstly, the big bit? sticking out is known as the main beam or more correctly, the main lobe.

A question I'm often asked is, "Does the beam have an end point, where it stops? on the chart you can see that it has an end.?"

This is actually a very good question.

If we are talking about a snapshot? of the signal at one particular point in time, the answer is yes.

If we are talking about a very large period of time [ provided that we are considering a signal in true free-space [ ie outer space !! ] with no obstructions, the answer is till yes, but the length of the main lobe has now become very, very long indeed.

So how far does the signal go ? Well, this guy had the right idea:

http://www.entertonement.com/clips/cdngcdfklv--To-infinityToy-Story-Tim-Allen-Buzz-Lightyear-

Imagine that we had three rocket ships. We put them side by side: 1,2,3. Each rocket ship travels at EXACTLY the same speed as the others. Now we fire off the second rocket. A short time later, we fire off rockets 1 and 3. Both 1 and 3 are a bit behind rocket 2. Both 1 and 3 are exactly level with each other. All three are travelling vertically upwards from the launch tower.

Now imagine that each rocket has trailed a very thin wire behind it from the launch pad. The rules given to the rocket's computers are that the angle measured from the pad between rockets 1 and 3 must always remain the same.
At any point in time, we can measure the distance that rocket 2 has gone [ relative to the launch tower ]. No matter how far rocket 2 goes, rockets 1 and 3 are always the same relative space with respect to the angles we mentioned. Numbers 1 and 3 will be close to number 2 close to the tower and will be far away as number 2 gets far away from the tower.

We can't take this analogy too far, but it shows that the rockets and their angle spacing remain the same no matter how far the rockets travel.

So, if we measured signal strength from our yagi right down the middle of the beam say 100 meters away from the transmitter [ launch pad ] and we moved off to the right and then to the left and checked the points where the power had dropped by half, we could draw lines to the transmitter [ launch pad ] and measure that angle [ called the half power beamwidth ]. We could check the power and angles 200 meters away, 300 meters away etc and find that the angle remains the same.

The polar chart then is a snapshot of how the beam would look physically if we could see it with our own eyes. At any instant in time, it has a definite end point as it is expanding, but it will keep expanding forever if there are no obstacles etc.

Now onto the next thing. What about those strange little bumps that you see on the side of the pattern ? Those are called sidelobes. Almost impossible to explain physically [ for the mathematically inclined , they are the result of the sinx/x distribution pattern of the antenna, just as sidebands are formed via a sinx/x function in the creation of an 802.11 signal - ?same math, different physical function ].

Sit and look straight ahead. You can see things pretty well right ahead, but as we get further away from dead center? things are less clear. We are on the periphery of our vision.?

Ever hear the expression "eyes on the back of his head"?? Well, in nature, some creatures have this. Some spiders can see behind themselves.

http://images.google.com/images?sourceid=navclient&rlz=1T4ADBS_enUS249US249&q=spider+eyes&um=1&ie=UTF-8&ei=Z46AS42hIsuztgeI7ayVBw&sa=X&oi=image_result_group&ct=title&resnum=1&ved=0CA8QsAQwAA

You could think of the antenna as having lots of eyes? when thinking about the receive function. One big eye in the middle and lots of little eyes around it. Some of the little eyes? can see behind themselves. The "Front to Back"? ratio of antenna tells us how powerful? or not these 'little eyes?? are.

http://www.astronwireless.com/topic-archives-antenna-radiation-patterns.asp

Dave.