How to Conduct a Wireless Survey Part 2


Visualizing Radio Waves

To give you time to assemble your survey kit, I thought that the next logical stop on our journey would be your understanding of how RF works in your environment. To do this, I am going to have you visualize how it would look if you could see the radio waves. You will have a mental picture that will always be with you.

Every beginning wireless class teaches that radio waves radiate of a dipole antenna in nice neat concentric circles progressively getting larger and weaker as they move away from the antenna; picture this: you standing at the edge of a perfectly calm lake and tossing in a rock. Visualize what the waves look like. This is what radio waves look like (only a great deal smaller); what radio transmission would look like outdoors in a parking lot or empty field.

Is that your environment though? Not hardly. There are all kinds of things in our world that scatter and deflect radio transmissions. What would it look like if we returned to our calm lake but tossed a rock under the legs of a pier? You would see the initial circle, but as it encountered the legs of the pier, some of the waves would get bounced back and dispersed, some would wrap around. You would see a disturbed radiation pattern of waves. This is a bit truer to our environment.

Now picture a city street with tall buildings. I just placed an Access Point at the intersection of two streets. Do you think that the radiation pattern will be circular? Well perhaps right around the Access Point there will be concentric circles, but if you looked at the pattern from above and could see how far the waves radiated, you would notice that they extend farther than they would in open air due to the buildings “herding” the waves down the corridors. The radiation pattern would look perhaps like an ‘H’ or ‘t’. Can you visualize that? This is how waves might move down a hallway in an office.

Mentally skip a stone across the water. Waves can skip across the air (sometimes that’s why we can hear radio stations far away from where they are transmitted from). Waves can bounce off of hard surfaces. They can be absorbed; in a warehouse with fairly equal coverage throughout a few pallets of peat moss can render a “hole” in the coverage around those skids. There is another effect called “radio shadow” where behind an obstruction the coverage falters the similarly to the way light casts a shadow on an obstruction.

Knowing how radio waves move is an important first step in conducting a site survey. With a bit of experience you should be able to walk into an environment and have an understanding of how the RF is working or what you need to do to enable a wireless environment.

I will revisit the topic later as we advance in our study. We will look at radiation patterns of various antennas.

Source by Gary Wesselhoff

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