ITKnowledge Exchange member "Doug D" had a question about directional antennas and resident wireless guru Lisa Phifer and other ITKnowledge Exchange members offered their advice. Here is a portion of the conversation.
IT Knowledge Exchange member "Doug D." asked:
I am trying to set up a wireless access point to cover a couple of apartments near an office on a small campus. I set up an access point in the office, but cannot get a connection about 50 feet away in one of the apartments due to the intervening walls. There are three walls that the signal needs to go through, two of which are 1 foot thick adobe. My question is, will a directional antenna with higher db output make much of a difference, or is that more about distance than penetrating through the walls?
Lisa Phifer writes:
Directional antennas focus a transmitter's output power into a narrower field. In free space, that makes radio signal travel farther. When obstacles like walls are present, the power delivered to the near-side of the wall will be higher with a directional antenna than with an omni antenna. If the wall merely absorbed RF energy, a higher-power signal would indeed have a better chance of propagating completely through the wall.
However, all walls are not equal. Some surfaces (like metal) reflect RF instead of absorbing it. For example, a window covered by steel blinds will reflect signal, while a stucco wall containing diamond mesh both reflects and absorbs signal, depending upon RF frequency (wavelength). This USC research paper describes attenuation measurements taken through a wide variety of building materials and radio frequencies:
E10589 Propagation Losses 2 and 5GHz
Check out the measurements given for brick, stucco, and cinder block. I have not seen any published specs for adobe walls, but your walls probably contain some type of wire mesh inside them, and of course nearly a foot of mud. A high-gain directional antenna might be somewhat helpful, but ultimately may not provide sufficient coverage where you actually want it in the distant apartment. You may have better luck pursuing another alternative -- for example, Ethernet over Powerline (www.homeplug.org), or connecting WLAN bridges to directional antennas placed just outside windows, aimed at each other.
Radio 101 - signal strength is signal strength and how you use it up to you. If you take the base definition of an omnidirectional antenna as being value 1, then the transmit power (output of access point) is distributed evenly in a spherical pattern with the WAP at the center. Ideal for a multi-story house with the WAP in the middle.
A 'directional' antenna gets its 'gain' from forcing more energy into a 'torus' (doughnut) shape, usually in the horizontal plane while reducing the signal up and down.
Unfortunately you said 'adobe' walls. Signal absorption (loss passing through walls) is a function of density of the material, metallic content, and water content. WAPs use frequencies readily absorbed by water (cell phone dropouts in the rain), metal studs (twice as limiting as wood), and crystal lattice structure (cement and sand in adobe). You may need an outside antenna at one or both ends and a WAP repeater.
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