Everything about antenna... [Archive]

Big T

05-30-2005, 05:48 PM

hello everyone... i have been doing lots of research on antennas but i'm still confused... Searching for "antennas" in this forum also doesn't help.

Could somone please explain the relationship between
Transmitter Power, Output Power, Receiver Sensitivity.

what does "54Mbps, 10% PER, -68 dBm" mean in terms of receiver sensitivity?

If transmitter power remain constant and I upgrade my stock AP antenna to a stronger DBI antenna, what does it to the coverage? (i understand the flattening donut effect of onmi)

I read somewhere that upgrading to a yagi antenna "focuses" the intensity of the signal, similar to a laser beam. Is that true?

Last question is if I upgrade the antenna, does that affect the "receiver sensitivity?" too?

Could you pleaseeee help?

Terence

ad5mb

05-30-2005, 09:56 PM

Transmitter power, output power are two ways of expressing one parameter: The power generated by the transmit side of the transceiver.

Receive sensitivity is the minimum signal strength at the radio input which provides a useable link.

Stronger dBi antenna. Clearly, you are unclear on the concept.

You read somewhere that... Probably this, which I posted at a Yahoo group...

There are 2 kinds of antennas: Omnidirectional and directional.

Omnis transmit in all directions, 360 degrees. Omnis with gain are made by stacking antennas and connecting them with a harness that makes them add their signals together. The limit to this is the loss of RF in the phasing harness; you get to a point where cable losses exceed the gains from stacking antennas.

Directional antennas work like those high-end flashlights with adjustable reflectors. Consider, as an example, the kind that lets you remove the head completely, so the flashlight acts like an expensive candle.

That's an omni. Screw the reflector back on, the same energy gets directed at a smaller portion of the sky. You gain energy in one direction, at the expense of other directions. The antenna flings its energy in all directions, like a bare light bulb.

Adjust the reflector to focus on a distant spot. The beam width gets very narrow. Adjust the reflector to bestow its blessing on a wide area. Less range, but you can see to the sides.

As a rule: More gain, less beamwidth. Use 6 db gain parabolics on posts in the corners to focus energy into your coffeeshop, and spread it throughout the place. Think of it as a floodlight. Use a 24 db parabolic for a long haul link to another parabolic on the other end. Think of it as a searchlight.

Types:

The dipole is the simplest antenna, just a short stick. If an antenna has gain, it has gain over a dipole. That little black stick that came with your AP is a dipole.

The Yagi is a directional antenna built on a short stick. If you get howling all night wind storms, and the prevailing wind is lined up with your distant connection like a weathervane, a yagi is a good choice. If you want a stealth antenna, the yagi is a good choice. Not relevant, but what the heck: The Yagi was designed by an engineer named Uda. Mr. Uda called it the Yagi-Uda array to honor his professor. Americans shorten that to Yagi. Mr. Uda would not mind.

The Patch is typically made on a printed circuit board. Very flat, easy to Velcro to a laptop lid. You get gain without having to carry a tripod, patch cables, and putting up with stupid questions. Outdoor patches exist, but they are like the sail on a square-rigged ship. A good choice if the istant connection is at right angles to the prevailing wind.

The Sector antenna is a dipole or patch in a small reflector. Not visually obnoxious, people who care about architecture won't get all twitchy. Typically low gain, wide beamwidth. Floodlight.

Parabolic dish: Big, ungainly, large diameter, short length. Good choice if the prevailing wind is at a right angle to the link.

Antenna gain is bidirectional. The gain benefits the receiving side as well as the transmit side.