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1) Which is a better measure of wifi antenna RECEPTION abilities?

2) Is the answer to question one a measure of both power AND reception? Which is it more a measure of.

Please don't tell me about the logarithms, just in understandable language please.

2007-07-05 13:20:28 · 2 answers · asked by Shellshock 2 in Science & Mathematics Engineering

2 answers

When someone talks about dB they really mean to say dBi or dBd, at least when it comes to wifi signals. The term "dB' just means decibel, but "decibel" is not specific enough to be useful in regards to antenna gain (i.e. the "power" of the antenna). The term "dBi" means "decible isotropic," and "dBd" means "decible dipolar".

You see, the reception capability of an antenna is actually measured in two different ways. One is to see how strong the reception of an antenna is given another antenna that sends out a signal in all directions. This is known as being measured from an "isotropic" source. Another way to see how strong an antenna is to use an antenna which radiates a singal in mostly one direction (of course, towards the recieving antenna). This second method, called a "dipolar" source is sort of like cheating -- an antenna that has its power focused directly at another antenna is 2.1 dB higher on average.

I don't ever recall reading whether wifi antenna power/range claims were based on isotropic or dipolar source, but if you know they used an isotropic source you can subtract 2.1 dB from their "dBi" rating to get the relative dBd rating. In other words let's say you have two antennas you are comparing, Antenna A is 6 dBi and Antenna B is 8 dB (but it mentions it's measured relative to a dipolar source); what is your power for both? Well Antenna A is 6 dBi and Antenna B is 10 dBi; the equivalent of which is Antenna A is 4 dBd and Antenna B is 8 dBd. No matter which way you measure it, Antenna B is the more powerful antenna.

In summary, manufacturers of wifi antennas (unlike professional radio gear or even professional wifi antennas) are probably just throwing numbers around. If it only says "dB" and there is no mention of isotropic or dipolar source, then for all you know it could be a dBi or a dBd rating -- you just don't know.

As for question two, dB is really a measure of an antennas power. But... an antenna's reception is tied directly to how much power it can radiate. The more power it can radiate, the wider area it can grab a signal from.

2007-07-05 14:37:25 · answer #1 · answered by figgypower 3 · 0 0

The measurement of an antenna in db or dbi is in its gain abilities. When speaking of 'db' in antenna gain it is short for dbd which references to a to a half-wave dipole antenna. Half wave dipole is more of a realistic wave pattern which one would expect in the real world (a figure 8 pattern from the antenna with the antenna in the middle of the intersecting pattern).

dbi references to an isotropic type of array (which is basically an evenly distributed pattern full circle from the eminating source...antenna).

The answer to your second question is power or 'gain'.

If you're trying to decide which of 2 similar devices to buy, but each is rated differently than the other for antenna gain. The db output rating is already 2.5 times greater than the dbi with the same power input applied. 10mw db rating has more reaching power (2.5 times) than a 10mw dbi rating.

The best way to understand is through pictures showing an antennas radiation pattern. see here: http://www.softwright.com/faq/engineering/FIELD%20INTENSITY%20UNITS.html

2007-07-05 14:10:52 · answer #2 · answered by mermeliz 7 · 0 0

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