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RF output Power? (URGENT)

1. What is the RF output powerin an Access point.

2. In the technical Speicafiaction the  RF ouput power is given in dbm and in some its given in mw

a. what is dBm and mW(mega watt)?
b.  what is the effect of higher or lower  dBm of RF ouput?
c. whats the standard  dBm of RF output?
c.  do Higher dbm effects the signals strength ?
d. is it a major factor to decide the  external anteena can be  attached to a device?
e. whats differnece it makes in decision of purchase of  external omni Anteena
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pawankk
Asked:
pawankk
1 Solution
 
jhanceCommented:
>>1. What is the RF output powerin an Access point.

This is usually defined as the amount of power produced by the RF (radio frequency) output circuit.

>>2. In the technical Speicafiaction the  RF ouput power is given in dbm and in some its given in mw

Yes, that's trued.  dBm is a convenient way of dealing with RF power.  The two terms are related by the formula:

Pdbm = 10 log(Pmw/1mW)

So 1 mW = 0 dBm, 10 mW = 10 dBm, 100 mW = 20 dBm, etc.  A standard logarithmic progression.  Similarly, 0.1 mW = -10 dBm, etc.



>>a. what is dBm and mW(mega watt)?

As above but mW is milliwatt, not megawatt.  An mW = 0.001 W or 1 x 10^-3 W.  An MW or MEGAWATT is 1,000,000 W or 1 x 10^6 W.  It's quite uncommon to use MW in RF except for high-power RADAR or commercial broadcasters.  MW of power in the 2.4GHz band (as with an 802.11 WiFi system) would be deadly even at long range...

>>b.  what is the effect of higher or lower  dBm of RF ouput?

Also as above.  Once again, RF power in mW and dBm are two ways of expressing the same thing.  Think of it as temperature in F or C.  It's the same temperature, just a different way of expressing the value.

>>c. whats the standard  dBm of RF output?

No standard, just what the manufacturer of the device designed it to use.  Many APs are 100mW or less.  Some are more powerful.  Just depends.  Most jurisdictions have regulations on the MAXIMUM.  In the USA the FCC rules govern this.

>>c.  do Higher dbm effects the signals strength ?

Possibly.  Signal strength depends on many factors but if all other factors were the same, starting with more RF output power at the transmitter would give you greater signal strength at the receiver.

>>d. is it a major factor to decide the  external anteena can be  attached to a device?

No, that's another story altogether.  The antenna takes the available output power and "focuses" it in a particular way.  So most external antennas specify a gain factor, usually in dBm.  So you might purchase an antenna that says "+7 dBm" gain.  If you start with a +1 dBm output AP and put a +7 dBm antenna on it, you get effectively +8 dBm.  But there is no "free lunch" here.  You get better gain with such an antenna but at the expense of directivity.  In other words, the RF output now is more narrowly focused and so some directions get more power and others get less, usually a LOT less.  

Whether this is good or not depends on what you're trying to accomplish.  If you are trying to cover a large area this will be counter-productive.  If you are trying to hit a single station at long distance, it would be a good thing.

>>e. whats differnece it makes in decision of purchase of  external omni Anteena

OMNI is short for OMNI-DIRECTIONAL.  These usually have small gain figures since they are designed to have a 360 degree radiation pattern.  Many of these provide gain (over the reference antenna - which is not always a standard) by reducing radiation in the "Z" plane (i.e. the up/down direction).  So you get a more "doughnut" shaped pattern vs. a "basketball" shaped pattern.  This is OK is all your stations are on the same level but if you have a multi-story building you could reduce your effectiveness.  See the radiation chart for the antenna you are considering and be sure it matches your installation.
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wylie_ukCommented:
quality answer
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gurutcCommented:
Yes indeed!  jhance definitely earned points on this baby.  I'm bookmarking the question so I can come back to this excellent reference.  Bravo!

- Travis
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pjtemplinCommented:
And disappointing that such a fine answer for an URGENT question would only be worth 50 points.  Ought to be an urgent tax...
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pawankkAuthor Commented:
jhance
no one have ever answered so clearly ever thanx buddy...

but some more words from u
. whats differnece it makes in decision of purchase of  external omni Anteena ?

1.my purpse is to broadcast internet connection to household within building, with  ariel area of 4 km i have purchased a micronet  ap with 16 dbm rf output and micronet omni of 12 dbi but it altogether is not working as its not broadcasting. some one have adviced me a sanyo device having 23 dbm(200mw) rf output but the problem is that  sanyo device is 802.11b (ie. 11mbps)  where as micronet one is ...802.11g (ie. 54 mbps) so whats ur words about the access point  purchase.

2. you have stated  "Many of these provide gain (over the reference antenna - ....... considering and be sure it matches your installation."

i want to know
a.how the omni signals travels any refrence(site links) with digramatic explaination.
b.what is radiation chart.?? and where to find it out.
c what  if  i give my omni ateena  at a tower height off 150ft wil ihave to ..give the same height at the other receving end.
d. you have stated  "This is OK is all your stations are on the same level but if you have a multi-story building you could reduce your effectiveness" whats the effectivenes will reduce and secondly any option to cope with  this drawback
 
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jhanceCommented:
What you are asking is an entire field of study in itself.  RF and most especially microwave (which is what WiFi @ 2.4GHz is) is very tricky.  Even the experts who make their living worrying about antenna design and signal propagation are often wrong.

There is no simply answer to what I think your question is.  You'll just have to make your best guess based on the information you can gather and try it out.  Once you have that, you can do some tests and tweak it.

If you really have no clue as to how all this works, get a consultant who has a proven track record in doing this kind of work.

Get information on antenna propagation from the antenna manufacturer.  If they can't or won't provide such information, forget them and choose a different vendor.  Antennas vary widely in their performance and their intended use.  An antenna that is perfect for one application may be next to useless in a different one.  It's not that it's a poor antenna, it's just an incorrect application.


>>>micronet  ap with 16 dbm rf output and micronet omni of 12 dbi but it altogether is not working as its not broadcasting.

What do you mean by this comment?  A 16 dBm AP is not very powerful.  So couple it with a 12 dBi gain antenna should give it an effective power of 28 dBm in the direction of the antenna gain.  That's equivalent to almost 1W but trying to push that signal out to 4km is a long way.  Is this pure line-of-sight or are there obstructions?  You might be able to get 4 km with line-of-sight with this setup but there are many other factors.  What about interference?  The 2.4GHz ISM band has many other users.  Not the least of which are microwave ovens and cordless telephones.  Any of these could be swamping your signal.  Remember that a gain antenna (on the receiver) will provide gain to both desired and undesired signals.  So you need to carefully orient your gain antennas to minimize such stray pickup.

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pawankkAuthor Commented:
jhance thanx buddy again

Jhance clearly mentioning i am Establishing a WISP (Wireless internet service provider) thorugh an AP and Omni  for starting with a area of 2 Km on start and then will expand to 8 Km gradually depending on connenctions i get.

For your  detail i have given my Broadcasting  Omni anteena on a tower height of 80ft. from ground may increase it gradually. In my area of broadcasting what i think  there is maximum to  3 storied buildings (ie. 40 ft upto max) so i think there is  no obstractions so ever.

But the line of sight:- i coudnt get the meaning what is it .. does it mean that the receiving end should be at the same height  (ie. 80ft from ground) but i m trying to catch the signals by clamping the recieving AP on a height of 20-30 ft. from ground but i am not geeting the signals..

at last  to  the point i want to know:-
1. What is line of sight means (same level of height)

2. What  are factors affecting the signal strength and whats their precautions.

3. Interface u asked but i think cordlees wil not give the problem if the height of tower is  80 ft. wil it give the problem if yes then how to handle the interface problem.

4.you said " Remember that a gain antenna (on the receiver) will provide gain to both desired and undesired signals.  So you need to carefully orient your gain antennas to minimize such stray pickup"

... i have connected my Omni gain ateena with an AP with a low loss RF cable:- what i know ..is that it will transmit the signals of the AP wil it  transmit undesired (others) signals to ? if yes then how to obstruct it from trnasmiting other signals. Secondly  u said to orient  my anteenas what orinetation means.. do detail me ..
 
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