distance of BNC connectors

Answered Question
Aug 20th, 2007
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All,


the Cisco APs have a short distance between the two BNC connectors for 802.11a and a longer distance between the BNC connectors for the b/g antennas.


I once read that these distances are chosen according to natural constants. Does anyone know the exact story of this?


--Joerg


Correct Answer by scottmac about 9 years 11 months ago

It's predictable pattern.


Assuming the usual "rubber duck" antennas:


If they were a half wavelength apart, they would nullify each other(180 degrees out of phase)


If they're a 1/4 wavelength apart, the pattern becomes enlongated between the antennas (think of the antennas as "goal posts" and the generated signal pattern is stretched int eh direction of the ball going through the goal posts).


At one wavelength apart, the general pattern is still generally circular / omni, and there is some wavelength magic math that suports better propagation.


So, with a cable-connected external antenna, not a big deal ... they could be any distance apart; with a direct-mounted rubber duckie or whip (or dipole, or , or, or) you get some wave magic (or not, depending on the antennas) by starting at one wavelength.


Note: if you want to PREVENT "coupling" when you mount an antenna to a conductive surface, you should put it (IIRC) at least TWO wavelengths away.


Good Luck


Scott


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ericgarnel Mon, 08/20/2007 - 07:09
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the closest I can find on this is loss/cable length


from http://www.cisco.com/en/US/products/ps6521/products_data_sheet09186a008022b11b.html


"Low-loss cables extend the length between any Cisco Aironet 2.4 GHz and 5 GHz radio and the antenna with RP-TNC connectors. With a loss of 6.7 dB per 100 feet (30 m) for the low-loss cable and 4.4 dB for the ultra-low-loss cable, this provides installation flexibility without a significant sacrifice in range"


It's been a while, but if you check the DBi (gain) of the anntennas, you will see that the 5 Ghz antennas have more gain typically, I believe this is due to fcc regulations; someone please correct me on this....

scottmac Mon, 08/20/2007 - 17:23
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I'm not sure about the regulatory stuff, but physically, I think the reasons are something along the lines of:


It's a shorter wavelength, so you can "get more antenna" in the same space as a 2.4Ghz ...


... which is necesary because 5.8GHz signals deteriorate faster, and requires more power to get the same legs as a 2.4GHz signal, which needs more power to go the same distance as a 900MHz signal (and so on downwards).


5.8GHz doesn't penetrate very well, but it bounces like a ... well, it bounces well I meant.


It's not exactly accurate, but conceptually:

think of one power unit (watt, milliwatt ...) as a three foot string.


Make a single wave from the string and call it one MHz frequency ... now make two full waves (representing 2Mhz) ... the distance from one end of the string to the other is much shorter (or you had to stretch it and reduce the height (amplitude) of the wave).


Continue doubling the waves ... to get the same (or nearly the same) distance, you have to decrease the amplitude of the wave ... to keep the same amplitude, you reduce the distance from one end to the other.


Frequency, power, and usable distance (with normal aparatus, Thank You Mr. "I can talk a zillion miles on half a watt" NASA) are closely related.


Like I said, it's not a perfect analogy, but for bosses and management in general, it has proven to be helpful ... ;-}


Cable also loses/attenuates more as the frequency climbs; there's more loss per connector, more loss on severe bends (because it deforms the coax and causes an Impedence Lump"), and more loss from moisture leach, so the extra gain comes in handy.


FWIW


Good Luck


Scott


with_joerg Mon, 08/20/2007 - 11:20
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That sounds reasonable. Why would it make sense to put the antennas at a wavelength distance?

Correct Answer
scottmac Mon, 08/20/2007 - 14:00
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It's predictable pattern.


Assuming the usual "rubber duck" antennas:


If they were a half wavelength apart, they would nullify each other(180 degrees out of phase)


If they're a 1/4 wavelength apart, the pattern becomes enlongated between the antennas (think of the antennas as "goal posts" and the generated signal pattern is stretched int eh direction of the ball going through the goal posts).


At one wavelength apart, the general pattern is still generally circular / omni, and there is some wavelength magic math that suports better propagation.


So, with a cable-connected external antenna, not a big deal ... they could be any distance apart; with a direct-mounted rubber duckie or whip (or dipole, or , or, or) you get some wave magic (or not, depending on the antennas) by starting at one wavelength.


Note: if you want to PREVENT "coupling" when you mount an antenna to a conductive surface, you should put it (IIRC) at least TWO wavelengths away.


Good Luck


Scott


retiredmarine Sun, 08/26/2007 - 05:19
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good answer, if you do the math, wavelength = speed of light / frequency in MHz, i think that is the formula, you can see the distance between the connectors is one wavelength, roughly 4.9 inches for b/g and 2.3 for a radios.



lucychiu Thu, 08/23/2007 - 14:43
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Will this rule still apply to the 11n capable a or b/g radio antenna connector placement?

scottmac Sun, 08/26/2007 - 06:29
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To some degree, yes. Physics is physics and waves will behave in specific ways for a given set of circumstances.


Using things like phasing of multiple radiators and separation distance between radiators has been around since early radio.


That is how radio stations (and satellites, and "smart panel" antennas ...) "steer" their signal. For any given positon, if you take the same signal, split it, then feed it to each of the antennas in a specific phase relationship, you can "deform" the pattern and "focus" it to a desired direction.


Yagi antennas do this passively. Remember that RF electrical energy will follow the path of least resistance ... conductive metal is easier than air.


A Yagi design starts with a dipole (frequently a loop dipole), then adds one or more shorter (< 1/4 wavelength) elements ahead of the dipole (called "directors") at very specific points, which correspond to particular spots in a wavelength.


At the same time, there is (at least) one element behind the radiator (the loop dipole)that is longer ( > than 1/4 wavelength, called a "reflector") that will, because of where it's located, and it's length, distort the wave back towards the front of the Yagi.


Because the locations of the Yagi's elements are related to specific phase point in a wavelength, Yagi antennas are built for specific frequencies / narrow bands of frequencies. If the passive elements (reflectors & directors)aren't in the right place to catch the signal in the right point, it won't have the correct effect on the signal (it get's pretty messy ... RF flying everywhere ....ugh)


In the long run, everything you want to do (or not do) to a signal relates to how it interacts with other signals (even if the 'other signal' a reflection or scatter fo the same wave) and the reflective or absorbtive propertise of things within the field of radiation.


The 802.11n designers are trying to take what used to be a negative property (multipath) and manipulate and use those same properties to improve coverage and distance.


RF / wave propagation, antenna stuff is pretty complex stuff ... well over my head. There's math involved I don't even know the name for.


That being said, what I mentioned above is described in very general concepts, and may not (for the "real" engineers out there") be 100% accurate in it's finer details. It's accurate enough for general discussion, but don't try to use it for you engineering homework ;-}.


Hope this helps!


Good Luck


Scott


Rob Huffman Tue, 08/21/2007 - 05:22
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Hey Scott,


I must add to Eric's endorsment of this great post! Your brain must have been in overdrive for this one :) A 5 point must!


Take care,

Rob


PS; Loved the interview - Congrats

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