Antenna Gains with 1240

Unanswered Question
Nov 21st, 2007

I have a customer with the habit to use Hyperlink Antennas with the Cisco Radios.

I would like to know the pros and cons to use for example a aironet 1240 with an omni 2.4Ghz 12dBi, cisco lighting protector and original cables versus the same 1240 with an hyperlink 8dBi 2.4Ghz omni antena, hyperlink cables and lighting protectors....

I need the calculation of the total gain on each case if is possible.



I have this problem too.
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Richard Atkin Wed, 11/21/2007 - 14:11

There are two concerns when using high-gain antenna, system gain, and EIRP.

System gain = Gains - Losses, so in your case...

System Gain = Antenna Gain - (Cable Loss + Connector Loss + Lightning Protector Loss)

EIRP is the total system output in dB or mW - this is restricted by law. EIRP = Tx Power + System Gain.

In the UK 2.4GHz EIRP is 100mW / 20dB. So for example, if I had a TX power of 5mw / 7dB, with a System Gain of 8dB, my EIRP would be 7 + 8 = 15dB, which is inside the EIRP limitation, so it's a legal solution :o)

In addition to this, antennae with different gains have different propagation characteristics; the vendor should be able to provide this information. With an omni antenna, you will typically see the highest gain is reached in a position that's perpendicular to the direction the antenna is pointing in. Ie, if you have a 'stick of dynamite' shaped omni, and it's orientated vertically, the highest gain is along the horizontal plain, which is where you client is most likely to be. As you move to a position under / above the antenna, the gain will reduce significantly.

Hope this helps.



Rafael Jimenez Fri, 11/23/2007 - 10:27

Looking in the documentation and tech notes I found that using a 1242g radio with surge protector and 50ft LMR400 cable the requiered antenna gain is at least 5 dBi, because the loss in the cable an other accesories is close to 4.4dB.

Eeach connection (0,5dB aprox) plus the 3,4dB in the cable plus an lithing protector (0,5dB aprox) (0.5+0.5+3.4=4.4)

Using hyperlink pigtails (N to RP-TNC jumpers) and other accesories increase the value in the final loss.

I know that is a little more complicate but its give me an idea.


scottmac Wed, 11/21/2007 - 14:16

The information provided is a little superficial for any in-depth discussion.

However, depending on the local laws and regulations (and enforcement of same), the significance of complete Cisco (as a system) versus Cisco with third-party parts usually boils down to certification.

In the USA, The AP, cabling, antennas, and peripheral parts are tested and certified as a system. If you substitute a substantial component with one that has not been tested as part of the overall / complete system, it may not be certified / authorized to operate without a specific compliance inspection.

The regulations were recently amended (last year or so) so that a third-party component that is substantially identical can be substituted without the need for re-certification.

So, IF there are similar regulations where this customer is, and IF they are enforced to a degreee that the customer would worry about it, then the 'Rules and Regulations" argument is probably your strongest lever.

Aside from that, parts made to operate within this kind of system *will* operate ... the question is "are they operating at an optimum level?" ... and there's no way to judge that without surveying the location.

Most folks that don't know any better just assume that if you swap in an antenna with twice the gain, you get double the signal strength and everything else stays the same, you know (I hope you know) that that is not the case; the entire coverage pattern has to change, at least on one plane.

So you might fix the system one way and break it in another ... only a survey will give you the true way it is.

Also, strike the phrase "Lightning Protectors" from your discussion; they cannot and will not protect any part of your system if it is struck (or experiences a "near hit") from lightning.

They are, at best, a static drain. They can assist in charge dissipation, which *may* SLIGHTLY reduce the chance of a direct hit.

A segment of fiber between the roof-mounted stuff and the inside stuff is always your best protection from a lightning hit.

Good Luck


Rafael Jimenez Thu, 11/22/2007 - 06:14

Thanks Scott, The customer has several year with a complicate wireless implementation. Is a loadig dock, with dozens of ships, hundreds of movil cars and thousands of containers in 1000x600Mts area. In my physical survey I found the hyperlink accesories (cables, antennas, conectors) and 1mW amplifiers. The first thing I suggest was a survey (with airmagnet) and the inconvenience of the amplifiers. Like many other customers I found a misconception of the diversity feature, the people think they can put an omni and other type of antena in the same radio.

Anyway your information is very useful.



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