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Hello everyone!

I would like to know more about dead spots on high gain antennas. I am reading this document:

http://www.cisco.com/en/US/docs/wireless/technology/apdeploy/2600_2600_3600_DG.pdf

On page 37-38 there is a paragraph which says:
"A high gain antenna may have a null or dead spot directly below it because the antenna element is often much longer and has less metal surface area available to conduct the radio wave if you are directly underneath it. (See Figure 50.) The further away you are from the antenna, the more surface is available, and the better it performs."

Could anyone explain this a bit further?

I don't like their description, especially where it says "...has less metal surface area available to conduct the radio wave."

Many antennas have nulls, even simple low gain dipoles - it's all relative.

What's important to see in Fig. 50 is that in the horizontal directions the pattern spreads out wider as you move away from the antenna.  At the bottom, where the null is, there is almost no signal (in that direction).

Hope that helps.

There is a fixed quantity of RF going into an antenna. A gain antenna does not increase the quantity of RF, it redistributes the RF in one direction at the expense of less RF in other directions. If you want to increase the RF in all directions horizontally you must decrease RF vertically. Omni gain antennas compress energy wasted on airplanes and earthworms into more energy in the horizontal plane for wifi users.

No gain ( 2.4 dbi 1/4 wave dipole ) radiation patern:

http://rcexplorer.se/wp-content/uploads/2012/09/pattern_hg2458rd.gif

Low gain  ( 8 dbi ) dipole RF energy distribution in the vertical plane:

http://www.l-com.com/copyrighted_images/pattern_hg2408u.gif

( l-com.com generously and wisely allows their images to be used in tutorials if you tell people where you found the image )

15 dbi omni vertical propagation:

http://www.l-com.com/copyrighted_images/pattern_hg2415u_pro.gif

An omni gain antenna is a collinear array, with two Ls. based on the same root as collide, collision: bringing two or more things together.

A collinear array is a stack of 1/4 wave dipoles and phasing harnesses. The phasing harnesses keep the RF signals in the same phase, so the transmitted signals add together instead of cancelling each other out:

http://learn-antenna.blogspot.com/2010/08/arrays-antenna.html

http://vk1od.net/antenna/ccps/

So: X amount of RF energy in a 1/4 wave dipole is analagous to a 1 ounce bagel:

http://upload.wikimedia.org/wikibooks/en/1/13/Dipole_pattern.gif

The radiation pattern of a 15 db gain omni is analagous to a 1 ounce crepe:

http://3.bp.blogspot.com/_u_YFCyIcLS4/R7T_7JoImBI/AAAAAAAAAHI/R8m7T6OBouA/s400/omni+radiation+coverage.jpg

same quantity, different distribution.

Addendum: 15 dbi is the practical limit of gain for a collinear. adding more elements requires adding more cable which adds more loss - 15 dbi is the point of diminishing returns. the people on ebay who advertise 18 dbi or more omni antennas are the same people who sell 4 watt USB adapters with palm sized 36 dbi antennas.

In plain terms, 

The higher the gain in a specific antenna the narrower the beam width is and thus far if you are closer to the rf antenna the less of a signal you will receive (depending on the radio in the laptop or device) because of less side lobes are present. In return, the further out you are the better experience you will have with signal disbursement and it also depends on the type of beamwidth (Horizontal vs. Vertical) Horizontal will give you more surface area but less of a travel path, Vertical beamwidth will give you longer distances but less coverage area. The higher the gain the more suseptable to dead spots the closer you are to the antenna. It highly depends on the beamwidth of the antenna. Thanks, Reggie (CWNA/CWDP)