Last Post: August 19, 2006:
The text and some of the practice questions sometimes refer to dipole antennas. Yet, it seems to me they are talking about what everyone else in the RF business calls a vertical or a whip. As the name implies, a dipole has 2 poles, or ends, and is generally center fed. It's a T shape. It has the same donut shaped radiation pattern as a vertical. What am I missing?
I am almost 100% positive this is your answer:
The dipoles referenced with 802.11 ARE center fed dipole antennas. They are for the most part mounted vertically. In effect, the coax runs half way up the physical antenna, then splits. The center conductor continues to the end of the physical antenna. The lower half is the other half of the dipole, which is connected to the braid of the coax.
A vertical antenna is a quarter wave length antenna. The entire physical vertical part is only 1/4 wave length. The ground (artificial ground plane or maybe the metal roof of your car when used with a magnetic antenna) HAS to be there to form the other half of the antenna system.
A dipole has center and ground 180 degrees apart from each other. A vertical has center and ground 90 degrees apart from each other.
Both have a donut shaped radiation pattern to the horizon. A dipole has two nulls (both up and down). A vertical only has one null (up).
Right you are!
I sacrificed one of our antennas to the wireless gods, and found that the coax extends through a metal cylinder to which the coax shield was soldered. The center conductor extended from that cylinder. The length of the exposed center conductor and outer cylnder were the same. I didn't have a measuring tape, but each half looked to be a bit longer than an inch, which would make it a hlafwave or longer dipole.
Notably, the outer, black, plastic shell extended almost an inch further - making it look longer than it really was electrically.
By the way, verticals can be almost any length. Quarter, 5/8, and half are common for lower frequencies. Full wave or longer are easily possible with VHF and higher. Ground plane is necessary for good efficiency. It's common for HAMs to use verticals that are much bigger than their car, and sacrifice efficiency. The ground plane might be as few as 4 leads sticking out.
I used to do radio in the Coast Guard, and just got my HAM license in case of another Katrina. Can you tell? ;^)
Thanks for the research!
Nice research for sure! Just to clarify though, it sounds as if your antenna is for 2.4GHz because if your elements are a bit longer than an inch, then this is for the 1/4 wavelength for 2.4GHz. You said half wavelength, and I think it should be 1/4.
On another note, I have a few extra (ok about 15) 9 dBi outdoor omnis that I have been wanting to disect. Where is the Dremel....
So Seymour, let me tell you what you are missing?¡é?€?|
You are missing ?¡é?€??real world?¡é?€?? verses ?¡é?€??book theory?¡é?€???¡é?€?| and, it seems, never shall the two meet.
If you intend to pass the CWNA, you need to forget everything you ever learned about RF, radio theory, propagation, antennas, site surveys, etc, etc?¡é?€?|
It?¡é?€??s not that the CWNA test is wrong ?¡é?€?¡° it is just a matter of ?¡é?€??theory?¡é?€?? not matching ?¡é?€??real world?¡é?€??, along with semantics..
Let me give you an example?¡é?€?| somewhere in the CWNA Practice Test is a question that goes something like this :
?¡é?€??Which of these are directional antennas??¡é?€?? And the choices include yagis, patch antennas, grids AND omni-directional antennas?¡é?€?| The correct answer INCLUDES omni-directional antennas.
Now, if you ask anyone who has been in the radio business for more than 6 months, they will tell you that an omni is NOT a directional antenna. Antennas are either Directional Or Omni-directional. If you go for a job interview and the boss asks if an omni is directional or non-directional, you had better answer ?¡é?€??non-directional?¡é?€?? if you wan the job - but that is ?¡é?€??real world?¡é?€??.
The CWNA test states that an omni IS a directional antenna (hence the name ?¡é?€??omni ?¡é?€?¡° DIRECTONAL?¡é?€??). And, in theory, that is correct. ALL antennas radiate is a direction, so every antenna is ?¡é?€??directional?¡é?€?? in the true sense of the word..
My point being there is a significant difference between ?¡é?€??radio as you know it in the real world?¡é?€?? verses ?¡é?€??radio according to CWNA?¡é?€??, and if you intend to pass the CWNA test, you must do a data dump, re-boot and recalibrate your thinking.
But, I could be wrong..
If there is a question on the test that says an omni-directional is a "directional" antenna, then the question is wrong. If that statement were true, then all antennas would be directional, and there would be no type of antenna that wasn't directional. I know I'm preaching to the choir, but I do think this one is wrong.