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As I am studying for the CWNA, I keep running across a question that makes me wonder a bit. Basically, asking what to use at a 4 mile bridge link. The correct answer is a semi directional, however, why would a highly directional be a wrong choice?

If it is strictly for a bridge link, why not use the more narrow beam width of the highly directional antenna. If the EIRP is too high for local regulations or something, you could use a lower transmission level or an attenuator. Perhaps I dig into these things too much but it peaks my curiosity.

Thanks a bunch!

Does the question address the bridge link as point to point or point to multi-point? What are the possible answers? The devil is usually in the details of the particular question and what would be the most correct answer will usually rule the day. Also, if the question does not contain additional information about the bridge link, revert to the black and white in the official course materials. The questions often have choices that look like legitimate answers. Even without exact question context, I feel very confident that what you are seeing here is  a well crafted question that uses what appears to be a legitimate answer but is factually wrong. It is what is referred to as a "distractor"

The types of antennae listed in the official study guide are omni-directional, semi-directional, directional and dish-grid.

Thanks for the reply.

Here is the exact question.

To establish a 4-mile point-to-point bridge link in the 2.4 GHz ISM band, what factors should be taken under consideration? choose all that apply

a. Fresnel zone with 40 percent or less blockage

b. Earth bulge calculations

c. Minimum of 16 dBi of passive gain

d. Proper choice of semidirectional antennas

e. Proper choice of highly directional antennas

So a point to point link, a and d are the official correct answers.

The way it asks, what factors should be taken into consideration, why would I not consider a highly directional antenna such as a grid or dish?

Thanks!

Because you have to give the best answer and in the official curriculum there is no such animal as a high directional antennae

The types of antennae listed in the official study guide are omni-directional, semi-directional, directional, and dish-grid. (dish-grid are a subset of directional)

Also note the first answer is a derivative of the official study guides requirement which states that the first Fresnel zone must be at least 60%clear of obstructions.  poTAY-to, poTAH-to.

Process of elimination.

a. Yes

b not required unless we are talking 6 miles or more. 

c.There are no minimum passive gain requirements  as far as the FCC is concerned if you antennae gain goes up your power must decrease accordingly.for the IR

d. Yes this EXACT type of antennae is in the course material

e. No such antennae in the course material. It looks good and makes you think it's legitimate but its not. 

I would bet that the creator of the question would also argue the course materials state Dish/Grid Antennae are the highest gain antennae and in this case given the presented options your bridge link is not long range, as earth bulge is not a factor thus you must use the semi directional.

If I remember correctly, you only have to worry about earth bulge if it is 8 miles or more, not six.   

You are both close, the CWNA course material states 7 miles is limit that mandates earth bulge calculations. I will have to check again about the highly directional antenna comment, I was quite sure there was that type but could be wrong.

Thanks!

According to the newest CWNA Sybex book, there are 3 antenna types, Omnidirectional, semidirectional, and highly direction. That is on page 113. The definition of them are antennas that radiate RF in a fashion similar to the way a spotlight focuses light on a flag or a sign.

 You must keep in mind when taking this test or any technical certification test you have to give the most correct answer. The question clearly states the bridge link is 4 miles. What you are terming a highly directional is limited to two antennae - Parabolic Dish or  Grid Antennae. These are used for bridge connections up to 35 miles in length. You question states that the link is only 4 miles in  length. Earth bulge starts to become a factor at 6-7 miles.

A semi directional antenna beamwidth can vary between 180 to 15 degrees A good dish grid or a parabolic will be in the 5 to 7 degree range on beamwidth with some cheaper variants which much higher beamwidths >14 degrees

Also consider the real world impact. lets say you had to 300 identical setups across the country of PtP's between 3 and 4 miles.

A parabolic dish with a 5 degree beamwidht will run about 300 dollars each so that 600 each w/o shipping and tax per site where as a a decent semi directional costs for two ants goes below $150 w/o shipping & tax. Dollar dollar bills ya'll...

With the greater beamwidt it is also much easier to align the antenna's