How far, is far enough?

Over the Christmas break, I wanted to compensate a few of my Proxim WiFi adapters so that I knew exactly how different they were when measuring RSSI. There are countless write ups detailing how and why we need to compensate our adapters, and the methodology behind doing so. The one thing that kept jumping out at me was how far do I have to be from the AP, in order to reliably compensate WiFi adapters? I read some articles that have said we need to be X distance, and other articles claim Y distance..so which is it? I live in an apartment and as such I don’t have a clear long 30′ hall to measure against in. Can I reliably compensate adapters at say, 10′ , but more specifically what is the actual distance required to be in the far field?

For starters, why do we need to be a certain distance from the AP to begin with? If an AP is mounted to a 10′ ceiling, is sitting directly under it too close to reliably compensate my adapters?

The answer lies in the math.

In order to properly be at the correct distance, we need to ensure that our receivers are located in what is called the Far Field. The Far Field is where we can predictably and accurately model the RF behavior with tools like CST, and its where the RF has “calmed down and normalized” – this is the zone that clients will live in.

Overall, the Far Field is the region that is far enough away from the antenna, that the behavior can reliably be modeled and calculated. This is the “normal operation zone” for antennas.

Lets explore the idea of far field so that we may be able to know weather or not we are truly in this “normal operating zone”.

In the world of antennas, there are lots of different types. From Patch Antennas, to Horns, Monopole, Dipole, the list goes on and on. For the scope of this post, I will concentrate around the traditional Half-Wave Dipole Antenna, its far field characteristics, & how to calculate the far field.

What exactly is a Dipole Antenna? This type of antenna configuration has 2 poles(ends) where AC current conducts through each pole 180° out of phase. A Half-Wave dipole is the most common type of Dipole utilized due to the physical space savings when compared to a Monopole.  The characteristic radiation pattern yields the main power lobe orthogonal to the radiating element.

Dipole Radation Pattern

 

Having the understanding of the basic radiation pattern, we can now look at the governing math behind a Hertzian Diploe.  The Hertzian dipole is a theoretical dipole antenna that consists of an infinitesimally small current source acting in free-space. Although a true Hertzian dipole cannot physically exist, very short dipole antennas can make for a reasonable approximation. The length of this antenna is significantly smaller than the wavelength:

small lambda

A surprising result is that even though the Hertzian dipole is minute, its effective aperture is comparable to antennas many times its size. This allows us to make calculations around characteristics such as the Far Field Conditions.

Field Regions

 

In order for us to know when we are actually in the far field, we have to actually find out where the far field is located.  We need to define the following;

  1. Wavelength λ @ 5.8GHz;
  2. Speed of Light; c = 3E8 m/s
  3. Frequency f5.8E9 Hz

lambda

Plugging in these variables into the above equation, we find that λ = .0516m, or 5.16cm. Half of this length is the dipole antenna length (as we are utilizing a half-wave dipole antenna) therefore, D~ 2.58cm

Far Field eqns

Being that were using a half-wave dipole, D= λ/2 = 2.58cm. For most cases, a half-wave dipole is going to have an antenna length between .33λ and 2.5λ. This means that we are finally in the far field region at 2.5λFor a 5.8GHz signal, 2.5λ= 12.9 cm. Thus, when we are right about 6in away from the AP, we are barely in the far field and will start to have predictable behavior as we move further away.

So what does all this really mean? Welp, you will see lots of heuristics out there that talk about how far you need to be in order to properly compensate WiFi adapters. Based on the mathematics involved, any distance greater than the 2.5λ value for a half-wave dipole should be fine for our receivers. Personally, I like using the 2-3m range. It’s relatively easy to eye-ball,  and my survey tripod just happens to extend up to 10′ – so this is my “minimum distance” that I use when compensate my adapters. It also just happens to be about the height of APs mounted to a drop ceiling in an office environment.

Happy Surveying!

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