Not getting good results with Wilson amp and yagi

powrby4d · EVDO User Joined: 24 Dec 2007 Posts: 27

I calculated (for instance) that 1 watt of transmission power on the 24dBi grid at 3 meters would be 8.94 volts/meter which would appear to exceed the FCC Part 15.209/231 Emission limits. I am a newbie at this and am perfectly happy to have all my numbers or even the place I'm referencing wrong, but more authoritative than anything I might have come up with was the response from Digital Antenna when I asked them exactly the same question on their 489-DB to check my figures:

"When using a high gain antenna such as the 489-DB, it should be a minimum of 6 meters horizontally away from people or a minimum of 6 feet vertically away from people."

Which leads me to believe that someone smarter than I am thinks that understanding of radiated ERP not only in the intended directional LOS but also in the adjacent area is a good safety practice when hooking up high-gain antennas as well. Haven't seen much info about that as a function of posts about folks hooking stuff up so was curious before I find myself getting strangely warm standing next to my grid or yagi.

Thanks for the reply. This is interesting stuff and I enjoy learning from the folks here.

n6gn · Posted: Fri Jul 18, 2008 2:59 pm Post subject:

Well, I'm not a lawyer but let me comment about part of this. Firstly, without spending some time at it, I'm not certain what part of the FCC rules mobile telephones fall under. They are certainly intentional radiators but it isn't clear to me they aren't an exception of the case not requiring individual licenses - which I believe they otherwise fit. I'm not certain what legal control the mobile carriers get with respect to the spectrum they have purchased. So whether it makes sense to try to apply conventional part 15 rules here or not is something I can't answer.

However, let me respond with respect to the safety aspect of transmitters.

While many people have the impression that large, high gain antennas such as dishes and the like "focus a medium or small amount of energy into a dangerous beam" or some similar thought, the OPPOSITE is actually true. Here's why:

Antennas have gain by virtue of their ability to redirect waves. Energy which might have been more omnidirectional with a small/low gain antenna tends to go all in a single direction. Note that energy is conserved. The antenna doesn't increase the total power or energy available, it simply redirects it. There is no way that an antenna can take .1 watt of transmitter power and cause it to boil, burn, mangle or mutilate with the same effect as some much larger amount of power.

Antenna directivity and gain is only obtained in what is called the "far field" of the antenna. This is a distance greater than about twice the antennas physical dimension squared divided by the wavelength. Thus a directive antenna only provides the improvement when one gets far away from the antenna. In the near field, the region closer to the antenna, the power/energy from the transmitter is actually SPREAD out more by larger antennas so a larger antenna creates SMALLER fields in the immediate vicinity.

While this is somewhat counter-intuitive it is still important. It means for example that one ought to be a LOT more concerned about leakage from the kilowatt-class (microwave oven) transmitter in the oven in the kitchen radiating(leaking) out a damaged door seal than of radiation from, say, a really big satellite groundstation with a really large dish. On the former, all the radiating energy is confined to an area of about one-eighth of a square wavelength-around 3 square inches or so for the microwave oven, while for the latter it is spread over the full area of the dish and even then not reaching it's effective maximum until a large distance away from and directly in front of that antenna. Thus, large EIRP from a small transmitter and a big antenna never produces fields that could be more dangerous than the same transmitter connected to a smaller antenna.

I don't know if this is of interest or not, but it's how things are.

n6gn