Practical experience with a Cantenna

"Jon Arbuckle" hath wroth:

I live in a very thick over grown oak, redwood and douglas fir forest and have considerable experience trying to shoot through the trees. I suggest you give up now. The water in the trees are like a brick wall. The trunks, are impervious to RF. At one mile, the Fresnel Zone is about 19ft radius at midpoint. That means you'll need to have a 38 ft wide hole at midpoint in order for the path to be reliable. It also means that your antennas at both ends will need to be at least

19ft off the ground.

I've also gone through the trouble of mapping the local access points and some clients in the area. Using this map, I've played with various antennas on my rooftop trying to see what works best for "drilling" through the trees. I've also tried to setup a WISP (wireless ISP) in a nearby town also full of trees.

What happens is that if you can visually see the other end of the link, it will work, but may not stay working. For example, we have a neighborhood WLAN/LAN with about 6 houses on the wireless system. All are line of sight, and most have at least one tree in the way. I can't go through the trunks at all. The leaves allow some penetration, but when it rains, those go dead. The only long shot that's stable is a 400ft shot with clear line of sight.

I also have a series of point to point links scattered around the forest using panel and dish antennas. They're also affected by the weather but have sufficient fade margin to prevent total loss of signal. The biggest source of unreliability over the years has been squirrels chewing on the cable and lightning hits.

Even if you get 24dBi dish antennas at each end of the link, you're going to have problems going through the trees. A can antenna is about 8-10dBi gain, which is insufficient for a 1 mile link even without the trees.

Isn't there some magic relationship between H2O and 2400Mhz? Like water absorbs the 2.4G RF most efficiently, and thus microwave ovens use this frequency.

Wifi isn't all that far away in frequency.

Actually that's urban myth. Microwave ovens use 2.4 GHz because its in the unlicensed ISM band.

[snipped from Microwave heating is sometimes incorrectly explained as a rotational resonance of water molecules, but this is incorrect: such resonance only occurs at much higher frequencies, in the tens of gigahertz. Moreover, large industrial/commercial microwave ovens operating in the 900 MHz range also heat water and food perfectly well.

Also,

Well, that certainly clears up a lot of bull I've been told over the years. Now if we can just get to the bottom of the Richard Gere gerbil caper....

Microwave ovens were using 2.4Ghz way before people jumped on the 2.4Ghz for unlicensed radio use. In fact microwave ovens were already listed as being potential interferers to 2.4Ghz radios.

has a high percentage of water, and water is famously H2O. The molecule of water has the O (Oxygen) in the middle, and the two H's (Hydrogen) stuck on it like Mickey Mouse ears at a particular angle (105o). The H's are positive and the O is negative, so the molecule has a + and - end. It has "polarity". Polarised molecules try to line themselves up with the electrical field, like compass needles trying to point at North. But because the electrical field is changing 2,450 million times a second the molecules don't quite have time to line up one way before they have to try to line up the other way! So, anything with water in it has all these molecules being moved this way and that by the electrical field, and heated up.

The microwave radiation produces heat inside the food in the oven. Heat is produced when the water molecules in the food vibrate (at a rate of

2,450,000,000 times per second) when the food absorbs the microwave radiation. The movement of the molecules produce friction which causes heat. This heat cooks or warms up the food. >

Microwave ovens were using 2.4Ghz way before people jumped on the 2.4Ghz for unlicensed radio use. In fact microwave ovens were already listed as being potential interferers to 2.4Ghz radios.

has a high percentage of water, and water is famously H2O. The molecule of water has the O (Oxygen) in the middle, and the two H's (Hydrogen) stuck on it like Mickey Mouse ears at a particular angle (105o). The H's are positive and the O is negative, so the molecule has a + and - end. It has "polarity". Polarised molecules try to line themselves up with the electrical field, like compass needles trying to point at North. But because the electrical field is changing 2,450 million times a second the molecules don't quite have time to line up one way before they have to try to line up the other way! So, anything with water in it has all these molecules being moved this way and that by the electrical field, and heated up.

The microwave radiation produces heat inside the food in the oven. Heat is produced when the water molecules in the food vibrate (at a rate of

2,450,000,000 times per second) when the food absorbs the microwave radiation. The movement of the molecules produce friction which causes heat. This heat cooks or warms up the food.

[snipped from Medical literature, which covers examples of items retrieved from patients' rectums in extreme detail, has never recorded a case of an animal being removed from a patient, nor of damage inflicted on a patient's insides due to rectal insertion of an animal.