seen so many , omni , yaggi , panel etc
id like a nicely explained site to shwo how each of their pros and cons and what sort of performance can be had , this is my latest hobby lol
This shud do it :
http://www.qsl.net/n1bwt/contents.htm
I doubt it, unless he wants to understand the theory behind antenna design and radio propagation. That site is designed for (and by) radio amateurs for, essentially, construction purposes.
weebroonieuk, if you're just looking for guidance on what to buy, it's not going to be as simple as a chart on antenna design models, I'm afraid.
For a start, think on this. Radio is a very large spectrum, from the extremely slow and low frequency stuff that can be used to talk to submarines at sea, or deep into the earth, right up into the microwave band, and that is why wifi resides.
Next, the design of an antenna depends on the frequency, because any antenna will only be absolutely optimal at one specific frequency, so if you're going to cover a band, you want that optimal frequency to be in the middle of the band.
Next, and this is a bit simplistic but broadly true, the size of the bit of the antenna that does the actual transmitting or receiving is going to be inversely related to frequency .... the lower the frequency, the larger the active element. An amateur HF beam is large enough that it makes many houses look small. We're talking many metres, and sometimes, tens of metres, even for a smallish Yagi design. A microwave horn, on the other hand, could fit in the palm of your hand.
The trouble is, as the frequency goes up and antenna size goes down, so extreme accuracy in manufacturing becomes more and more important. If I were to tell you that the most effective and efficient design is a horn, it won't help you much because a poorly made horn could be hopelessly inefficient.
Yagi antennas generally aren't popular at microwave, because, mainly, of the factors I've mentioned. All a Yagi is, broadly speaking, is a boom with a series of elements placed along it. Each element will be of a given diameter, a given length and at a given distance from other elements. One element will be the actual radiating element (o.e. the bit connected to the transceiver), and it will have one reflecting element behind it (a kind of radio mirror) and a series if directors in front. How many directors there are determines how directional the who antenna is. The more elements, the better the antenna will be (all other things being optimal) at concentrating transmitted power in a given direction, or in receiving from a given direction.
That, of course, has one obvious implication - you need to know which direction to point the antenna. So, either you're interested in a specific direction, or you have a rotator capability to change where it's pointing.
But, with a Yagi, the fact that you're at very high frequencies gives some manufacturing problems, in that those elements need to be very thin, and very close together, and that makes the whole thing very fragile.
The dish idea (like a satellite dish) is effectively one active element (the LNB, the bit on the end of the little arm). The dish is a parabolic reflector (again, a radio mirror) which collects signals over the surface of the dish, and being a parabola, reflects them back at a single point, that being precisely where you stick the LNB. Again, manufacturing (and assembly) tolerances are pretty small, but the sensitivity of the whole assembly will depend, in part, on the size of the dish. The dish is like a fishing net .... the bigger the net, the more fish you'll catch. Unfortunately, also like a fishing net, the bigger the net the bigger the amount of the wrong type of fish you'll catch. i.e. higher collection of noise too.
So, while perhaps both dishes, a radio telescope or early warning radar station will be a different size, and design, to a high frequency, point-to-point telecoms link.
Even performance isn't easy to judge. You'll often see manufacturers quoting gain figures in dB, but since dB is a relative measurement, it's a meaningless figure unless you know what it's relative to. dBi and dBd are different, for example, where dBi is relative to an isotropic element, and dBd to a dipole. Given that an isotropic antenna is a theoretical point source radiating squally in all directions, a dBi figure tells you a limited amount about actual performance, because it's ion relation to a non-existent hypothetical antenna and is calculated mathematically, not by comparative measurement. It's good idea in theory, because it gives you a way to compare different designs, but in practice, you're depending entirely on manufacturers methods of doing the calculations, and manufacturers would never sex up figures to hype their product, would they?
Essentially, unless you have a very specific usage in mind, a couple of simple points will be all you need. First, you typically, and up to a point, get what you pay for. Secondly, any directional design (horn, dish, Yagi, etc) will be directional. Some are more directional than others. A directional antenna is great if the other part of the system is in the specified direction, and hopeless if it is not. So if you're after a link between yourself and a mate's house a few hundred yards away, directional is the way to go. But you'll effectively need clear line of sight.
If, on the other hand, you're after something to boost signal in your house, but you mean all over the house, then you really need something omnidirectional, because a directional antenna is likely to be great in one direction, and totally useless in other.
Finally, you're likely to find that where you put an antenna has more impact inside a house than the design of it. You want to be as close to the centre of the house as you can get, and you want the antenna fairly high up in the room, and you want to ensure you have no large, dense objects between it and wherever you'll be connecting from.
Just as with size, the ability of radio to penetrate objects in inverse to frequency. The higher the frequency, the less able it is go get round, or through, objects in the way. If you put a microwave repeater the other side of a large fridge freezer, then you can probably assume you won't even be able to tell it's there, much less connect via it. If you're connecting to a mate's house, try to avoid pointing via trees, especially when they're wet. It's best to avoid walls where possible, or at least, to minimise the number of them. For an external link, site the dish outside and run cable inside, but also be aware, the length of capable has an impact, as does it's quality. At least with wifi, you're talking about very low power levels, because cable for high frequencies that's carrying any real power can get seriously expensive, and mechanically very awkward to use, because impedance becomes a major issue, as does mechanical connectivity.
So, if you do find any "simple charts", treat them with a degree of caution.
PS. For any purists or radio types out there (and I know we have some on here), yes I know I've taken some outrageous liberties with gross oversimplifications. I'm just trying to get the points across in broad terms.
watercooled (16-07-2010)
It was the OPs comment about it being his new hobby that prompted me to send him there. I had visions of him making baked bean can antennae. Personally, for wifi, if I wanted to "go mega" Im guessing Id be looking at a phased dipole array, or a co-linear ? Saracen, u certainly seem to know a heck of a lot bout radio
But yes, I do agree with (most) of your points... I spose being a radio ham myself does tend to give me a slight bias... Are u one of us too ?
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