Antena Omnidirecional 17 dB de ganho - Wirelink - Polarização horizontal
Antena omnidirecional com polarização horizontal para uso em provedores de internet. Ideal para uso em localidades onde existam vários provedores operando na polorização vertical.
Antena omnidirecional com tecnologia "Waveguide Slotted" de polarização horizontal é considerada por especialista a melhor antena para provedores devido o seu ótimo desempenho e baixo ruído. O seu funcionamento na polarização horizontal a torna superior a qualquer outra antena omni existente no mercado devido
ao baixo ingresso de espúrios e interferências (Ruidos) de outras antenas transmitindo na vertical. Fabricada em alumíno escovado proporciona-lhe alta resistencia a chuvas e maior durabilidade sobre as convencionais.
A WLAN receiver can use any of these channels, and can automatically hop from channel to channel if interference is encountered. An 802.11b antenna for the US and Canada should radiate well between 2410 and 2460 MHz.
Slotted Waveguide Antennas
Unlike wideband antennas like the biquad, slotted waveguides are resonant antennas, and have a relatively narrow operating frequency range. The designs I am describing on this page have an adequate bandwidth for any WLAN, but they have been carefully designed and must be equally carefully constructed.
The major attraction of a slotted waveguide design is its simplicity. Once you have built the first one it is very simple to build many more.
The gain varies little across the 802.11b spectrum, dropping a little bit at the extreme ends.
How we Produced These Designs and Charts
These slotted waveguide designs are the result of lengthy simulation using Zeland Software's Fidelity and IE3D electromagnetic simulators. Fidelity is much better at modelling waveguide structures than my favorite simulator, NEC2, but it is quite an expensive package, with a long learning curve...
Simulation can give you much more information about the performance of a microwave antenna than you get from building it. This is because there are severe limitations in the accuracy of measurements at microwave frequencies. Simulation makes it easier to see subtle interdependencies that would be very difficult to measure. In this case, I used the simulation to tell me how the antennas should behave, and then verified the performance both in my lab and on my antenna 'test range'. The results were surprisingly accurate - and attest to the quality of the Zeland Fidelity software.
How a Waveguide Antenna Works
Uma Antena Slot é uma linha de transmissão de baixissima perda. Ela permite nos propagar sinais atraves de um certo numero de pequenas anteninhas (slotes) construidas internamente em sua estrutura. O sinal é acoplado ou colocado dentro da Guia de onda com uma simples ponta de um cabo coaxial ou pino guia, e a medida que o sinal trafega ao longo do tubo guia , este transpassa os slotes micrometricamente cortados e inseridos no tubo. Cada um destes Slotes permite que um pouco da energia seja irradiada para fora ou penetre adentro na perfeita proporção, por isso é uma antena ressonante de banda estreita perfeitamente sintonizada em nosso laboratorio com equipamentos de primeira linha , os Site Maters.
At left we can see a graphical representation of the E field intensity shortly after starting excitation of an 8 slot waveguide. The slots are to the left of the image. The coaxial probe is at the lower end of the image, and the field can be seen to be clumped at maxima every half wavelength as they travel up the waveguide. The waveguide airspace takes up the middle 1/2 of the bluespace, the rest is air infront of (to the left) and behind (to the right) the antenna.
If you click here you can bring up a Windows Media Format Movie version. An MPEG-1 version is at this link. You can see the wave travelling up the waveguide from the probe. The intensity of the E field is given by the color. Here we have primarily blue colors, about -40dB on the final (red) intensity which is achieved once the resonance is fully excited. When the signal first gets to the top and starts reflecting back down the air column it is still green, about -30dB from its ultimate intensity. Reflections are also occuring from the plug at the bottom of the airspace, and the sum effect of all these, including continual drive from the coaxial probe, allows the intensity to build up through yellow and red (0db) signal levels. You can see the signal radiating out the slots at the left of the image. The radiation intensity is less at the top than at the bottom in an 8 slot design, it is hard to radiate perfectly with such a limited number of slots.
Unidirectional Waveguide Antennas
I am going to describe two unidirectional designs. The first has 8 slots and is about 30 inches long. The second has 16 slots and is about 5 feet long. Simple to construct, the 8-slot has been provided as a good starting point for an antenna novice. I built my 8 slot prototype using only hand tools.
High Performance 16 slot Unidirectional
The 16 slot design has been made to radiate over a wider beamwidth by the addition of "wings" to each side of the guide, flush with the front (slotted) surface. It is, of necessity, higher Q, and the higher gain is obtained over a narrower bandwidth. They can be expanded aluminium or sheet, and should extend 9.6 inches beyond the sides of the guide. They act as a ground plane for the slots. Do not change this dimension, it is two electrical wavelengths.
Omnidirectional Slotted Waveguide Antennas
The slotted waveguide has achieved most of its success when used in an omnidirectional role. It is the simplest way to get a real 15dBi gain over 360 degrees of beamwidth.Horizontal Polarization in a wide area network can often double the number of users that can interconnect without interference. When using horizontally polarized BiQuads, or Patch antennas (provided that they have been tested for good cross-polarization performance) at the client site, these omnis will be 20 dB stronger than the signal from a similar vertical collinear. Conversely, vertically polarized receiver antennas will prefer the vertically polarized colinear over the slotted waveguide by a similar amount. Transmission on an immediately adjacent channel, (say, channels 5 or 7), normally not permissible because of interference, is now possible. So a judicious intermingling of horizontally polarized clients can talk with a horizontal central station on the same or adjacent channels that other clients are using with vertical polarization.
To make the unidrectional antenna radiate over the entire 360 degrees of azimuth, a second set of slots are cut in the back face of the waveguide. When looking stright at the face of the waveguide you will be able to see straight through both slots.
Unfortunately, unless a lot of slots are used, the antenna becomes more like a bidirectional radiator, rather than an omnidirectional. This antenna was invented in the 40's, and as our simulation and measurement technologies have become more accurate it is apparent that the slotted waveguide designs we have used in the past are far from optimum. The most common defect is a 'tilt' in the radiation pattern at the extreme ends of the frequency range. This occurs when the wavelength of the signal travelling down the guide differs from the slot spacing.
My current favorite uses 32 slots to get 15dBi of gain, radiated in a uniformly omnidirectional manner. The large number of slots makes it easier to dissipate the energy from the waveguide. Like with the 16 slot unidirectional, two sets of "wings (one set at each slot surface) are required to get equal radiation of energy over a full 360 degrees. Note that a higher Q is necessary to get all the slots illuminated evenly.
Note that the gain vs frequency curve is peaked at 2440, and it radiates well over all 14 channels.
Highly Directional Slotted Waveguide Antennas
Sometimes it is useful to have a highly directional antenna. For example, when installing a point-to-point link between two buildings it is not desirable to have a wide angle of coverage. Any interference from other 802.11b devices (or microwave ovens) that are in the radiation zone will affect your link integrity.
The ideal antenna for such a situation is a dish, such such as the Primestar dish. When using my Biquad feed, it is possible to reject interference outside the dish's primary 5 degree cone by 30 dB or more.
But, if a 16 slot waveguide antenna is turned to a horizontal position, parallel with the ground, it will radiate vertical polarization. Its directivity in this plane is extremely good. As you can see from the diagram to the left, most of the spurious lobes are more than 20 dB down from the main signal, and they are very narrow
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