Beamforming : what is the key to maximum gain ? - Page 1
Forum section: electronics / rf-microwave

author: jujun
author_rank: Regular Contributor
timestamp: December 16, 2018, 11:00:08 am
content: Hello,These days I read about beamforming. I wonder what is the main factor to have the highest gain.To me it sounds like the best way is to have a maximum amount of antennas, but what about the space between them ?Maybe with a good space pattern between antennas it's possible to improve the gain ?Do you have any ideas about it ? or any pointer to be able to know more about it ?Thank youJ

author: mark03
author_rank: Frequent Contributor
timestamp: December 16, 2018, 05:28:08 pm
content: The beamwidth (which I guess is inversely proportional to gain) is ultimately limited by only one thing: the aperture of the array, measured in wavelengths. Imagine replacing your array of antennas with a single big elliptical dish, having the same overall dimensions. The major and minor axes of the ellipse determine the beamwidth in the corresponding plane. There is no arrangement of antennas which can beat the gain of this dish, if the antennas fit inside the imaginary ellipse.Far away from your antenna array, the radiation pattern in azimuth / elevation is the 2-D Fourier transform of the array. Careful thinking about this can help us understand factors like the required spacing between antennas. E.g. antennas too far apart => "grating lobes" at radiation angles far from the desired forward direction. Generally the spacing requirement is 1/2 wavelength for point sources, to avoid grating lobes. A situation like an array of VHF/UHF yagis will be somewhat more complicated, of course, because each array element has its own directivity (not a point source). Then they can be spaced further apart.

author: TheUnnamedNewbie
author_rank: Super Contributor
timestamp: December 17, 2018, 12:18:44 pm
content: Quote from: mark03 on December 16, 2018, 05:28:08 pmI'm not 100% sure this is correct - horn, Yagi-Uda and Vivaldi antennas, for example, all show gain and an effective aperture that is larger than the physical one if my memory serves me right.The gain of an array (G) can be approximated as\$ G = G_{el} \cdot AF\$where Gel is the gain of an isolated ellement and AF is the array factor. This shows that in order to increase array gain, either the gain of an individual element can be increased to increase gain (more directive element), or the array factor (generally achieved through more elements). In many cases, for real beamforming, we want to be able to control the direction of the beam somehow (either passively in the feed network, or with active phase shifters, which would make it a beam-steering array). In the latter it's common to use somewhat broad-beam elements so we can steer the array better.

author: GeoffreyF
author_rank: Regular Contributor
timestamp: December 26, 2018, 06:44:46 pm
content: "Gain" in an antenna refers to more gain in one direction than others. Decibels is always in relation to something. In sound it is relative to a minimum amount of sound (a Bel). In RF it is usually DBi or DBd. DBi is relative to a hypothetical antenna that radiates equally in all directions and is called "Isotropic". DBd is relative to an actual antenna called a Dipole that ideally radiates in a donut shape, but where that shape is relative to the ground and other obstructions. An ideal dipole has a gain of 2.1 times an Isotropic.Seeking maximum gain from something such as a yagi actually has diminishing returns and cannot be fully accomplished. There has to be some amount of energy in the opposite direction.An antenna can also be less than resonant in which case power from the transmitter is lost to heat. Standing Wave Ratio is the measurement for antenna impedance matching. Power is also lost to heat in the feed line or other devices along the way.Putting all this together, how effective an antenna system is has to do with all these factors together. Beam forming while ignoring impedance matching is also a fools errand.

author: Howardlong
author_rank: Super Contributor
timestamp: December 26, 2018, 07:37:45 pm
content: It's been some years since I did any practical work on phasing Yagis (and axial helices) specifically, but optimum spacing depends on gain of the component antennas, with larger gain (or aperture) meaning wider spacing to reduce side lobes.The same very much applies to patch antennas that I have much more experience with.essentially, once you have your component antenna's effective aperture area and wavelength you can get to the spacing.Here's a spreadsheet I used to play with, and will also deals with steerable arrays, unfortunately it only deals with an odd number of elements, but you could re-do it to deal with even elements.https://www.microwaves101.com/uploads/Phased-array-101-Rev1-0.xlsIn practice, it's always a classic engineering practical compromise. You have a list of requirements with varying priorities. Here are a few.In non-steerable arrays using passive power splitters, increasing losses in the transmission lines become a key element in larger arrays.Patch array antennas can be cheaply integrated with transmission line transformers and power splitters in the microwaves and can be fabricated very cheaply and accurately onto PCBs assuming the substrate's not too lossy at the given frequency.This contrasts with Yagis that need close engineering tolerances if they're to perform well at higher frequencies, not to mention the complexity and cost of implementing power splitters, which at lower frequencies will be a spaghetti of power splitters and phase matched cabling. However, Yagis have comparatively low wind load, so tend to be chosen at lower frequencies.In some scenarios placing an LNA and/or HPA at each driven element will offset the transmission line losses, and for steerable phased arrays this is often the norm anyway.One further note is that any solution is application dependent. Although passive antennas themselves are reciprocal devices, there are sometimes different priorities for receive systems and transmit systems. For receive we are looking for maximum S/N, i.e. high gain and low noise, so greater front to back ratio and side lobe minimisation may be more of a priority for receive than for transmit. This is especially true for space communications, where the warm earth is typically behind the antenna, and will add a lot of noise, offsetting the raw forward gain performance. For transmitting, typically forward gain is our single most important parameter, with front to back ratio and sidelobes far less of a concern.

author: mark03
author_rank: Frequent Contributor
timestamp: December 27, 2018, 06:29:24 pm
content: Quote from: TheUnnamedNewbie on December 17, 2018, 12:18:44 pmYou are probably right. I was hoping someone else would post to clarify this as it's not quite intuitive to me yet. I've got a ham license but my main point of reference for beamforming is acoustics (ultrasound). There we are dealing with simple radiators with backing material to ensure radiation only comes out the front. I suppose the trick with yagis (at least) is that you are taking some of the backward radiation and redirecting it forward. This can be done without increasing the effective aperture, and it provides gain.In any case, the same basic principles apply to array construction and spacing: the more directive your individual antennas are, the farther you can space them apart. Referring back to the Fourier transform rule, array construction can be viewed as an exercise in sampling theory (sampling in space not time). When your samples are too far apart, higher frequencies will be aliased (== grating lobes at large angles from the array normal). If you  low-pass filter the signal before sampling (by using antennas with higher gain i.e. a narrow beamwidth), then it is ok for the samples (antennas) to be further apart.