Efficient Feed for Offset Parabolic Antennas for 2.4 GHz
Dragoslav Dobričić, YU1AW
2.4GHz 抛物面天线的高效馈源
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摘要
This article examines some of the possible solutions to the problems of efficiently illuminating SAT TV offset parabolic antennas in the use on WLAN frequencies of 2.4 GHz..
本文探讨用于2.4GHz频段的卫星电视偏馈抛物面天线的馈源的解决办法。 Introduction
引言
The problems that occur while illuminating shallow offset parabolic dishes, in addition to those related to the efficient use of parabolic dishes generally are additionally aggravated by the specific geometry of the parabolic mirror itself. [2] Feed positioning in the way that its phase center exactly coincides with the focus of the offset parabolic dish and its aiming so that the radiation maximum falls in the geometric center of the elliptic reflector surface are not intuitive at all, as in classic parabolic antennas. Therefore, there is much confusion and many wrongly positioned feeds that do not correctly illuminate offset parabolic dishes, decreasing their efficiency and gain.
这个问题时常存在于一些浅照射的偏馈抛物面天线,除了和抛物面天线有效利用有关之外，抛物面反射镜本身的具体尺寸[2]又使之更加严重。馈源的定位方式——其相位中心必须严格与偏馈抛物面天线的焦距保持一致，而且对准。这样，其最大辐射值落在椭圆形反射面的几何中心，是不直观的，正如在经典抛物线天线那样。因此，有许多混乱，许多错误的馈源定位，不能正确地照射偏馈抛物面天线，降低了它们的效率和增益。
The optimal feed for some given parabola has to fulfill several important characteristics:
对于给定抛物面天线最佳馈源必须具有几个重要特点：
1.The radiation angle of the main beam, between the points in which the gain is -10dB in relation to the maximal value, has to match the subtended angle. The feed radiation angle, both in horizontal and in vertical plane has to be the same, regardless the ellipticity of offset parabola.
1、主束的辐射角度，在辐射功率为-10dB的点到最大值之间，必须与张角相配。馈源的辐射角在水平和垂直都应当相同。而不管偏馈抛物面的椭圆率如何。
2.The phase center of the feed has to be well defined and stable with changes of frequencies within the working range. The change of the phase within the whole angle of illumination has to be as small as possible.
2、在其工作范围内更换工作频率时，馈源的相位中心必须精准、稳定。整个照射角度的相位变化必须尽可能地小。
3.The feed characteristics must not change much in the presence of the parabolic reflector and carrier structure. 3、在抛物面反射器和托架前面，馈源的特征不能有太大变化。
4.Feed radiation diagram has to be very clean, i.e., with low side lobes and rear lobes.
4、馈源方向图必须非常干净，即，旁瓣和后瓣应非常低。
5.The feed structure has to encroach as little as possible into the focal cone, i.e. in the space between focus and the antenna surface. Therefore, it is good when the feed phase center is on the front edge or directly in front of the antenna structure.
5、馈源结构必须尽可能地少地伸进焦点椎体中，即，焦点和天线表面之间的空间。为此，馈源相位中心最好在前沿，或直接在（馈源）天线结构的前面。It is not easy at all to accomplish all these demands. The efficiency and gain of parabolic antennas directly depend on the mode of accomplishing these demands. Therefore, in practice, it is common to make good feed first and then to choose or make a parabolic reflector with a F/D value that fits the best with the feed. [4, 5] However, if you want to use the cheap production of SAT TV offset parabolas for the work on HAM or WLAN frequencies; you have to try to construct a feed that matches those parabolic reflectors. SAT TV antennas usually have F/D in band from 0.7 to 0.9.
达到这些要求并不容易。抛物面天线的效率和增益完全取决于实现这些要求的方式。为此，在实践中，通常首先要制作一个好的馈源，然后再选择或制作一个F/D值最大程度地适合馈源的抛物面反射器。无论如何，如果你想用一个低制作成本的卫星电视偏馈抛物面天线，工作于业余无线电频段或者WLAN波段，你必须制作一个与现有抛物面反射器相配的馈源。卫星电视天线的F/D一般都在0.7-0.9范围。
For efficient illumination, we need a feed with a clean diagram that has equal width of the main beam in both planes and gain of about 12-14 dBi. This fact at the start excludes some antennas as efficient feeds for SAT TV offset parabolas. Among them is, for example, the coffee can (simple open circular waveguide) antenna that has gain of about 6-7 dBi and is very inefficient as a feed for offset parabolas. It is acceptable only for parabolas that have F/D less than about 0.5. The bi-quad antenna is somewhat better, with its gain of about 10 dBi, and its optimal version with evened diagrams in both planes and a gain of 11 dBi is even better.
为了高效照射，我们需要一个有干净的方向图的馈源，主束在水平面和垂直面要等宽，增益大约12-14dB。这实际就把一些做为卫星电视偏馈抛物面天线的高效馈源排除在外了。它们是，例如，咖啡筒（简单的开口圆形波导）天线有大约6-7dBi的增益，做偏馈抛物面天线的馈源是非常低效的。可以接受它的只有F/D小于0.5的抛物面天线。菱形天线稍微好一些，大约有10dBi的增益，其最佳版本的水平和垂直方向图相等，增益大约11dBi甚至更高。
