วันพฤหัสบดีที่ 17 มิถุนายน พ.ศ. 2553

Antenna Patterns — What Do They Mean? รูปแบบการแพร่กระจายคลื่น เขาหมายถึงอะไร ?

บทความที่เกี่ยวกับสายอากาศส่วนมากแล้วจะประกอบด้วยรูปแบบการแพร่กระจายคลื่น แต่ว่าเราจะตีความหมายของรูปนี้ได้อย่างไร?

Most antenna articles include these diagrams — but how do you interpret them?
บทความโดย Joel R. Hallas, W1ZR จากหนังสือ QST (ฉบับ December 2009)

Atruth about antennas is that all real antennas radiate better in some directions than in others. It is thus important for hams to know which directions will work best, so the appropriate antenna, antenna height and orientation can be selected. A good way to describe how antennas perform is to plot their response. To some of us, the meaning of such plots is intuitive — but perhaps that’s not true of all our readers. In this short article, we’ll try to take some of the mystery out of this topic.

For a horizontal half wave dipole, the radiating field that leaves with maximum strength is in a direction perpendicular to the dipole. The field strength is reduced as we move in the direction of the ends. Near the earth, the field cancels at the horizon and increases to a maximum at some angle above the horizon depending on how high off the ground the antenna is.

Plotting Antenna Response

Rather than discuss this in general terms, we can represent the directional effects of the antenna in plotted form that can be easily interpreted. We could plot the field strength as a function of azimuth angle, the angle moving from, for example, far left to far right at a particular elevation angle, using familiar rectangular coordinates. We have plotted both the relative strength of the radiated power of such a half-wave antenna in Figure 1.











Figure 1 — Calculated relative field strength shown in rectangular coordinates.



Polar Plots

While a representation such as Figure 1 is correct and useful, it is more common to show the information in something called a polar plot. This kind of plot represents the intensity in a particular direction by the length of a line from the center of the plot to the curve at any angle. This gives what seems to me to be a more intuitive view of the performance of the antenna as a function of the angle.















Figure 2 — Polar plot of modeled relative
power vs elevation angle. The end view of the wire antenna is shown as a dot. The green arrow points in the direction of maximum signal strength and its length represents the maximum signal amplitude. The red arrows point to the elevation angles at which the relative power has dropped to half that (–3 dB) of the maximum signal strength; the angle between them is the 3 dB vertical beamwidth.

Figure 2 is a representation of the field strength from a thin 40 meter dipole 60 feet above typical ground. This is shown as a function of elevation angle taken in the direction perpendicular to the antenna wire, the azimuth angle of maximum output. Note that the field at the horizon is zero, as we would expect for a horizontal antenna over real earth. Note also that at this height of 60 feet, the radiation upward is reduced and the maximum radiation is at an elevation angle of 32° above the horizon. It is easy to see from the rings, shown in decibels, that the response straight up is down 10 dB from the maximum. This will be different for different heights. Although this antenna is bidirectional, only one set of arrows is shown.




















Figure 3 — Polar plot of modeled signal strength in decibels (dB) as a function of azimuth angle at the maximum elevation angle, 32° for this height. The antenna orientation is shown, so consider this looking down from above. As in the elevation pattern, the green arrow points to the azimuth angle of maximum strength at 90°, or perpendicular to the wire, as we would expect. The red arrows point to the azimuth angles at which the power has dropped to half the maximum, the 3 dB horizontal beamwidth.

Figure 3 is a polar plot of the radiation from the same dipole at an elevation angle of 32°, the angle of maximum radiation. The angles correspond to the same angles as shown in Figure 1 and represent the relative strength in different directions going around the antenna from 0° at a wire end all the way around clockwise to the same point, 360°. The relative power is generally shown in decibels (dB), a convenient logarithmic representation that makes it easy to add up system gains and losses. Note that the dB scale is compressed to show additional detail in the more significant portion.