✔ 最佳答案
Since you have just extracted two sentences in your post without giving any description of the antenna, I presume that your question refer to the type of TV antenna similar to the one given in the below picture.
http://www.selteka.eu/tv%20antenos_FFA_4538.jpg
The dipole is the part on the antenna where electromagentic waves are received. Behind the dipole, there is a reflector. The purpose of the reflector, as its name implies, is to reflect signals back to the dipole such that more signal strength would be received by the dipole. The dipole and the reflector is at a distance of a quarter of a wavelength apart.
Now, consider two waves that originally travel together, the first wave is caught by the dipole. The second wave continues to travel until it is caught by the reflector (i.e. it travels a distance of a quater of wavelength further from the dipole to the reflector), and then reflected to reach and caught by the dipole (i.e. a further of a quarter of wavelength from reflector to dipole). The seond wave therefore travels a total distance of half a wavelength more than the first wave.
Because during reflection, electromagentic theory shows that there is a phase change of pi radians, which corresponding to half of a wavelength. Hence, the second wave (reflected wave) travels a total distance equivalent to a full wavelength. When it meets with the first wave (the one that reaches the dipole direct), the two waves are in phase, and constructive interference occurs and thus gives a strong received signal.
