Principles and differences between PIFA and monopole antennas

1. PIFA antenna evolved from a micro-band antenna. A lot of English documents introduce patch antenna. We suggest you look at the basic principles. The simplest patch antenna is a metal plate placed on the ground plane in parallel and fed with the same axis or microline. Its radiation mainly relies on edge fields. Assuming that the antenna is placed in parallel to the earth, the shape of the antenna is rectangular, the long side is placed on the left and right, and the length of the long side is 1/4 wavelength. If the field on the left edge is from patch to ground, the right edge is just reverse from ground to breaking the electric field on the left and right edges into horizontal and vertical components, and you will find that the vertical components are offset, the horizontal component is enhanced. This will generate a line polarization far field parallel to the ground plane. For mobile phones, the main polarization of the PIFA antenna is generally parallel to the main ground plane of the mobile phone. At this point, we can get two basic conclusions: 1) the resonant wavelength of the antenna is four times that of the long side of the patch (in practice, consider the shortening of the wavelength of the media, proportional to 1/SQRT (epsilon); 2) the radiation of this antenna depends mainly on the edges. While the edge field is tilted outward, the better the radiation (open field ). That's why the height of the PIFA antenna is so important.

2. After adding a ground film (many of which are near the feed), the current on the patch will change from the microscopic point of view, and some current will flow back from the right side and then return to the ground. In this way, the resonance frequency of the antenna will be reduced. Generally, the wavelength will be 4 times the sum of the long side and short side of the patch (the wavelength shortening effect should also be considered ). From another perspective, the feed column and short-circuit column are two-line transmission lines. It will transform the impedance of the antenna. It is a transformer effect, which changes some of the resistance to inductance, so that the entire antenna is resonant. The longer the line (the maximum length is 1/4 wavelength), the more obvious the change effect (the more sensitive, the higher the antenna height ). The principle of transmission transformation should be clear. When changing the horizontal size of the feed column and short-circuit column or the distance between them, you are actually changing the characteristic impedance of the transmission line. The section of the square in the transformation formula is changed accordingly. This is why we often say that the changes in feed power and short circuit will greatly change the impedance of the antenna. It is also why PIFA antennas can be optimized without matching the circuit (in fact, adding matching sometimes reduces the antenna's transmission performance ).

3. The explanation of this problem is that the configuration may be clearer. There are indeed some problems that require a solid theoretical foundation and practical experience to be deeply understood. We recommend that you be solid at work. In my personal experience, the data in actual work needs to be carefully analyzed, and why should I find the difference. The difference here may be that this result is different from the previous one, or the result is different from what you think, or the result is different from others. When you find out why it is different, you will have a deep understanding and make progress. Sometimes it takes a lot of other knowledge to find out the difference. You also need to find a way to learn (review) and increase your knowledge accordingly.

The Chinese meaning of monopole is a single pole, but its actual working principle is not the same as its name.

Starting from the short dipoles, the alternating current (field) is generated when the charge on the two arm is positive and negative and returns a sine. The half-wave vibrator has 1/4 wavelengths for the upper and lower arm. The current size of the upper and lower arm has the same symmetric flow direction (in pairs of positive and negative charges). The current intensity distribution is gradually increased from the center feed point to the two ends. The current at the feed point is the largest, and the resistance is the smallest (because the resonance has no impedance. Such an antenna is a balanced antenna (the current is balanced between the upper and lower arm of the antenna ).

Now, the one arm of the dipolar antenna is removed, and the other arm is replaced with the infinite Earth. The earth reflects the field. According to the image principle, a positive charge will sense a negative charge in its image. At this time, the upper arm of the antenna will generate an image where the current distribution is exactly the same as that of the lower arm of the dipolar antenna. In this case, we call this antenna a single pole antenna. For infinite Earth, the radiation map is equivalent to a dipole. If it is gradually reduced, it will not be able to travel the ideal image, and the current distribution below will change.

Today's mobile phones are getting smaller and smaller, and the body cannot be regarded as the earth. The smaller the body, the larger the single pole antenna in the mobile phone is affected by the mobile phone, because the mobile phone has become another arm of the antenna. This antenna is a non-balanced antenna. The current distribution on the antenna (system) will be significantly affected by the mobile phone body. When the maximum cell phone size is less than 1/4 wavelength of the minimum system frequency, the antenna system will not be able to produce resonance at this frequency point. For example, in the design of the AMPs and PCs dual-band systems, the AMPs frequency band requires the length of the body to reach at least 1/4 wavelength of MHz. The smaller the mobile phone, the more likely the antenna is to be affected by the flip, and so on...