“Everything should be as simple as possible, but not simpler.”
As a senior engineer focus on 4G/LTE over 10 years, I met many kinds of problems about signal transmit. But as my summarizing, many problems are caused by antenna design. Different position, different antenna type, different orientations, event different size……
To sum up, in the this article, I will systematically share with my experience about the antenna design.
1. Antenna design flow
Typically, the design flow of the antenna is shown below.
In the process, we should follow the key steps, describing as follows:
1) When the structure is stacked, the antenna supplier has to join the work, evaluate the antenna structure, and initially determine the antenna type, so we can save many time to change.
2) When designing the PCB, it is necessary to reserve the space for the antenna according to the type of the antenna. But if the PCB revised, the antenna needs to be re-commissioned.
3) When the antenna supplier debugging the antenna, determine the shape of the antenna according to the previous evaluation and the actual environment, so we can determine the antenna matching circuit value;
4) Antenna test verification is mainly divided into passive and active. Passive includes indicators such as pattern, gain, input impedance, and efficiency. The active includes transmit power and receive sensitivity indicators.
Antenna design is art. How to draw it beautiful, it’s art.That is to say, it’s about creation:
1) In terms of appearance, most antennas have a certain geometry, and for different products, even if the antenna has the same operating frequency range, its shape is quite different. This means that the antenna is a component that is closely related to the structure. In most cases, changes in the product structure often affect the performance of the antenna, therefore, we have to re-tuning the antenna.
2) In reality, many products have not fully considered the impact of the structure on the antenna design at the beginning of the design, which makes the design and debugging of the antenna in the later stage more difficult, and even the antenna function cannot be realized. Therefore, it is important to consider the antenna design as early as possible in the design process of the product. I suggest the antenna supplier join the design at the beginning of product design.
2, Antenna design considerations
1). Before the antenna debug, at the first, we have to determine the working frequency band. Different frequency bands, different judgment standard and different form.
2) Do you know how many kinds of antennas? Generally, depending on the location, the antenna has two types of built-in and external types. Built-in antennas include: Monopole antenna, PIFA antenna, patch ceramic antenna, FPC antenna, etc. External antennas are: rod antenna, whip antenna, spiral antenna, car antenna, etc., as shown in the following figure.
3. Four tips tell you how to select antenna in different situation:
1). Harsh environment
Such as more metal objects around the product, the signal is easily blocked, like car products, wireless meter reading products, wireless public telephone products, etc., therefore, I suggest external antenna.
2). With metal structural parts inside
This structure is not conducive to the realization of the built-in antenna, therefore, I suggest external antenna.
3). With many strong interference sources or particularly sensitive circuits inside the product
Such as embedded application processor circuit, memory circuit, high-speed digital signal processing circuit, clock oscillation circuit and switching power supply circuit, etc., therefore, I suggest external antenna, too, with away from strong interference sources and sensitive circuits.
4）. If built-in antenna, I suggest to consider the structure, cost and requirements of performance. For example, there are 5tips about selecting:
A. Keep open around the antenna, avoid large-volume metal devices;
B. Do not use metal spray on the outer casing of the product corresponding to the antenna area;
C. Place antenna away from the handheld area;
D. If product installing on a metal countertop or wall, pay attention to metal close to the antenna.
E. Place away from the strong interference source inside the product. Shielding, filtering, and other way, therefore, to avoid the interference
4. Match impedance of the antenna
If the port impedance of the antenna is not 50 ohms, then impedance between the transmission line and the antenna end will be mismatched. We’d add a matching network to the antenna input. As a result, I suggest pi type antenna matching circuit.
5. Antenna PCB design
The antenna part PCB design mainly includes the following points:
PCB layout and routing
1) When placing the PCB, place the RF output near the antenna to minimize the length of the RF trace.
2) The RF trace is 50 ohm impedance control, and the trace has a complete reference ground plane;
3) adjacent layers of the RF line prohibit any other signal lines from crossing or crossing;
4) Multiple ground holes on both sides of the RF line to shorten the signal return path and enhance signal integrity;
Process PCB in antenna area
Differences PCB processing for different antenna forms:
1) I suggest to use RF connector, includes HRS/Murata’s ultra-small RF coaxial connector, MM8430-2610 RF coaxial connector with switching function. On the other hand, for cost savings, pad connections could work, too , recommended size is 1.5mm*1.5mm
2) If the slab structure is larger than two layers, the pad directly refers to the last layer, but all the layers in the middle are hollowed out to keep the 50 ohm impedance continuous.
3) During soldering , the RF line should be on the same line with RF trace on the PCB to reduce the reflection loss of the RF signal.
1) How many antenna feed points? any other requirements ? Please listen to the antenna supplier’s recommendations;
2) The size of the antenna feed point is appropriate. That is to say, my opinion is 2mm × 3mm, and the pitch between the feed point and the ground PAD is 2mm.
3) The antenna feed point and the ground PAD are placed close to the edge of the PCB board;
4) The antenna area takes up as much space as possible. As a result, if multi-band design, the antenna area should be bigger;
5) Recommended PCB length is greater than 85mm;
6) In the form of ceramic antenna or FPC antenna, please consider antenna area and position on the PCB.
Regardless of the type of antenna, the antenna supplier should join the PCB design earlier. That is to say, any change about the construction, or many function blocks on the motherboard and multiple antennas, so, please note antenna factory in time.
Antenna performance confirmation and judgment criteria
After antenna debugged, the antenna supplier should provide report (including the antenna matching circuit value, antenna active performance). The confirmation of antenna performance is mainly examined from two aspects: passive testing and active testing. But please pay more attention on test results of active performance. It can reflect the performance of the antenna. So I share 8 tips about the performance and criteria.
Passive performance of the antenna
The passive performance focuses on: the radiation parameters of the antenna, gain, input impedance, efficiency, and standing wave ratio of the whole antenna.
1) The antenna pattern
This is generally a surface pattern in a three-dimensional space. The engineering often uses two cross-sectional views on the mutually orthogonal principal planes. It can describe the directivity of the antenna. Generally the horizontal plane and the vertical plane is the principal plane. But because the constraints of the structure of the whole machine, the surrounding environment, and the transmission power, the antenna pattern is basically a three-dimensional irregular surface. That is to say, for a quasi-omnidirectional antenna, the antenna should have radiation in all directions in three dimensions to allow the signal to propagate in all directions.
As a result, the following figure shows the comparison of the quasi-omnidirectional antenna pattern. The comparison of the quasi-omnidirectional antenna pattern (a) is better (b) poor
What is Gain?
This is a parameter that measures the ability of the antenna to transmit and receive signals. Certainly, this is one of the important indicators of the antenna.
The function of Gain
The gain of an antenna refers to the ratio of the power density of the signal produced by the actual antenna at the same point in space as the ideal radiating element at the same input power. It quantitatively describes the extent to which an antenna concentrates the input power.
The unit of gain is dBi and dBd. dBi is the gain relative to the point source antenna (omnidirectional antenna), and the radiation in all directions is uniform; dBd is the gain relative to the symmetric matrix (dipole) antenna. They have the following conversion relationship: dBi = dBd + 2.15.
How to set Gain when design
Antenna manufacturer’s report use dBi. That is to say, in general, the test report includes peak gain and average gain. The peak gain is the maximum gain value of all angles at the antenna operating frequency. The average gain is the average of the gains of all angles at the antenna operating frequency.
But in practical applications, the antenna gain is not as large as possible. It is limited by the MPE (Maximum Permissible Exposure)  standard and the SAR (Specific Absorption Rate) standard in the FCC certification test.
3) The input impedance
What is impedance?
It is defined as the ratio of the signal voltage at the antenna input to the signal current. The input impedance has a resistance component and a reactance component.
The presence of the reactive component reduces the extraction of the signal power from the feeder from the feeder. Therefore, during commissioning, the reactance component must be made as zero as possible. So that the input impedance of the antenna is pure. But in fact, even antennas in well designed , it always contain reactive component values in the input impedance.
The antenna input impedance depends on its structure, size, and operating wavelength. For any antenna, the debug antenna matching circuit always control the impedance. The reactance component of the input impedance is small within the required operating frequency range. And the resistance component is close to 50 ohms, so that the input impedance of the antenna is Zin = Rin = 50.
In Europe, this is necessary for good impedance matching between the antenna and the feeder.
4) Antenna radiation efficiency
It is the ratio of the radiated power of the antenna to the input power. It indicates the ability of the antenna to effectively radiate electromagnetic energy. Generally, the radiation efficiency of the antenna is required to be more than 60%. But due to other factors such as structure, the radiation efficiency of the antenna will be reduced to varying degrees.
5) In the case of mismatch,
Both incident and reflected waves are present on the transmission line. the voltage standing wave ratio (VSWR) means the ratio of the antinode voltage to the amplitude of the node voltage is called . The ratio of the amplitude of the incident and reflected waves is called the return loss (RL) and is expressed in dB.
In mobile communication systems, the standing wave ratio is less than 1.5. To sum up, I list a table about the comparison of standing wave ratio with return loss.
6) I have introduced several important passive performances of the antenna, as a result, I give the table about these performances:
Antenna active performance
The active test focuses on the radiation performance and receiving sensitivity of the whole machine. It tests the transmission power and receiving sensitivity of the whole machine in all directions in three dimensions. We test it in specific microwave darkroom. The active performance of the antenna directly reflects the radiation characteristics of the whole machine. Above all, the antenna active performance tests mainly include: TRP and TIS.
1) TRP: the total radiated power of the antenna,
This is abbreviation of Total Radiated Power. It obtains dividing and averaging the transmit power of the entire radiating sphere. For instance, where the terminal conduction power is constant. So it is related to the antenna radiation performance.
2) TIS: the total isotropic sensitivity of the antenna,
This is abbreviation of Total Istropic Sensitivity. It is an indicator of the sensitivity of the whole machine to the entire radiation sphere. For example where the terminal conduction sensitivity is constant. Therefore it is related to the antenna radiation performance.
Antenna design is a very important part. Through the detailed introduction of this article, you can get a more comprehensive understanding in the design process of such terminal products. And also you can learn some basic performance of the antenna and its judgment criteria. This paper puts forward some opinions on the antenna design of the terminal products and the test of the antenna performance.
Wish my article provide help and guidance to the antenna design of the customer.