PCB and system design when RF signals in the GHz-range are involved is often termed as “black magic” in the electronics design industry. WiFi PCB trace antenna design is no exception.
Is it really all that tough? It truly depends on the complexity of the design you are trying to accomplish. Modern simulation tools have made it very easy for engineers to tweak standard designs with ease – and fast! But hey, even if you handed someone with thousands of dollars worth of simulation software and test equipment – let’s face it – if they did not pay attention in their microwave engineering classes, they won’t be able to do much.
Look at a little low pass network we have here, who would imagine this is a low pass filter? This type of design technique is very common in RF systems because you would rather not add components on the RF path. We will see why!
Fortunately, WiFi PCB trace antenna design is easier!
Let’s stay well away from all the science involved in RF black magic. If you are someone who is not experienced with RF design and is not sure what all has to be taken care of when spinning out your own PCB trace antennas, there are certain things you need to be very aware of. We are listing the design considerations involved and what/how they affect RF designs in very English words (some nerds will hate it). But here we go…
The VERY VERY basic stuff
No magic in this section. Just some basic knowledge of transmission line and RF design related conventions. This section will help you understand antennas better. They are definitely not just wires carrying a current, and they are definitely not rocket science.
Visualizing an antenna
The knowledge of basic transmission line theory is VERY helpful in understanding antennas. In very crude terms, you may think of an antenna as a means of radiating energy from an electronic circuit out to space.
How do you radiate energy out to space?
You need to transfer as much energy as possible to the antenna. And the antenna should pass on as much energy as possible to space. That is, your circuit should be able to radiate power using the antenna.
Now, how do you transfer as much energy as possible?
Consider the maximum power transfer theorem. It states that the source output impedance and the load input impedance should match for maximum power to be transferred across from the source to the load. So that is what we will do here. Match the output impedance of RF source to the input impedance of antenna.
So what are our basic antenna design objectives?
Firstly, the antenna must match the output impedance of the RF source. Secondly, the antenna when attached to the RF source must make the effective impedance of the RF source equal to that of free space. This ensures maximum transfer of energy through the RF source-antenna junction and antenna-space junction.
Better matching also means better reception of signals.
Matching an antenna
What do you need to know about matching an antenna to the RF source? Just 2 things!
- Output impedance of RF source
- Input impedance of the antenna
Finding output impedance of the source is the easiest part. Just look at the datasheet!
The tougher part is to find out the antenna impedance and make sure they match. Before getting into antenna impedance issues, we outline what influences antenna impedance. This is very important in practical design consideration and is something that most books on RF theory do not explicitly list.
What affects PCB trace antenna impedance?
The PCB trace antenna is just a PCB trace on a PCB substrate. The most common PCB antennas can be modeled as a microstrip line feeding the antenna section (which is supposed to radiate energy). What commonly affect antenna impedance most are:
- PCB substrate dielectric properties
- Thickness of PCB substrate
- Length, width and thickness of every trace RF waves pass through
- Bends in RF traces
- Dielectric properties of other media surrounding antenna
- Dielectric properties of solder mask, if present over antenna
- Uniformity of PCB cross section
- Any copper present near the antenna on the PCB
- Vias on the RF trace
- Component footprints present on RF trace
- Active components present near RF trace
- Size and shape of ground plane of the PCB
More coming up soon...
A complete sample MIFA antenna design and fabrication process for ESP8266 and ESP32 will be made online soon for your reference.
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