This small form factor means the built-in ground is small, and whatever ground is connected to the USB stick becomes part of the overall antenna design. Next, we investigate a small 868MHz antenna for a USB-size device. It also enables us to study material tolerances and select the right PCB material for the job, with the required tolerances. We can’t expect good system performance without taking the interaction of antenna and motherboard into account.ĮM simulation enables us to design for a real world antenna environment early on. It is quite obvious from this example that placement and environment matters. For position 2, the antenna at the edge of the motherboard, we see a more omnidirectional coverage, but one minimum at phi=0° where the stand-alone RF board had a radiation maximum. The only difference is the -6dB reduction in realized gain due to mismatch. For position 3, the antenna centered on one side, we see a dipole-like “8” shape similar to the stand-alone RF module. The worst result (red) is with the antenna overlapping the motherboard: realized antenna gain -16dBi and less. This results not only in a strong resonance shift, we also observe a change in antenna pattern. The only change was mounting the radio board to a motherboard. Remember, we started from a radio board that was tuned perfectly to VSWR=1 at 868 MHz. With the antenna at the center of the PCB at position 3, we find a stronger frequency shift and VSWR = 10 at 868 MHz. This position 2, on the edge of the motherboard, results in some de-tuning of the resonance frequency and VSWR = 5 at 868 MHz. Perfect radio board, but wrong placement.įor better performance, we move the radio board so that the entire antenna area is clear from any PCB traces below. VSWR is >100 which means almost total reflection in this environment. RF board ground is connected to motherboard ground.įor position 1, with the RF board overlapping the motherboard, the antenna fails completely. This carefully tuned RF board (left in the picture) is then placed on top of a motherboard at different positions, at a vertical distance of 5mm. The antenna is tuned to target frequency using a matching network with 2 SMD components, with a resulting VSWR=1 at 868 MHz. 868 MHz antenna placementīaseline for the first example shown here is the 868 MHz antenna from Texas Instruments Design Note DN024 In this document, we will have a look at some of these external influences, and their effect on antenna performance. Besides the PCB, surrounding structures and packages can easily de-tune an antenna that used to work fine in other environments. Especially for small antennas, PCB ground acts as an integral part of the radiating structure, and any change to the PCB shape might be critical. Placement and PCB size have a strong influence on antenna performance. It is easy to find appnotes with PCB antenna examples, but copying an existing design to your own PCB can result in disappointing performance.
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