Ultra broadband dipole based near field probe with integrated amplifier

An ultra broadband near field probe operating in the frequency range from 2 GHz to 11 GHz was developed. The high bandwidth of the active probe allows to measure the electrical near field of UWB antennas. Due to its small size of 7.6 mm times 56 mm the probe can be set up easily in a near field measurement system and has little effect on the electromagnetic field distribution of the antenna under test. To protect the probe, a Teflon housing was designed. The components on the PCB of the probe are shielded. So, there is reduced influence from external fields which can couple into the stripline based matching network. A broadband differential amplifier with a measured on-wafer gain of about 15.5 dB in the frequency range of interest is used to transform the output voltage of the dipole into an unsymmetrical signal. The output port of the amplifier is matched to 50 Omega by using striplines. Only one single RF cable connected to the mounted SMP plug is required to supply the amplifier with +5 V DC and to carry the probe signal. In the frequency band from 2 GHz to 9 GHz, the reflection coefficient of the prototype is better than -8 dB and the conversion factor is nearly constant. The probe reaches a decoupling of the electrical field components of 10 dB at 2.5 GHz increasing up to more than 20 dB at 10 GHz. The measured common mode rejection of the dipole with its feeding lines is higher than 10 dB at 7 GHz.