Variable attenuators are important circuits that can be employed in many radio frequency (RF) applications, e.g., in automatic gain control (AGC) amplifiers, broadband gain-control blocks at RF frequencies or as broadband vector modulators. For any applications, low insertion phase shift and low power consumption are of interest. A way to implement variable attenuators is using the RF PIN diode. The PIN diode is characterized by a low doped (I = intrinsic) semiconductor region between p- (P) and n-type (N) semiconductor regions. Besides the variable attenuators, the PIN-diode is used in other RF circuits, such as RF switches, limiters and phase shifters. This project presents the design of variable attenuators at 7.5 GHz and 500 MHz frequency bandwidth for ultra-wideband (UWB) applications using two different PIN diodes. The variable attenuators have a topology based on 90° hybrid couplers. The design is performed using Advance Design Systems (ADS) from Agilent Technologies Inc. After presenting the PIN diode and its equivalent circuit, the theory of the 90° passive directional branch line coupler and the operation principle of the variable attenuators are presented. As the selection of the appropriate PIN diode is a critical step in the design, special attention is dedicated to this aspect. It follows the design of the variable attenuators with extensive descriptions of the simulations in ADS. Firstly, both series and shunt attenuators are presented. However, as these circuits normally offer narrow band variable attenuation, the 900 directional branch line coupler is used in the attenuator for broader band operation. At the end, a double hybrid coupler is found to eliminate the ripple in the high attenuation state of the single hybrid coupled attenuator. So the final topology of the variable attenuator is a double hybrid coupler variable attenuator- Moreover, in this project, different PIN diodes are investigated for variable attenuator applications. Different manufacture companies are currently providing different kinds of PIN diodes in terms of parameters and packages. Every type of PIN diodes are providing different sort of advantages to the designers. That is why it has become more difficult for the RF designers to choose the right device for the specified application. Beside the design of the variable attenuator using PIN diodes, some considerations in form of a guide line to the designers while they are using the PIN diode for designing the variable attenuator. In this work, the used PIN diodes are a beam lead PIN diode and chip PIN diode. The beam lead PIN diode is used because it is manufactured for high frequency and it produces excellent electrical performance and isolation at high frequencies. On the other hand, the chip PIN diode eliminates the problem of package parasitics. However, printed circuit board (PCB) manufacturing limitations at the university laboratory incline the balance in the favor of the beam lead PIN diode, HPND- 4005 from Avagotech, instead of the also considered chip diode MA-COM MA4P202.