Conventional approaches for microwave breast tumor detection are limited by the imaging resolution due to the low operating frequency. The objective of this work is to provide a proof of concept for radar-based detection of breast tumors in K-band using the temperature-dependent permittivity of the tissue for contrast enhancement. The innovation of this work is given by i) investigating higher microwave frequencies for breast cancer diagnostics and improved resolution; ii) exploiting variations in tissue temperature as a non-invasive approach for contrast-induced radar imaging eliminating the need for contrast agents such as nanoparticles; iii) using a well-defined setup with the breast compressed similar to mammography; iv) eliminating the need for coupling liquid through the usage of ultra-wideband bow-tie antennas operating from 16.55 to 40 GHz for a reflection coefficient lower than −10 dB; v) validating the experimental findings through numerical modelling. The experimental setup in this work consists of a single-pixel transmission setup with the antennas placed in a 3D printed container. Two different tissue mimicking phantoms have been studied that both model the temperature-dependent permittivity of biological tissue. The first phantom represents homogeneous fatty tissue properties and the second phantom simulates fatty tissue with a tumor inclusion. A uniform phantom warming is realized through a water bath combined with a continuous monitoring of the phantoms temperature. We show that a homogeneous phantom without tumor can be distinguished from a heterogeneous phantom with tumor in the temperature range of 28 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$^\circ$</tex-math></inline-formula> C to 38 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$^\circ$</tex-math></inline-formula> C.