| CPC G01R 33/345 (2013.01) [G01R 33/3614 (2013.01); G01R 33/60 (2013.01)] | 11 Claims |

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1. An insert for an EPR probehead, the insert comprising:
a directional coupler, wherein the directional coupler is configured to receive microwave power from a source at a first port and to transfer a portion of the received microwave power to a second port for transmission to a sample space, and wherein the directional coupler is configured to receive a first microwave signal from the sample space at the second port and to pass a majority of the received first microwave signal to a third port as a second microwave signal;
an amplifier having an input and an output, wherein the input is arranged to receive the second microwave signal from the third port of the directional coupler and to produce an amplified version of the received second microwave signal at the output for transmission to a detector; and
wherein the amplifier comprises a high electron mobility transistor.
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