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We have developed a new handy electrooptic (EO) probe to measure signals whose frequency is lower than 100 MHz. A distinct feature of the new EO probe is a long metal rod attached to an EO modulator. Measurement sensitivity does not depend on geometrical structures of devices under test (DUTs) with our new EO probe, whereas it does depend on geometrical structures of DUTs with a conventional handy EO probe. The sensitivity is at least 18 dB higher than that of the conventional one. In this paper, we qualitatively explain the effects of the rod on sensitivity and show experimental results, which agree well with the explanation. The advantages of signal measurements with an EO probe are also discussed, and two applications, where an EO probe plays important and essential roles, are proposed.