The Aaronia AADCBLK DC Blocker is for use with all Aaronia Spectran spectrum analyzers in the HF and NF series, and prevents the RF-input of the spectrum analyzer from being destroyed by the DC-voltages. The DC-Blocker must be used for measuring active powered antennas like GPS or satellite dishes. It prevents damage to the input of RF analyzers by blocking up to 18 VDC. Simply connect the DC-Blocker to the RF-input of the spectrum analyzer and you can measure directly on GPS, satellite antennas or even DSL/ISDN lines.