DC block, prevents any DC voltage (e.g. from an amplifier with line power) getting to the aerial. These are particularly recommended with Log Periodic aerials.
See article on line power corrosion problems.
Male F connector to Female F connector (CoAx adapters are available)
Loss is less than 1dB at UHF (up to to 1000MHz)
P&P is cheaper for this item if you phone the order through !