A calibration apparatus for an array of antenna elements. In one step, a message is generated by a content provider. The system also includes control means 62 arranged to monitor the error signal to derive an indication of the frequency deviation of the VCO, and, in accordance with this derivation, to maintain the frequency deviation of the VCO substantially constant. The top end cap of the filter cartridge has a hole with radial sealing gasket, which seals against the top of the standpipe to prohibit flow from bypassing the filter cartridge.