This antenna has the connector for an Elster V100 water meter inbuilt. This enables the AT301 to receive the water meter signal data and display the consumption and cost on the Control Panel in almost real time with a 10 minute delay. This short antenna is used if you wish to connect the AT301 to the water meter and the radio signal between the Control Panel and Control Unit is not affected by any shielding. If the signal is poor and the AT301 Control Unit is being shielded, then you will need to use the Part # ANT2-3B which extends the antenna by 3 metres so it can be placed in a spot where it will receive the radio signal more clearly.