on an external surface of the balloon 1 but may alternatively be formed on the internal surface of the balloon the antenna is connected to an electronic circuit u201c electronics u201d 3 for providing radio frequency communication between the electronic circuit 3 and a remote communication terminal the balloon 1 is filled with a fluid which forms an electrically insulating substrate for the antenna electrode 2 the fluid is of fixed composition and of fixed mass the fluid comprises a liquid which is substantially electrically insulating and which has a relative permittivity greater than 10 in the specific example illustrated the fluid is water but other liquids and gases may be used distilled fresh and sea water all have relative permittivities substantial equal to 81 and dielectric loss tangents substantial equal to zero relative permeabilities are substantially equal to 0 99 and magnetic loss tangents and magnetic saturation are substantially equal to zero however sea water has a conductivity of 4 s m and fresh water has a conductivity of 0 01 s m which is generally unacceptably high and would result in excessive losses distilled water has a conductivity of 0 0002 s m and is therefore very suitable for use as the fluid substrate of the antenna in general the fluid is required to be cheap and to be inert and easily handled in addition when the antenna is to be used as a medical implant or for other medical purposes the fluid must be substantially non toxic distilled and or deionised water are therefore suitable for such applications although other fluids also fulfill these requirements when the antenna is.