Radio Frequency (RF) & Wireless Design

Created by Steven Minichiello on 27 April, 2018

Why a plastic PVC, bamboo, fiberglass, or other non-metallic mast are better supports for antennas

While most people understand that the metallic portions of the antenna are the receiving (electro-magnetic resonance) and transmitting (electro-magnetic radiation) are tuned to a specific center frequency (channel) or across a band (e.g. wideband), little detail has been spent on understanding the EM coupling effects of the attachment mechanisms; in this case the mast.


While it is true that the vast majority of towers and masts are made of metallic such as steel, aluminum, and other alloys, the radiation pattern of an antenna does become affected by these close Near-Field structures.


There are many articles on the use of a balanced twin-lead line feed (https://en.wikipedia.org/wiki/Twin-lead) as the preferred method to reject common mode coupling from the antenna over a coaxial feed to the antenna. However most articles will tell you that for the balanced pair to be effective, they must be far from metal objects !!


There are other articles that call out the use of a BalUn to reject and suppress the influence of the antenna radiation on the external shield of the coax (https://en.wikipedia.org/wiki/Coaxial_cable#Common_mode_current_and_radiation ).


However there is little in the way of calling out the preferred use of non-metallic mechanisms to support the antenna to improve the overall performance of the antenna both in transmitting and in receiving (tuning).


To understand this effect one simply has to understand the operation of a Yagi-Uda antenna where there is a single dipole radiator and other parasitic elements (reflector and many directors) - https://en.wikipedia.org/wiki/Yagi%E2%80%93Uda_antenna


Since the overall directivity and gain of the antenna are in direct relationship to the parasitic element geometry, it is easy to understand that the size and proximity of the mast is a critical factor in how this antenna performs.


Therefore in any application where an antenna is going to be installed, make sure you understand that any metallic objects in the close proximity of the antenna will alter its frequency response as well its operational performance.