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A monopole antenna requires, in addition to the radiator, a ground plane as an essential antenna element. In order to work properly, typically an existing platform to which the antenna is attached to can be used as such (e.g. roof of a vehicle that is made of electrically conductive material like steel or aluminium). Monopole antennas will usually be smaller in height and slimmer in design than dipole antennas. This becomes especially important with antennas for lower frequencies as their size can be of the order of several meters.
Dipole antennas don’t require a ground plane for proper operation. They are typically larger in height and bigger in design than monopole antennas, since they have the essential second conductive element already built-in.
Some COJOT manpack antennas feature a shock absorption spring that is made of rubber instead of steel. The beneficial characteristic of this new type of shock absorption spring is that it makes the antenna up to 30 % lighter and better attenuates oscillation of the antenna. See the difference yourself in this short video.
We recommend to always use the gasket that is supplied with the COJOT antenna. Firstly, it helps prevent water ingress to your installation. And secondly, some high power COJOT antenna models, like the WB10DXM, require special gaskets that enable the antenna to achieve the maximum specified power rating. These special gaskets are made of highly heat conductive material, which increases the heat dissipation capability of the antenna base by allowing it to take full advantage of the mounting surface (ground plane) to which the antenna is attached to. (When using the gasket, please also refer to the above given question ‘How to ensure proper capacitive grounding?’)
At radio frequencies the grounding can also be achieved by capacitive coupling even though there is no contact at DC. If you are using the rubber gasket which comes with the antenna (and which is in general non-conducting) and tighten the antenna well to the ground plane using the bolts that come with the antenna, then the capacitive grounding should be good enough for the antenna to perform as specified in the data sheet even though no DC path exists (assuming that the thickness of the layer of paint on the ground plane is of the order of tenths of millimetres). The use of any thicker non-conductive gaskets may, however, deteriorate the antenna performance starting at the lower frequencies.