Frequently Asked Questions

Antenna technology basics:

What to consider when using a monopole antenna?

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.

What to consider when using a dipole antenna?

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.

Installation issues:

What to pay attention to when having to use a ground plane with the antenna?

The antenna ground plane has to be of electrically conductive material like steel or aluminium (electrical isolators like fiberglass are therefore not suitable). It should be noted that the ground plane needs to be large enough compared to the size of the antenna for the antenna to perform properly. In addition, ground planes with a symmetrical shape are usually of advantage as an unsymmetrical ground plane may affect the radiation pattern of the antenna at certain frequencies - making the antenna radiate more to certain directions while decreasing its radiation performance in some other directions.

When using monopole antennas one should be aware that the exact installation location on the ground plane may have a very significant effect on how the antenna radiates in different directions. The best possible performance with a monopole antenna is typically achieved when having the antenna installed in the centre of the ground plane. Dipoles are less sensitive to a possible ground plane below them but may nevertheless be affected.

What to consider when selecting the installation location for an antenna?

In order to achieve good radiation performance (coverage), it is crucial that the antenna is properly installed and its location carefully selected. When installing an antenna or especially several antennas, for instance on top of a vehicle, there are potentially many factors that can have a deteriorating influence on the radiation performance of the antenna:

  • The close-by surroundings (near field) of an antenna should be kept free of any objects (especially electrically conducting ones), as the antenna may otherwise not work at all. The size of the near field is roughly a function of the frequency and the antenna size. For instance, a 2 meter antenna has a near field with an approximate radius of 0.8 m at 30 MHz.
  • Objects blocking the radio wave propagation may attenuate the signal or cause unwanted reflections, refractions etc. which would cause the system to not operate properly. Therefore the wanted transmission directions as seen from the antenna should be kept clear of any obstacles.

How to ensure proper DC grounding?

Proper grounding is achieved through a solid, short and non-inductive conductive path between the ground plane and the antenna base. COJOT antennas that require separate DC grounding have for that purpose a threaded bolt (M4) at the antenna base. A suitable grounding lead (length: 25cm) is supplied with the antenna. We recommend ensuring the DC grounding by connecting the grounding lead between the antenna base (through the 4 mm ring terminal at one end of the grounding lead) and the vehicle body (through the 5 mm fork terminal at the other end) to avoid the build-up of large static voltages and the resulting possibly harmful uncontrolled discharge of these.

How to ensure proper capacitive (RF) 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.

Why to use the gasket that comes with the antenna?

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?’)

Antenna features:

What is a rubber spring in COJOT manpack antennas?

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 video.