Frequently Asked Questions

• Output power: COFDM versus CW

  Generally the output power of our power amplifiers is specified for a single carrier CW signal. If you'd like to use them for COFDM / digital modulated signals you can expect an output power of about 10 ... 20 % of the CW power. This depends on the type of modulation you want to use and the required signal quality and specifications.

• What is Monitor Output?

  Most of our power amplifiers include a built-in directional coupler for detection of the forward output power. So the output power can be observed by a simple DC voltage - it is proportional to the RF power, but not calibrated and no linear function.

• No bi-directional functionality / no built-in coaxial relays

  Our modules are not designed for bi-directional applications and don’t include any coaxial relays. The switching between transmitting and receiving must be done externally by the customer – there’s no RF switching implemented! 

• Sequence Controller

  We strongly recommend the use of a sequence controller for your amateur radio system. Many coaxial relays have too low isolation between the ports during the changeover. If the power amplifier (in a transmit-receive system) is switched too early, this may lead to damage or destruction of the input transistor in the LNA or converter. With a sequence controller this trouble can be avoided. The sequence controller provides a control signal for the coaxial relay and it switches the voltage supply for the power amplifier. There is a time delay between the two signals to guarantee safe switching.
  Our sequence controllers are intended to control a power amplifier and an antenna relay - not to control a transverter module. Your transceiver must provide a ‘PTT to GND’ signal to switch the transverter module into transmitting mode. If you don’t use a power amplifier it is not necessary to use a sequence controller.

• When do I need an isolator for my power amplifier?

  An isolator protects the power amplifier in case of bad antenna matching as it prevents reflected RF to go back into the final stage. So if you have a good antenna situation where the VSWR is always fine, you really won’t need an isolator. But if there is the chance that the antenna might be forgotten or damaged it is really worthwhile to protect the power amplifier. Generally the VSWR of load is specified with 1.8:1 for all of our power amplifiers.

• Conductive silver adhesive

  Unfortunately we cannot offer conductive silver adhesive, please check out the web for CircuitWorks CW 2400 which is provided by many suppliers (Bürklin, Conrad, Voelkner, Farnell, Mouser, etc.).
  
  Another (but expensive!) solution is the material EPO-TEK H20E which is supplied by the following company: www.jpkummer.com

• 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.

• RF power of Bias-Tees

  Our bias-tees KU BT 6001 N/SMA are intended for voltage supply of Low Noise Amplifiers (LNA) and Low Noise Converters (LNC), but not for Power Amplifiers (PA). The KU BT 6000 N/SMA is specified for the following maximum RF power of 1 W. If there’s only one frequency band required we can probably offer you a special version with more RF power. Please specify the required power and the frequency.

• How to dimension a heat sink

  Many questions are: “How big does a heat sink have to be?”
  
  Unfortunately this question couldn’t be answered in one sentence. However, the proverb “A lot helps a lot” is surely appropriate in this case. Amplifiers built up of semiconductors always works best at moderate ambient temperature due to the physical properties of the transistors. Exceeding the max. working temperature given by the manufacturer will cause the termination of the transistor. Furthermore the lifetime of semiconductors will detach enormously at high working temperature.
  
  Here are some hints to mind helping you dimensioning and working with heat sinks:
  
  What is the max. air temperature for a heat sink to work well? For example, in southern Spain the ambient temperature is much higher as in Norway.
  Is the device continuously active or just for a short time? At short times of activity a smaller heat sink can be used as the device cools down during the off-time.
  When using heat sinks with a profile and without additional cooling of a fan, it’s very important to assemble it vertically because the air must go unhindered through the cooling ribs.
  Black anodized heat sinks will cause a better cooling effect as bright surfaces.
  When using heat sinks out of doors, direct solar radiation should be avoided. This can lead to an exceedance of the max. working temperature of the amplifier even when it’s not operating.
  RF power amplifiers usually will be supplied in aluminium or copper cases with a flat floor space. The mounting area of the heat sink should also be flat to enable best heat transfer.
  The use of thermal paste improves the thermal resistance between the amplifier and the heat sink. Silver-bearing pastes like ARCTIC SILVER 5 have a better heat conductance value as common products.
  
  Here is an example for dimensioning a heat sink:
  
  Max. air temperature: Tair = 25°C

   

  Power loss of the amplifier: Tv = 60W

  (Power consumption of the amplifier minus output power)

   

  Max. case temperature of the amplifier: Tcase = 50°C

• 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.

• 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?’)

• 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.

• 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.

• 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 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.

• 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 — 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 metre 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 desired transmission directions as seen from the antenna should be kept clear of any obstacles.