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Interference generated in antennas, masts, connectors and filters

Nature of the problem. Where and how does interference occur?

Interference generated in antennas, masts, connectors and filters

Nature of the problem. Where and how does interference occur?

 

It has been established that interference (intermodulation) may occur (be generated) in antenna systems. Such intermodulation may even be of various orders.

 

"Antenna systems" comprise everything from duplex filters, connectors, cables, combining devices, antennas, mast nuts and bolts, wires and other metal objects adjacent to the antenna where corrosion has occured.

 

 

Antenna systems

 

Antenna systems

 

This type of intermodulation is the most difficult to treat since it may occur sporadically depending on relative humidity. It can be difficult to detect and impossible to predict its magnitude since corroded connections may be numerous and have different characters.

 

Corrosion involves the formation of various metallic salts due to the atmosphere’s content of sulphur, salt and other substances that readily react with various metals. These metallic salts are formed where two metals meet. An unfortunate effect of these metal salts is that they act as rectifiers – diodes – and can thus mix different frequencies – in other words generate intermodulation products.

 

Such mixing will occur if the transitional resistance between two different metals becomes high – because of poor assembly or corrosion penetrated in between the surfaces. By this "short circuit" of the diode disappears – and intermodulation products may occur.

 

 

How to identify corrosion interference

 

How to identify corrosion interference

 

A simple way of ascertaining whether corrosion interference occurs in the duplex filter or the antenna system is to connect a long coaxial cable with about 10-20 dB attenuation between the output of the duplex filter and the antenna. If the intermodulation products are reduced by means of the connected cable, IM products are generated in the antenna system.

 

Dummy loads used in connection with circulators – 3 dB couplers and similar devices or components - can also generate intermodulation products. A method to determine whether the dummy load is causing problems is to connect a coaxial cable with about 10-20 dB attenuation between the dummy load and the device. If interference is reduced, the intermodulation products are generated in the dummy load.

 

The risk of intermodulation products in dummy loads increases considerably when the output limit of the dummy load is approached. Consequently, it is important not to use undersized dummy loads.

 

Due to impedance changes in the antenna system the emitted energy of an intermodulation product generated in a corroded connection in an antenna may be greater than the output returned to the duplex filter. So it may be necessary to use a selective antenna and a selective receiver to track down corrosion intermodulation products generated in antennas, antenna masts, wires and other metal objects adjacent to the transmitter antennas.

 

 

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