FILTERS:
Filters are a necessary part of radio. Not only for us as hams but for virtually every aspect of radio communications. They can prevent front end overload, reduce harmonics from a transmitter, narrow a receiver passband, attenuate signals above or below a designed cutoff frequency, or, well you get the idea. They can do many many different things; even help reduce interference.
There are also many different type of filters. Here are some clickable links to descriptions to some of the more commonly used filters:
Filters can come in various forms including:
Interdigital
Stripline
Waveguide
Lumped constant
Crystal
A filter can be designed with very high Q (quality) which provide for narrow band operation and maximum attenuation of unwanted frequencies. On the other hand, if a filter needs to be a bit more broadband (pass wider range of frequencies) it can be designed with a lower Q. Each type has its advantages and disadvantages. Signal loss is also an issue since no filter passes 100% of the signal presented to it.
Filter adjustment can vary from build it and use it to hours of adjustment time. For example, a lowpass filter for 80 - 10 meters can be easily built from an existing design and simply installed in the transmitter line. On the other hand, a Band Pass (Bp) and Band Reject (Br) filter for a 440 MHz repeater can take hours to initially adjust. Adjusting the Bp/Br filter also requires a lot of sophisticated test equipment including a spectrum analyzer. That is not to say that it can not be done without a spectrum analyzer; only say to it would be very time consuming at best. Final results would most likely not be as good either.
The higher in frequency you go, the more concerned you need to be about filters. Things like silver plating become much more important at microwave frequencies then they are at say 2 meters. Skin depth also plays into the equation when you start plating different filters for microwave use. Since the depth RF goes below the surface of a conductor decreases with increasing frequency, less plating is required at higher frequencies.. To put this another way, the plating depth for a 6 meter filter would need to be much thicker then a similar filter used at 3.4 GHz. When you get to 24 GHz or higher the required plating thickness gets very thin. There is no use in plating any thicker then required, especially if you are paying someone to do the work.
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