[SOLVED] what is the use of band rejection antennas if filters will be always there ?!

[B4M]

Hello friends!
I have a trouble in understanding the use of implemented methods of band rejection in printed antennas such as slots...etc , "if the use of filters in a communication channel is always needed".
Could you mention any advantages of these methods?
thank you !

 

[SLK001]

A lot of antennas do not have the rejection that a dedicated filter can be designed to have.

 

[Dan Mills]

A lot of the art of radio design is in rejecting as much of what you don't need as early as possible.

Consider that a wideband antenna feeding a LNA will expose that amplifier to many signal that you are not interested in, possibly causing the amplifier to become non linear and causing distortion to the wanted signal (Or even burying it in intermod to the point that you cannot even detect it). This is why a LNA at the antenna should generally only have sufficient gain to amke up cable losses, and even that can be problematic.

Sure you can place a filter ahead of the amplifier, but then the filters insertion loss directly impacts the overall system noise figure.

Considerations like this apply right down the chain, you start off with as narrowband a signal as the antenna can be designed for, use a tuned amplifier, filter it some ahead of the first mixer, filter it some more at the IF, do some more in the DSP.... At every point overload becomes less of a problem, as the bandwidth narrows in on what you care about.

73 Dan.

 

[dick_freebird]

Band-reject antennas leave you less to filter or make
your designer filters more effective.

They also are a layer of protection against high power
out-of-band threats which could, say, fry your LNA and
anything before said filters in the lineup. This is a very
important concern in the "Spy-vs-Spy" world of EW and
satcom.