MICROWAVE FILTERS - Spurious Non-Harmonic Related Responses
Military Bases, Radar, et., cause Interference to FUNDAMENTAL FILTER FREQUENCIES because of unforeseen NON-HARMONIC SPURIOUS RF RESPONSES:
How can much higher frequency military base, airport RADAR, weather RADAR, your own STL, or other microwave transmitters cause interference to UHF and microwave receive systems? The answer is that virtually all Receive System filters have spurious responses to frequencies much higher than the frequency they are tuned to, THAT MAY NOT BE MULTIPLES OF THE INTENDED RECEIVE FREQUENCY.
The electronics in the receive system can then produce intermod at the receive, image, or IF frequency, because the device has a high ft , but low IP3, causing non-linear operation.
Because of spurious filter responses, you might also see degradation in 7 and 13 GHz systems caused by a 23 GHz transmitter. Many microwave devices can amplify signals many times higher than the frequency they are operating on (high ft ). Even those that don't can be overloaded by strong input signals.
Most spurious filter responses are never a problem because of the upper cut-off frequency of waveguide. Waveguide can save the day, even if your filter can't!
At frequencies where waveguide is not normally used, such as a 2 GHz ENG Site, RADAR and other high frequency, high power RF becomes a problem. That is why we make our SuperFilter/LNA Systems so selective, because no one knows what tomorrow will bring, and only a select few know what is being tested today.***
Below is an actual plotting of a typical Hi-Q, 5 Cavity filter, tuned to 700 MHz:
Below is the overall (and spurious response) of the same 700 MHz filter, swept from 500 MHz to 3,000 MHz:
Below is an expanded view of the spurious response of the same 700 MHz filter at 2,050 MHz:
Below is the expanded view of spurious response of the same 700 MHz filter at 2,750 MHz:
Conclusion: Different filters are going to have different spurious response frequencies, depending on their physical structure, number of sections, and type (cavity, inter-digital, et.) These responses are not necessarily multiples of their tuned operating frequency. A filter tuned to 2 GHz might have a spurious response in the 5.5 or 10 GHz Radar Region.
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*** New technology allows high-power microwaves to give a push or boost to a "solar sail" powered spacecraft, or create a minute barrier on supersonic airships to prevent surface heating and reduce drag coefficient.
A solar sail craft is predicted to travel about 250 times faster than conventional rocket-powered craft, with the added benefit that lasers or microwave energy can be used to provide a "push," but microwave energy can be generated more cheaply.
A "trip" that would take 2500 years could be theoretically made in 10 years by a solar sail craft. I leave it to you to determine how many megawatts of microwave energy is needed to "push" a manned spacecraft, provisioned to support several people for a 20 year round trip!