Antenna frequency hopping

[sonnet]

Dear All
I should design a system which is capable of scanning the frequency spectrum in a wide range (from 400 MHz to 4 GHz). I found few antennas that can cover this range, but this question come to my mind that does antenna have a limitation regarding the frequency hopping rate?? I mean as I should scan a wide range, I should change the frequency of synthesizer very quickly, so does it make any problem in antenna reception ??
Rationally the synthesizer should be able to support fast frequency hopping, but as I'm not an antenna expert, I though it's better to ask you guys.
Many thanks

[Bob]

Hi Sonnet,
A tuned circuit like an antenna or a resonant LC circuit does have to settle to the final response value when the frequency is changed. The time required depends on the bandwidth (Q) of the circuit. Antennas are usually not very high Q so their settling time is relatively short. A few milliseconds will be enough time to wait before the measurement is made. The measurement system itself will also have some settling time, so that has to be considered too.

Some circuits like high-Q LC circuits or crystal filters may need a longer time to settle.

To determine if settling time might be a problem in a particular case, try using different frequency step sizes and sweep rates and see if the data is the same at each point.

This settling time effect is commonly noticed when using a spectrum analyzer. If the sweep rate is too fast, the amplitude of the response is reduced. This is due to the narrow bandwidth of the tuned circuits in the IF of the spectrum analyzer.

--73/ Bob