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   

If your chosen antenna achieves the wide bandwidth through passive means (no tuning, no moving parts, no electronic tuning, etc.), then it essentially covers all frequencies at once and there would be no limitation on frequncy changes at all.

Obviously, antenna systems used for reception don't "know" the reception frequency unless there's an interface to provide that information.

It's very simple to cover this range with one antenna, unless you want a very small antenna.

All depends on your physical spec's and antenna coverage area.
Omni coverage vertical polarization is simple, use a bicone.  

In general, if something has a bandwidth of X Hz, its time delay in having a signal go thru it is going to be maybe 2 x 1/X seconds (i.e. 2 time constants).  So for a wideband antenna with 4 ghz of bandwidth you should expect the received signal to be available at pretty much full strength around 500 picoseconds after it shows up in the air.

That said, some antennas achieve their broad bandwidth by a parrallel connection of may narrower bandwidth elements, so you would have to look at the bandwidth of each of those elements and how tightly coupled it was to the main arm to figure out the signal rise time.

If you have a receiver, and the signals are all present in the air, and you simply tune the receiver to the new receive bandwidth, the signal was present at the antenna minutes ago, so there would be zero rise time in that application.

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