Make a miniaturized and wideband Monopole at low frequency (200~500MHz)

Hi, can anyone share experience or techniques that can make a wideband and/or miniaturized monopole?
Thank you very much.

An electrical small antenna (l < λ/4) can't achieve wideband matching. The smaller the antenna, the lower the bandwidth.

Thanks FvM very much.

My monopole is a spiral monopole, and It's a small antenna but bandwidth is narrow. (5 MHz)
Do you have experience in this case: How to make it wideband?
Any paper suggested?
Many thanks

I can only refer to an application note that shows why the bandwidth of an electrical small antenna can't be increased without reducing the effciency. http://www.ti.com/lit/an/swra088/swra088.pdf

The AN is quoting an IEEE paper which derives the theory behind the statement
J.S.McLean: A re-examination of the fundamental limits on the radiation Q of electrically small antennas, IEEE Trans Antennas Propagat, vol 44, pp672-675, May 1996

Thanks FvM very much. You look have many experiment in this field. Thank a lot