hi.......
can any one suggest to me how I can lower the lower cut-off frequency of a bowtie antenna to MHz range without the help of resistive loading and keeping the antenna size small?
You could look at making the antenna active. In that case it would be receive (or transmit) only. Don't forget that fundamental physics still applies! An active receive antenna can operate down to frequencies lower than its passive counterpart but the performance comes at a price in terms of complexity and limited dynamic range.
Make it really BIG. Or you can load it with a dielectric material with high eps_r - the price you pay is, naturally, a lower gain.
UWB is obviously going to be tricky, because of the wide bandwidth requirement. There are many articles about this online. Most seem to use planar antennas. This article is worth a read.
http://mwrf.com/passive-components/novel-microstrip-antenna-aims-uwb-applications
The antenna shown in Figure 3 can perhaps be scaled up, for lower frequencies. Although UWB does not extend to the UHF band, at least not according to the US Federal Communications Commission (FCC).
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