For the typical scanner Discone rated for about 100 to 800MHz, the pattern starts to shift upward in the 400-500MHz range and by 800MHz the antenna is pointing more to the moon than the horizon. I might be exaggerating a little on the moon thing but I've measured them about 10dB down at the horizon at 800-900MHz compared to 500MHz or lower.
I think its best to use a Discone where it works best in the 118-512Mhz range then diplex in a gain type antenna for 700-800 or higher to supplement it. At my location I use a commercial/military Discone for 118-512MHz then a tiny Discone rated about 400Mhz to over 3GHz for higher stuff and combine them with a diplexer and some filters, amps, etc.
What kind of problems above 800?