Antenna type to use

[Dave_s]

I'm brand new to this group. My questions seem to fit into several of your forums.
I cannot receive DTV ch 11.1 and 13.1, very weak at zip code 91402 using 4 bay roof mounted bowtie antenna. Signal strength varies randomly and quickly from 20% to zero back to 15%, etc.

On what channels do DTV stations 2.1,4.1,5.1,7.1,9.1, 11.1,13.1 transmit in Los Angeles? LA Mount Wilson is 18 miles south west of 91402 with Hollywood Hills preventing line_of_site.
What antenna and distribution amplifier do you recommend?
Do you sell a device to measure DTV signal strength?
Thank you, Dave_s