Looks like a good 7/800MHz band antenna would be what you need. If you use the scanner for VHF/UHF also, there's some good multiband antennas that work well.
And RG-174 coax is pretty lossy at the higher frequencies. Couple that with picket fencing and other things that impact performance, and it can be an issue.
I'd strongly recommend one of these two antennas:
Tri-band antenna covers 150-174, 430-520 & 750-870, 100 Watts, Whip Length 16.5"
theantennafarm.com
Tri-Band 150-162/450-490/763-870 MHz IP67 Rated Roof Mount Antenna
theantennafarm.com
Both Larsen and EMWave are well established companies that make excellent products. I have 30 year old Larsen antennas that work like new. EMWave is becoming my new favorite. Either one will work well and give you excellent VHF/UHF and 7-800MHz band performance.
As for the magnetic mount:
Manufacturer: PCTEL Maxrad; Description: Magnetic Mount For use with NMO style antennas; Size: 2 3/8"; Model #: BGMSMAI; Pull Strength: 55lbs; Frequency:
theantennafarm.com