Pierce Co WA
Hello.
I don't presently live in Pierce county, but I am from there and visit often as I have family there. I am fairly familiar with the area/etc.
My first thought was in wondering why you selected a directional antenna. Sure you'll be able to point it directly at some site, but your reception of other sites will likely be less. Are you only interested in what is on 700MHz (and not 450, 150, etc)? This depends on specifically what you intend to monitor. If only Pierce County, OK. How about WA SP? There are a few systems I monitor when up there and many conventional channels as well.
When it comes to the physical installation I believe the issues are (not in any order): location, antenna type, downlead (coax). You're antenna needs to "see" the site you want to monitor. You want an antenna that is suited for the frequencies being received. And you want the best coax you can get (lowest loss at higest desired receive freq).
The other poster is correct with suggesting LMR-400. I am using a generic version of the same coax air-802. Keep the run short (30 feet is better than 40). Figure out where you can install the antenna outside to have the least obstructed view and provide the shortest run for the coax to your radio.
Also: I planned on installing a discone for my dad, but after a little research here on RR I found out he was only 2.5 miles direct shot from one of the towers. While there are several sites, he gets good enough reception with his indoor antenna. If you're up high in Gig Harbor, you might have similar luck.