Being inside a structure = an automatic hit on the signal strength and believe it or not I use basically the same homemade type of 1/4 wave ground plane you've got there yourself - in fact I have several of 'em and use them if necessary. My primary monitoring antenna is an
OCFD made from 1/2-inch copper pipe per the instructions and it's worked relatively well for me for a while now. Having said that my particular "monitoring position" is smack dab in the middle of the entire Las Vegas metropolitan area and this is a valley with many transmitter sites sitting on the mountains that form the "bowl" around us so I get great coverage because of my location.
If you're willing to take a chance, the antenna that prcguy mentioned in another thread about 700-800 MHz antennas (
this antenna) might be worth trying. prcguy just posted his opinion of it in that other thread saying it was pulling in signals with good to full quieting (about as strong as you can get) and it was receiving signals that the stock antenna on his Whistler TRX-1 handheld wasn't receiving at all and declared the Shure one a huge difference.
I'm probably going to order one of those myself here soon as I can't imagine it can be worse and the design appears to have some advantages over typical duckies as well so again it might be worth taking the chance to get one.
As far as building something better that's always a possibility but the time and effort (and materials) you might put into something like that may not prove worth the effort. If you know precisely where the tower(s) are located (and 5-6 miles isn't that far even for 800 MHz signal propragation) there's always the option to build a Yagi antenna which is a beam antenna that will focus the signal gain in one specific direction aka it's something you'd literally point to the tower location and pull in the best signal. The one situation where that kind of antenna could be detrimental would be with a simulcast system with multiple transmitter sites obviously but even so if you can get a rock solid receive signal from at least one tower you should be ok.
Building a Yagi is a much more involved process than the simple 1/4 wave ground plane and there are plans out there for constructing them, here's one that's not too hard to put together (my opinion, of course):
http://forums.radioreference.com/build-your-own-antenna/125567-homebrew-850mhz-yagi.html
Old post but the basic principles at work are still valid and should work just fine if you decide to build something like that. Personally, if you have the ability and the funds, I'd say order that Shure antenna and see how it performs, if it turns out to be not up to snuff in your particular situation then look into constructing a Yagi.
Last ditch: mount an antenna outside, even one of those little 1/4 wave ground planes, or get it higher if possible. There could be some obstructions in the signal path between your location and the towers, and 800 MHz frequencies can be affected by that sometimes pretty strongly (trees, buildings, houses, etc) but even putting one of those just outside the structure you're in could make a dramatic difference in terms of reception.
Good luck...
