There's really no such thing as a best antenna unless you mean something that is specifically cut to a precise frequency and it's usage it receiving on that frequency and nothing else - and when I say that I mean any other frequency, even ones that are close to the exact tuned frequency the antenna is cut for. Somewhat complex way of saying it but, it does actually make sense.
I've heard decent things about the Spectrum Force antenna you have, never thought much about it myself because I've been using the very popular RadioShack 800 MHz antenna for decades now without any problems. Of course, I have to add that the majority of the time over those decades I've found myself in situations where monitoring the comms I was interested in was somewhat easy as I'm a city person and I almost always live someplace near the metropolitan "center" of whatever towns and cities that I've lived in. Like here in Las Vegas, for example, I'm literally a few blocks - a few hundred feet - from the geographical center of this entire metropolitan area and because we're in a valley surrounded by mountains where most of the radio towers in use are actually built on then again I'm in a central location where coverage would be considered excellent by default here on the valley floor.
If the specs on that antenna you have are accurate - from 700 through 868 MHz - that means it's cut for the center of that 168 MHz swath which would be about 784 MHz (168/2=84 so the low end + 84 or the high end - 84 either way) and it would make it
better for 700 MHz operation (which typically has activity in the 768-775 MHz range) than for 800 MHz operation (851-861 MHz range). It's closer to being tuned for the 700 MHz public safety band(s) than the 800 MHz so, again, if those specs are accurate then you're probably not going to do much better.
The RS 800 was designed at a time when the 800 MHz public safety band was 851-869 MHz long ago but was rebanded down to 851-861 so by nature it's actually cut for about 860 MHz. It still functions quite well for the 700 MHz range too but obviously not as well as what it's tuned for. I mentioned recently in another post
why finding an antenna cut specifically to the 700 MHz band (roughly 768-776 MHz so cut to about 772 MHz on the nose) is not really something you'll have much success with and you can read that post here if you're interested:
http://forums.radioreference.com/sc...ing-base-antenna-700-800-mhz.html#post2710164
Now yes that thread was related to getting a base antenna but the principle still holds: because of the interoperability requirements if radio hardware works in the 700 MHz bands it must also work in the 800 MHz bands so the antennas in use will by necessity have to be cut to a central frequency between both ranges - in other words again the antennas will be "dual band" by nature so there's no getting around that.
The only way to truly get a precisely tuned antenna just for the one band or frequency you want is literally making it yourself. You could - theoretically - take a paperclip (seriously) and cut it to a 1/4 wavelength for 772 MHz which would mean it's 3.64 inches or 9.25 cm or 92.5 mm in length and use just that and get a pretty good idea of what you're working with. Antennas don't necessarily have to be that complicated in terms of construction, but they can be pretty complex based on some designs, sure.
I recommended to someone recently that perhaps they get one of the Motorola 700/800 MHz antennas like this one:
700-800MHz Antenna For Motorola XTS1500 XTS2250 XTS2500 XTS3000 Portable Radios | eBay
along with the necessary adapter (if required) and that's about as good as it's going to get. You could also consider contacting a manufacturer like
Smiley Antenna and asking them to make you an antenna cut to 772 MHz (or whatever frequency you want) and see if they'd accommodate you, can't hurt to ask. They have tons of various models and connectors but they don't seem to make much of anything for frequencies over 512 MHz these days so perhaps it could open new business opportunities for 'em, who knows.
Having said all that what I'll finish with is this: antennas don't really have "dead spots"
unless they are directional by design especially monopoles like duckies happen to be (ok they have some aspects of a dipole depending on the internal construction sometimes) that can either be a straight element or perhaps a coiled one or a mixture of both because they are omnidirectional by design and will receive RF energy from all directions equally, they're non-directional by nature. If you've been in a situation where you're portable with the scanner and you lose signal it's not because of the antenna more often than not but literally a dead spot where the system's signal strength is just weak by design.
Also realize that by design scanners are wideband receivers and they do the very best they can with the given technology but they'll never match what the actual commercial hardware designed to operate on those specific frequencies with vastly tighter tolerances to spec are capable of. I've been in situations where I'd be standing next to a LEO and his radio is working as expected (belt-mounted unit with speaker-mic but the speaker-mic was not an antenna capable one like some of the Motorola Jedi/Astro units are) and yet my scanner - when I owned one which was long ago - wasn't getting anything at all. The commercial hardware has higher sensitivity (from what I gather) and better selectivity by design so it just works better, period.
Anyway, I doubt the antenna you have is the problem or even part of it, I'd suspect the actual system in use in your area just has some dead spots more than anything else, but it can't hurt to get another antenna like the RS 800 or the Remtronix clone of it (they run about the same price) and see how things work out with some testing on the go.
