And again, since we're talking about antennas designed for scanner duties aka they will not be used for transmitting then power capabilities and even SWR information bears no actual relevance in the decision to buy one.
I'm sure it's a good antenna, no doubt, as Laird does have a pretty good reputation for making them but at this point in time if I just had to have an antenna specifically designed/tuned for 770 MHz operation (meaning it's centered at that frequency or something very close to it and not down at 752 MHz either but obviously pretty close) I'd go look for a Motorola 700/800 MHz model like the ones most of their XTS public service hardware uses (tuned for 764 to 870 MHz so probably centered about 817 MHz)
Grab an adapter as required for the connection to the scanner since it uses a reverse connector on the Motorola units and go, that's my advice. The actual part number is 8505241U11 and it can be found
on eBay for $10 sometimes with free shipping so definitely a bit less than the Laird but basically about the same cost in the long run.
Either one of them would work fine I'm sure, and I'm somewhat surprised the RS 800 isn't working so well for the OP but then again it does have a tuning of about 860 MHz from what I've read (I've never tested such a thing myself, just seen that center frequency mentioned in discussions). The RS 800 has been around for quite a long time, long before 700 MHz systems were probably even on the drawing board so there was no reason to have it tuned lower - actual trunked 800 MHz radio systems usually have their input frequencies in the 806-826 MHz range (roughly 50 MHz lower) so those public service hardware antennas in those days 20+ years ago were tuned at about 838 MHz to cover that swath of frequencies.
Since the RS 800 is not designed for any transmitting purposes then it wasn't necessary to tune it for the lower end hence it being centered at about 860 MHz which was smack in the middle of the 851-870 MHz range originally used for public service but obviously that's gone through changes over the years as part of the rebanding process among others.
Anyway, either one should work great for such purposes.
