Hi y-knot, personally I don't use, and really haven't found a need for anything but the built in antennae. About the only time I loose the signal is going through tunnels, and occasionally if surrounded by very tall buildings. It's LOS, Line Of Sight, so if it can't see the sats it will alarm and say signal lost. This LOS is true of all GPS. Downtown Chicago and places like this can be a problem if you're really trying to navigate to destination you don't know. Took wifey to Lake Powell, didn't take the Garmin, but use a Lawrance GPS/Depth finder on the toonboat. Up most those canyons GPS doesn't have a chance, and they keep forking. Really can lose your way unless you scribe a rough map on way in to help return. Some of them go in for miles. I have a much greater problem dropping XM radio signal, everytime I pass a semitrailer, so I scoot right on around that puppy. I'm so cheap I haven't even gotten an external antennae for my cellphone. Haven't needed it so far, but have been places where is displays "no service". Most the time around any kind of population on the Interstates I'm good to go. Off road and in the out back is a totally different story. Tree canopy is enough to block GPS. I used my Street Pilot to deliver a mail pickup route I helped my sis out on yesterday. Out of 20 Post Offices only had 2 that it didn't have the actual address in the software. Had one close enough I got there though. If I hadn't had the Pilot I'd have been much later returning to terminal than the hour and 20 minutes I was. Since I was a new temp employee, one Post Master refused to give me the Express Mail, and took him a half hour to tell me this. Another Postmaster *****ed at me for 20 minutes telling me I'd never make it to terminal on time. I called sis and told her I was willing to help her out until she found a replacement driver, but I hated doing the route. I'm OTR driver, routes suck as far as I'm concerned. I'm home today. . . she found somebody to run the route. I told several Postmasters along the way that I'm certain there's a "Special Place In Heaven" for Postal Carriers. They all laughed, said it only took you one day to find that out, I said yup, all it took. Many in my family either workd for or have retired from the US Postal Service. That work sucks in my opinion. Don't know how they do it 30 years.
Regards,
Steve J.