I operate the latest R8s unit, and it seems to me that the internal antenna is weak, since i cannot lock radio signal in small distances. Either that or theres some jamming or signal interference. Do you think an upgrade to an external radio/antenna could fix, improve the situation? Should i start considering such an option
The first step should be to find someone with a radio scanner to see if the channel frequency is already in use in your area, or to change your frequency setting to see if the symptoms change.
The base/rover set-up works best with an external radio, the R8's are great but have a limited distance using the radio in the base (R10's are better but also limited). Line of sight will work for a long way (2-3 miles) but any obstruction will shut down the range quickly. Adding a repeater radio is SOP with those units. It will make a huge difference. They are usually placed on a good radio spot, they can be miles from the base and still work well. The interesting thing about them is that they will pick up the base signal where the rover has no chance of hearing it. So you can carry it with you and put it out even over a hill and still hear the base using the repeater to continue to work RTK. Honestly, I would not buy an R8 base/rover unit without a repeater radio.
And as Bill said, if you are getting walked on the radios will not work well.
Sometimes when we go to a remote location where we want to minimise the amount of gear we take but get as much range as possible, we run the base off its internal 0.5W radio, but have a 3m extension cable to the aerial port. We find a long stick or cut a tree branch and set that in the ground next to the base and get the aerial - which would otherwise be on the underside of the base unit - as high up as possible, like this:
Other times, particularly when ranging far observing control stations, when we are running an external radio at the base but can't get reception at the rover, we use that same cable at the rover and just hold it up at arms length and that usually does the trick.
I heard somewhere that Trimble and the like went to 0.5W internals to keep the power below the "FED's radar". Having said that, I also have it in my mind that Topcons have 2W internals.
We use the TDL 450H. It will work for what you need, however it may be more than what you need. It will broadcast at different wattages, I can't imagine you would ever need it at 25 Watts.
There are other cheaper less powerful options. I'm not sure what would be best for you for such a small area but the TDL 450H should cover the entire island. If you have some deep canyons, you would still need to move it around. It operates independent of the Base location, you can move it anywhere as long as it can hear the base.
What you really need is the little repeater Pac Crest used to make. It was 2 watts and about the size of a couple of cell phones. It would power up from two camcorder batteries. I can't say if they are still available but ours no longer works with Trimble's newer equipment. I don't know if there are some small repeaters anymore, that would be perfect for you.
Be very careful that what you are buying will do what you need.