Line of sight is the primary limitiation even at much lower power. 300mW on 2m was sufficient to reach from Argonia to AB0XM (100miles+) but only at altitude:
KC5WSV-9>APT202,AB0XM-10*,WIDE,qAo,KA0MR-15:!3710.49N\09744.25W^061/020/A=007118
Ditto yep,
I used to track high altitude balloons not as a participant but as a "hanger on". It was when I just got my Ham license and starting with rockets. The HAB's used milliwatts to a couple of watt APRS trackers. There was a digi-peater with an antenna on a 100 foot tower just 1200 feet away from my house.
I recall a balloon at 100,000 feet transmitting a position from over 425 miles away. I don't remember the tracker they were using but by reading the data it sent out that the 100 foot digipeater tower was receiving the signal directly.
There was a website that used to announce when and where balloon launches were to occur. I'd always check and see if there were any launches nearby especially my state of Illinois but also neighboring states. One time I read where a group was going to fly a Kenwood D7A(g) Ham radio handi-talkie. Personally I thought they were nuts to risk a pricey (at the time) radio in a balloon. I knew danged well that radio had messaging capability and lo' and behold, when flown it hit the local digi-peater and hence my local Xastir laptop. I messaged back to the balloon a couple of times with some silly stuff ("I see you!") and of course my Ham radio callsign. I knew the messages wouldn't be seen until and if the balloon was retrieved. Sure enough, 10 days later I get an envelop with a CD of all the pictures the onboard camera took at altitude and a metal ID tag that commemorates the flight. They thanked me for messaging their balloon with my location.
High altitude ballooning doesn't seem as popular as it was in 05-07. Perhaps the cost of helium has put the kibosh on it though I did read where some folks started using the infamous hydrogen. To the uninitiated, the balloons would be filled with a calculated amount of H or He and as the balloon rose the gas would expand as the balloon gets to altitude. It eventually would burst due to the expansion of the lifting gas and the degradation of the envelop from the vicious UV radiation at altitude. An inline parachute allows recovery of the payload.
An extreme example of this is
https://amsat-uk.org/tag/party-balloon/ Leo Bodnar flew party balloons with GPS trackers around the world.
http://www.leobodnar.com/balloons/ As I recall, one of them went around something like 5 times before going down off Iceland. Leo has a webpage and
sells radio items.
Tracking balloons got me more addicted to putting GPS trackers in rockets to recover them (an investment) a lot easier than depending on eyes alone or RDF.
RDF is good but one has to have a good final bearing before the rocket goes out of range. Also off-bearing tracking to go around obstructions can be problematic. With RDF one wants a great a range as possible with the rocket on the ground to have the largest Rf footprint as possible. This it to compensate for possible off-bearing tracking. Off-bearing tracking is the rocket is down, one has an RDF final fix before loss of signal. One has to jump in their car or truck and try to drive on roads (unless this is the playa where one could drive the bearing to the rocket) to get close enough to the rocket to receive the signal and home in to get the rocket. Believe me, I've seen people lose rockets with RDF trackers because they lost the bearing fix when the rocket landed a long ways away.
With GPS trackers, one may get a position fix with the rocket still 25, 50, 75 or 100 feet in the air depending on the distance away. That position fix is likely very close enough to get one into Rf range of the tracker/rocket. If the GPS receiver antenna is facing close to skyward, one will get a "final lie" of the rocket and can then pick it up. Rf ground footprint is a bit less important as it is with RDF trackers as off-bearing tracking is easier because one knows where to go for the last position fix. More likely a signal will be received at that point if the rocket isn't already seen. GPS makes recovery a lot easier.
Yeah I understand some rockets don't have room for a GPS tracker and RDF is the only way to go. It's probably a bit more economical to get a Ham Radio Tech license and get the bits together (H/T, tracker, attenuator and Yagi antenna) rather than buying a commercial tracker. A commercial system will be more dollars but is well tested and one technically doesn't need a license to use. One pays for that privilege though.
Sorry to drift off topic here but I wanted to make clear that GPS tracking is a reliable means to track rockets and has been proven in some really noxious environments. 100k feet is pretty cold and many times heating has to be done to keep the batteries warm.
Kurt KC9LDH