I actually use a combination of 300-ohm line and 450-ohm window line to feed my primary SOTA chasing antenna. This loop antenna is approximately a full wave at 14.06 MHz - about 70 feet of insulated wire. It’s mounted in the vertical plane - it hangs from a tree limb about 40 feet above ground. The feed is at the bottom center of the kite-shaped loop, so the polarization is horizontal. The feedline length is reduced by having the feed point down low - about 11 feet off the ground. The antenna is supported with elastic cords, and it has been through more than a year with many big winds and snow events.
This type of antenna is inherently more durable than a dipole with the high feed suspended at the middle of the span. The pattern is dipole-like, with a wave angle suitable for most of my contacts here in the states.
I have about 45 feet of 450-ohm line from the antenna to the entry into the house, and then there’s about 25 feet of 300-ohm line to the tuner in the shack. I decided to use the 300-ohm section - after much deliberation - because the feedline runs close to my computer, suspended about 3 inches below the ceiling, and I wanted to minimize radiation inside the house.
This is ham radio, and this is a compromise for sure! I ran Eznec models of my system, including the two feedlines, and I saw the losses add up on some of the bands. I recommend modelling these kinds of systems with Eznec, because the models allow trying out different solutions in order to evaluate the various trade-offs.
I use homebrew balanced tuners to feed my antennas. These use old designs from the 1930’s, with link coupling, series-tuned-primary, parallel-tuned-secondary, balanced split-stator variable capacitors, and large air-core balanced inductors. I usually run only 100 watts, so the losses in the lines are not high enough to cause real trouble. I don’t really care much about the actual impedances on various bands, the line SWR, or the Smith Chart - even though these determine the loss on the feedlines.
This antenna has exceeded my expectations, which were based on Eznec models. The plane of the loop is N-S, so it generally radiates E-W. Since I’m in Colorado that’s fine for many activations. The loop tunes broadly on 20M and gets out well. On 30M it also loads and performs well - I often work activators on the East Coast. On 17M and 15M it tunes nicely as well. The bipolar pattern holds from 30M through 15M.
Initially I swore never to load up this antenna on 40M, because it’s a half wave, and the feed impedance at the loop is many thousands of ohms! However, I tried it, and my tuner was able to match the feed easily - apparently the mismatched feedline transforms the crazy feed impedance to something reasonable. The loss in the 300 ohm and 450 ohm lines is so high at 7 MHz - several db - that there’s no danger of the loop feed flashing over. Some time ago I learned the hard way that a half-wave loop can be a tricky devil, if fed with real power!
As long as you have a good tuner, the additional mismatch of the 300-ohm-to-450-ohm line transition is no big deal. Most of the “450 ohm” window lines are really closer to 400 ohms anyway. When feeding a single wire or loop on several bands, you will have much greater mismatches and losses to consider.
When a loop is fed on non-resonant frequencies, the predicted SWR’s on the feedlines seem high on some bands, but the losses are reasonable, if the feedlines have relatively low loss and are short. Just don’t use coax, and don’t expect miracles when feeding a half-wave loop.
I’ve achieved Super Shack Sloth mostly with this vertical loop in my cottonwood tree. For some reason it picks up less noise on most bands than my other wire antennas. The ability to jump from band to band with one tuner is helpful when chasing. The cost of the system was - and is - so low that it’s not worth thinking about. The wire in the tree is almost invisible.
I encourage doing these kinds of experiments. Think seriously about loops in various configurations! The 20M full-wave vertical loop needs very little real estate.
Every time I make a SOTA contact on 30M or 40M with this mismatched system with the 300 ohm and 400 ohm lines soldered together I smile!
Of course, if the tuner was located at the loop feed, the system would provide higher efficiency - maybe next year!