[TowerTalk] Auto tuners, Baluns and open wire feeders
mspencer12345 at yahoo.ca
Tue Jul 17 19:20:47 EDT 2018
Thanks for the on and off list replies.
To add a bit more info / answer a few questions:
The site is a summer cottage with numerous tall trees that can serve as antenna supports along with cleared areas where masts / poles can set up.
The antenna in question was basically a dipole / doublet approx 66 feet long over all, fed in the center with open wire (two conductors approx 1.5" apart with spacers every 4 to 5 feet.). The ends of the dipole / doublet were approx 24 feet above ground and the center probably sagged down to 20 feet or so above ground level. For this test one end was supported by a tree the other end was supported by a mast. There was maybe 15 feet of nylon rope connecting each end of the antenna to a support.
I am basically looking for antenna that works reasonably well on 40M and ok on 20M and 15M. I could probably double the height and overall length of the antenna with a lot of extra effort. I want to sort out the feed arrangements before figuring out how to arrange for a higher and or longer antenna. I also have an inverted L antenna (along with a bunch of counterpoises on the ground) fed via another tuner that seems to meet my needs on 80M so I don't really need to worry about coverage below 7Mhz for this dipole / doublet antenna. (I suspect making it long enough to be easy to match on 80M might cause lots of lobes on the higher bands.) I also like the idea of conceivably shorting the open wire feeder together and possibly using this antenna on 80M as well in the future.
I really don't want more than two HF wire antennas at the site.
I'd like to be able to leave the antenna up year round and the overall weight of the antenna (and any feed line or other items that are suspended) is a concern for me. In the event a tree limb falls on the antenna and breaks it, the weight that may come crashing down to the ground is also a concern for me. All this is pushing me towards a dipole / doublet style antenna fed with open wire.
I don't expect to ever run more than 100 watts thru the antenna and its feed system. The site is reasonably RF quiet.
At this point I am reasonably satisfied the antenna "works" but leaving a large manual tuner on top of a step ladder isn't a long term solution (:
I am tempted to just try feeding the antenna via open wire with an auto tuner after applying chokes to the RF and DC power leads. I'll have model this and see what the impedances might be both at perhaps 24 and 48 feet above ground, and see if there would be any benefit in having an transformer on the output of the tuner (especially when operating on 20M.)
I just have a nagging sense that things might work better with a Balun on the output of the tuner as well (especially if I also decide that having a transformer on the output of the tuner is helpful.)
If I get ambitious enough I might take an oscilloscope with me to the cottage and look at the voltage coming out of the tuner and try measuring the currents in the feeder with a simple clamp on current probe.
Thanks again for the various suggestions. Much to consider.
mspencer12345 at yahoo.ca
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