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1-4 of 4 messages
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To Balun or Not To Balun feeding a full wave loop?
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by K8LEC on June 22, 2004
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I am putting up a full wave loop for 80 meters, which I also hope to use on 40 with a tuner. I want to use coax to feed it. I have read conflicting reports on whether to use a 1:1 balun to trasition from the coax to the wire, or just connect the coax directly to the wire. I have a Unadilla W2AU 1:1 balun which I assume would be a decent choice if a balun was used. Or would a different type of balun work better? Seems like using a PL-259 on the end of the coax and soldering the loop to either side of the balun would make for a better and sturdier connection, but I'm new at antenna building and looking for advice. Thanks in advance!!
Lars
K8LEC
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RE: To Balun or Not To Balun feeding a full wave l
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by WR8D on June 22, 2004
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Hello Lars,
I've used the full wave 80 meter loop for years. I have also cut dipoles for separate bands and compaired them on the bands at the same time against the loop and the loop always beats them. I used to feed mine with 450ohm wire and a 4:1 balun. I've now made some homemade 6inch spaced wire feeders out of number 12 stranded wire. This works much better than the commercial made 450 ohm "ladder wire". You can use coax but there's a 75 ohm matching stub that you have to put in first at the feed point then after that you hook up 50 ohm coax to run on into the shack. This only works well on 80 meters though. If you want to really have a good performing loop feed it with the commercial 450 ohm ladder line and you will need a 4.1 instead of a 1.1... The feep point impedance is around 600 ohms on the loop so the 1.1 just won't get you in the ballpark. You'll also always have to use an antenna tuner with this setup. You can get a good match with the 75 ohm stub and all that coax on into the shack but that limits the antenna to just 80 meters. I've worked japan on 20 barefoot with mine using the 450 ohm stuff into the 4:1 balun and it really plays on 40. It also needs to be installed from 20 to 40 feet. If you put it above that it changes the radiation angle. Its a real "sky hook" i'm sure you'll love it. I've had mine up now for about 10 years and just quit experimenting on wire antennas to try to beat it.
Good luck with the loop!
John WR8D
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RE: To Balun or Not To Balun feeding a full wave l
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by KI8DJ on June 22, 2004
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I use a 40 meter loop,horizontally oriented.It gives me about a 1.2 to 1 on 7.160 mhz while being fed directly with 50 ohm coax. I don't notice any rf in the shack. It is my understanding that the feedpoint impedance of a full wave loop a halfwave up or higher is approximately 100 ohms. My loop is lower only about 15 feet off the ground so the impedance falls nearer to 50 ohms. I suspect an 80 meter loop at 30 ft or so would behave the same way. A full wave loop is also resonant on even harmonics so 40 and 20 and 10 meters should be very easy to tune even with coax.A low antenna like this will be a good nvis antenna on the low bands with lower angle radiation if you tune it up on the higher bands.If you place the antenna higher off the ground and use coax for a feedline you might consider a 2 to 1 balun or a 75 ohm coaxial matching section.Good luck Gary ki8dj
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RE: To Balun or Not To Balun feeding a full wave l
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by WB6BYU on June 22, 2004
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All of the above...
I've used loops with direct coax feed, a 1 : 1 balun,
and a 4 : 1 balun. Plus open wire line, 300 ohm
twinlead, dual coax cables, and even zip cord (you use
whatever is available in a logging camp!) To be
honest, the physical environment, height above ground,
ground conductivity, and other factors make a bigger
difference in performance than the feedline type.
If you read through the articles on horizontal loop
antenna from W4RNL:
http://www.cebik.com/radio.html
You will find that the input impedance is usually over
100 ohms at resonance, and often in the 200 to 300 ohm
range on the higher harmonics (for a loop up 50').
This means a 4 : 1 balun may actually be a good choice
in many installations. One particular loop configuration
I tried with a 4 : 1 balun had an SWR under 1.5 : 1 at
some point on all bands 160 - 10m. Especially for
loops at lower heights, direct 50 ohm coax feed may
give a usable SWR over parts of the 80m band without
a tuner, depending on how sensitivie your rig is.
Certainly on 40 and 80m the SWR will be low enough
that a built-in autotuner should match it and
feedline losses will be low.
So it comes down to a personal preference. At low
heights the 50 ohm coax is likely to give you a better
match on 80m, while with a higher antenna a 4 : 1 may
have a lower SWR on 40m. But for initial testing
you can just put it up with direct coax feed (50 or
75 ohm) and use it without a balun.
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