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Author Topic: "Backwards" balun question  (Read 1090 times)
WD5ABC
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Posts: 37




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« on: March 22, 2008, 03:57:18 PM »

I built a homebrew 40m vertical, 30 ft tall with some top loading, it works great on 40m and 30m.  I'm feeding it with coax and it has 4 - 33ft radials.  Here's my question.  I'd like to use it as a non-resonant antenna on some other bands just for grins to see what it does.  I've used my tuner to tune it and it works but there's going to be a lot of loss in that coax, especially on the higher bands.  Have any of you guys ever tried feeding a 50 ohm (more or less...) vertical with twin lead through a balun?  It's a little under 50 ohms at 40m but the impedance on other bands is going to be all over the place.

I just put this antenna up last weekend and haven't had time to do any real tests with a FS meter but contacts on the air comparing it to my low dipole and my 4BTV seem to indicate it's better than both of them on 40m and 30m.

Have any of you guys tried something like that?  Just curious before I start playing with feedlines and baluns.

Tnx,
Kerry, WD5ABC
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KV9U
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Posts: 166




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« Reply #1 on: March 22, 2008, 05:37:18 PM »

Before I purchased a Butternut HF-9V vertical that is ground mounted, I had a homebrew multiband vertical in the same location. At one time I did feed it with open wire line, a few feet off the ground. I then used a 4:1 balun to change to coax just before it entered the house.

The antenna was around 30 feet high, but I also added some extra top hat wires to make it work better on 75 meters. It would have high angle radiation on the higher bands though even though I could tune it up most anyplace.

Although I am not a DXer, I was able to hear a fairly weak 75 meter KH6 phone station in the evening hours and was quite surprised that I was able to work them on the first call. I have never been able to do that with a low dipole from my QTH here in SW Wisconsin.

The advantage of using the Butternut is that it has a very clever design to use most of the antenna to advantage on a number of bands and allows you to operate on higher bands. With a 40 meter vertical, the radiation on 12 and 10 meters, and probably 15 meters,  may not be optimum.
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K7STO
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Posts: 45




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« Reply #2 on: March 22, 2008, 07:18:10 PM »

I have a homebrew 33 ft vertical with 32 radials of various lengths, which range from 11 ft to 20 ft as I have limited space for the radials. I used AD5X ‘s design concept for a no trap multi-band vertical. The antenna is a 40/20/10 direct connected thru a 4:1 balun at the base of the antenna. I use an external tuner at the shack for all other bands. I built the 4:1 balun using one of Jerry Sevick’s designs. This does help at matching the antenna impedance with the coax (RG-213).
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WD5ABC
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Posts: 37




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« Reply #3 on: March 23, 2008, 01:39:14 PM »

Ok, great, both of your answers are helpful.  I was thinking about running twin lead (or some variation of it) from the antenna to the shack then use a 4:1 balun at the shack, maybe just use the one in the tuner.  My other option was a 4:1 balun at the base of the antenna hooked up with the low z site facing the antenna.  I haven't ever tried feeding an unbalanced antenna with balanced line.  At least I know I'm not the first, maybe I'll give it a shot.  For now it's working great on 40m and tunes up well on 30m with a little help from the tuner in the rig.

Thanks for the input guys!

73,
Kerry, WD5ABC
PS  Never thought about feeding a commercial vertical that way but it might be a good way to eliminate some feedline loss even though the antenna should be approx. 50 ohms on all bands.
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K8GU
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« Reply #4 on: March 24, 2008, 06:27:57 AM »

Here's an example of someone doing just what you describe very successfully!

http://www.k2kw.com/4m1x/4m1x.html
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WD5ABC
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Posts: 37




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« Reply #5 on: March 24, 2008, 08:00:29 PM »

I've never seen that site, thanks.  You're right, they're doing exactly what I was talking about but I don't need anywhere NEAR that much feedline!  I'm thinking less than 75 ft.  They did it to reduce feedline loss due to the distance, I'm doing it to reduce loss due to swr but the outcome is the same.  I'm glad I'm not reinventing the wheel!

Thanks for the link.

73,
Kerry, WD5ABC
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KL7AJ
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« Reply #6 on: March 25, 2008, 08:07:39 AM »

Should work fine.  A balun is a passive, reciprocal device.  In this application, however, you're using it more as a straight transformer than as a balun.  But certainly worth a shot.

eric
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WD4HXG
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Posts: 182




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« Reply #7 on: April 16, 2008, 03:33:34 PM »

Kerry

A 4:1 balun for a specific impedance will exhibit narrower bandwidth when used off the spec impedance. So if you use a 4:1 balun speced for 50 to 12.5 ohms and attach the 12.5 ohm side to a load other than 12.5 ohms you can expect a bandwidth reduction. Since baluns are usually viewed as a wideband device the performance impact can be substantial.

Ferrite core baluns can be a real problem as the core often winds up dissipating the lost power, heating the core beyond the Curie point and worse cracking the core due to thermal stresses.  

I would use an impedance matching network at the base of the antenna and then use the balun between the feedline and the input of the matching network.
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