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Marconi
Advanced Member
Username: Marconi
Post Number: 549
Registered: 11-2001
| | Posted on Saturday, January 21, 2006 - 3:50 pm: | 
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This question has come up in a discussion, and I need a good definition and a description of exactly what a balanced antenna is. I am interested only in 11 meter base antennas in this regard, among them include the Imax, A99, I10k, Wolf .64, coilys new base, the V5/8, Penatrator, Astroplane, Starduster, 1/2 wave dipole, a folded dipole, a gamma fed yagi beam, a yagi with a hairpin feeder using an insulated driven element, a quad beam, and a quarter wave ground plane.
What makes one antenna a balanced antenna and another not a balanced antenna?
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Tech833
Moderator
Username: Tech833
Post Number: 1162
Registered: 8-2002
| | Posted on Sunday, January 22, 2006 - 2:20 am: |
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Balanced or unbalanced refers to the way they are fed and the element layout. A center fed dipole (8 feet either side with twinlead feeding at the center) is balanced in every sense of the word. balanced fed, balanced elements, balanced current distribution, everything.
You can center feed a dipole with unbalanced coax using a hairpin feed, coax balun, or some other means. In this case, the antenna is indeed balanced, but the feed is not.
Any end fed antenna is unbalanced. Any ground plane antenna is unbalanced. |
Kj7gs
Junior Member
Username: Kj7gs
Post Number: 39
Registered: 12-2004
| | Posted on Tuesday, January 24, 2006 - 8:18 pm: |
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So no common mode currents if I have coax feeding an end fed vertical? |
Tech833
Moderator
Username: Tech833
Post Number: 1167
Registered: 8-2002
| | Posted on Wednesday, January 25, 2006 - 12:06 am: |
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In a perfect world, no. However, this is an imperfect world. With an end fed vertical, you will get stray current in some cases. |
Marconi
Advanced Member
Username: Marconi
Post Number: 558
Registered: 11-2001
| | Posted on Wednesday, January 25, 2006 - 5:58 pm: |
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Some years ago a friend of mine read the following article: http://www.arrl.org/tis/info/pdf/8004019.pdf
I'm not sure, but I may have discussed this issue before on this forum. If so, I apologize for any duplication of effort.
In this article it briefly discusses a bazooka balun, front page, last column. In our minds at that point in time we saw this as being similar to the sleeved setup already used on a Starduster antenna. The problem was that this antenna was, what we called then....RF dirty. We felt we saw the problem using a simple little mobile field strength meter. We decided to use this idea as best we understood it and modify the SD mast to accommodate a 9' jumper and maybe solve this problem since we felt this article was discussing such events. The problem was, currents flowing on the outside of our feed line or mast for the antenna and acting bad in the neighborhood.
We shortened the owner supplied antenna mast to a bit less than 9' so that we could get to the end of the jumper where we attached it to the balance of the feed line going to the radio. We used a simple barrel connector the coax. We then attached the barrel connector using a SS hose clamp physically to the supporting mast to bring the coax back to ground at 1/4 wave length away from the feed point.
It was in our minds that this was similar to the bazooka described in the article. Excepting that the bazooka was electrically insulate at the top where the hub and mast came together, and we could not figure an easy way to do that.
After installation we did see a remarkable reduction in RF below the antenna and on the feed line. And after a while using the modified antenna the owner felt the antenna responded much better both locally, with distance, and the noise and TVI was mostly improved.
833 and others what are your thoughts on this idea?
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Kj7gs
Junior Member
Username: Kj7gs
Post Number: 41
Registered: 12-2004
| | Posted on Wednesday, February 08, 2006 - 10:58 pm: |
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Marconi, I didn't read the article, but it sounds like you did what you should have done according to my copy of the ARRL Antenna Book. Quarter-wave multiples of feed line give you the best results as far as reduction of common mode currents (would this give credence to the 18' "rule"?), and you may have had an existing half-wave multiple (or something near that) of feed line in place that you simply modified by adding the jumper. |
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