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Author Topic: Four Bands Off Center antenna  (Read 507 times)
W4XKE
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Posts: 67




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« on: June 22, 2001, 08:20:59 AM »

After looking over several antenna styles, I chose to put up an antenna that was described in the ARRL’s Wire Antenna Classics book, originally featured in QST (February 1996).  It would be nice to have a single antenna that could be used on 40, 20 and 10 meters without a tuner.  According to the article, 15 meters could also be used by changing the 4:1 balun to a 1:1 balun.  Actually, the three bands would suit my desires quite well.
I put the antenna up at 51 feet high, today.  It has 23 feet of wire on one side and 46 feet of wire on the other.  It is fed with forty-nine feet, two inches of 300 ohm TV ribbon (foam) and matched with a 4:1 voltage balun.  RG-8 makes up the remainder of the feed line to the transceiver.  It was tuned by adjusting the length of TV ribbon with an MFJ-249 antenna analyzer.  About 10” needed to be pruned from the recommended 50 feet to make the antenna come in at 7.260 MHz on 40 meters.
I expected 20 meters and 10 meters to come in at their respective frequencies, according to the article.  This has been a very disappointing project.  The others actually indicated 15.100 MHz and 26.50 MHz respectively.  Oh well, I’ll just have to use a tuner… not what I planned but still, it would work okay.  I quickly discovered that the 10-meter bandwidth was as narrow as a knife’s edge.  Another big disappointment followed.  Like it is often said, “You can’t work them if you can’t hear them.”  With this antenna, I sure can’t hear them.  Only the strongest of signals were received… very few of the 20 meter maritime net call in’s were received.  I didn’t bother to transmit.  There were probably dozens of QSO’s out there that I would interfere with that were not strong enough for me to hear.
Apparently, Bill Wright (G0FAH), the author of this article was more successful with the design than I have been.  I really cannot account for the dismal performance.  I was careful to follow the instructions very closely but it has all turned to trash for me.  I wonder if anyone else has installed a “Four Bands Off Center” antenna?  I keep thinking that I must have made a blunder somewhere, but how could such a cut-and-dried (proven?) design pose a hidden obstacle that could possibly be overlooked?  Next week I will look into a classic, flat top, mono-band dipole.  Surely, I can’t foul that up.       Johnny W4XKE
 
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AC5UP
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Posts: 3844




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« Reply #1 on: June 22, 2001, 11:57:43 AM »

I've used the 136' version with great DX success and tried the shorter cousin for a GP SWL antenna without complaint. (not the best on AM broadcast, but it's FB above 3 MHz or so) The first thing I'd do with yours is build a 4:1 dual-core Guanella CURRENT balun and hang it directly at the feed point. Coil up your twin lead and store it for another project. Feed the antenna directly with RG-8X. (RG-213 is usually too heavy)

A current balun encourages equal antenna currents on both sides of the feed point. Although the antenna is asymetric, you'll have better luck and more stable tuning with a current feed than with a voltage balun. Best guess as to why yours is so touchy on SWR is reactance in the twin lead. It's pretending to be a tuned stub. Try a direct coax feed and don't be surprised if you have to re-prune the length.

Wire is fairly cheap, think of it as a low-cost education...
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NB6Z
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Posts: 550


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« Reply #2 on: June 22, 2001, 12:38:51 PM »

Johnny,

Don't be too discouraged! I had the same type result when I built my first off-center fed mullti-band wire antenna. I modelled my first attempt after the old design in the ARRL hand books for a "windom" using open ladder line (I used outdoor TV twinlead). Even with a tuner at the transciever, it was problematic... Tuning was too sensitive and loads of RF in the shack.
I re-grouped to try again... This time I tried using the dimensions and materials described for a "Carolina Windom" marketted by Radio Works. I even used one of their line chokes at the end of the twin lead. I tried several winding ratios on the balun coil I made to connect to the coax. This design was more succesful, but the performance was not as good as a dipole I used for refernce and there was still some RF in the shack. (I assume the commercial antenna would have worked better than the one I built.)
The concept of this type antenna really appealled to me because it would "fit" my QTH sitiuation so well. So on my third attempt I went back to the basics; and came up with a center fed design this time. A 20 meter Extended Double Zepp (EDZ) would fit (80 feet) and could be tuned for the other bands with a tuner. (The basic EDZ antenna is described in the ARRL handbooks.) Since it was center fed, there was no more RFI issue and I could count on all the radiation to come from the horizontal wire. This antenna worked great with a simple MFJ tuner (balanced output) for several years until a storm brought down one of the trees holding the wire up.
On my fourth attempt I got real serious! I bought an antenna modelling program and went to work on designing the ultimate wire antenna for my situation. Some of the results I came up with may be of interrest to you since they use the same materials that you already have on hand. I have documented the construction and performance of the NB6Zep antenna on my website at:
 http://www.teleport.com/~nb6z/nb6zep.htm I also have several other wire designs you might like to look at before you start your next construction.
Keep trying; the answer is out there...

73, Griff
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