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Author Topic: dipole or just a long wire???  (Read 762 times)
K6RAS
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Posts: 78




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« on: February 08, 2001, 11:12:04 PM »

You didn't mention the elevation of your site with respect to sea level but I'll tell you something about my own set-up just for reference.
My QTH is at 2000 feet in the Sierra Foothills; mixed pines (real tall pines  -  those 100 foot varieties) and some oak.  The house is in a shallow valley, between three hill-tops which are perhaps 60 to 100 feet above the house.
I use a simple dipole which points NW/SE, situated at 45 feet (running between two trees) into which I pump about 150 watts or less for both CW and SSB.  The performance is fantastic.  Of course, I did trim the antenna 1/4 inch at a time to achieve maximum performance but an exercise that I'd do under any circumstances so it wasn't extra trouble.
Keep in mind that a long wire radiates differently than a dipole (or anything else you might care to build) so you should make a decision on what radiation pattern will best suit your needs before you get to deep into your design.
Perhaps WB2WIK or WB6BYU will sign on one evening and share their perspective with you.
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N7ZDR
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Posts: 68




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« Reply #1 on: February 08, 2001, 05:18:10 PM »

Well we live in the woods on a clif overlooking the creek about 75 yards down to the water. thinking about running a long wire off the porch and down across the draw?Huh We do have tall pines but I would be over the house?? ( tin roof). Someone have any hints about what would work or what will not work. Thank you Larry
ps the draw is about 100 yard across!





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VA3EP
Member

Posts: 38


WWW

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« Reply #2 on: February 08, 2001, 09:27:02 PM »


I did quite a bit of research about long wires vs. dipoles before I put up a limited space dipole in my back yard. I only have about 70 feet from the back of my one story house and a utility (cable / phone) in the back corner of the lot.

I was going to put up a random longwire, what stopped me was that research showed I needed a GOOD RF ground. There is a lot of theory behind this (the ground acts as a "reflector" to complete the other half of the dipole, the long wire acting as the first half). The other way to explain it is that the feed for the long has to work "against" something to crank out a signal.

Simply connecting to the ground of the wall outlet is not enough as at RF frequencies, due to distributed inductance and capacitance, this is NOT really a ground and you get a lot of RF in the "shack"; i.e. the radio (and probably you) act as the other half of the dipole and radiate like mad :-)

Bottom line is you need a good RF ground to connect the outside of the coax against (center goes to the long wire) and this can be as many as possible and as long as possible radial wires, either buried or laid on the ground surface. The other good ground is 4 to 6 eight foot copper ground rods driven into the earth about in a circles 6 feet apart and bonded with heavy copper wire.

You then can feed the long wire (center conductor) and ground (braid) from a tuner and use the long wire on a number of bands.

The better solution (for me) was to string up the longest dipole possible (I ran it diagonally across the back yard from eave rain trough (about 12 feet high) to the utility pole (about 20 feet high)) and run the dipole center feed coax to a tuner (AT-11). Even thought I only have a 66 foot dipole the AT-11 can load it up from 80 to 10 meters, I just checked into a net 500 Km away with that setup on 80. Also worked a local 10M FM net the other night. It would probably be better if I fed it with ladder line and a balun at the tuner (less losses, as the VSWR on a non resonant antenna is high, and coax is more lossy with hign VSWR).

Do yourself a favor and get the ARRL handbook and read the section on wire antennas, or get the ARRL antenna handboodk. I also found a couple of good papers

   "HF SSB Users Guide"
   "Smart tuners for Stealth Antennas"
at
   http://www.sgcworld.com/publications/pubs.htm

Bottom line, get as much wire up as high as possible, the dipole if probably better in most cases, if you have to use a long wire lay a lot of radials or use a lot of ground rods, and feed it with a tuner, ladder line being better than coax.

Have fun!

Eric VA3EP
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WB6BYU
Member

Posts: 13485




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« Reply #3 on: February 09, 2001, 12:49:54 PM »

I guess with an opening like that, I should say something...

I've had quite good results with long wires, and feeding them
needn't be a problem.  However, as they get longer, the pattern
becomes more directive (towards the ends of the wire).  This can be
a problem with a sloping wire on the higher bands, as maximum
radiation ends up going into the other side of the valley instead of
off into space.  I recommend that long wires more than, say, 2
wavelengths be installed horizontally, unless you verify that the
resultant radiation angle is greater than the slope of the wire.
(The ARRL antenna books will show you the radiation patterns for
different lengths of longwire antennas.)

My recommendation would be to use a dipole at (or near) the top
of the slope.  By taking advantage of the sloping ground, you may
be able to get a good low-angle radiation even if the antenna is
not too high off the ground.  

On the other hand, long wires are cheap, and it may be interesting
to string up one temporarily to see how it works in your situation.
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AC5UP
Member

Posts: 3927




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« Reply #4 on: February 09, 2001, 01:31:34 PM »

I've played with longwires, dipoles, loops and extended double zepps. Every longwire I've tried reminds me how much I like extended double zepps and loops. Could be a whole different story at your QTH, but I've had much better luck with antennas that are inherently balanced to ground. An extended double zepp will beat the pants off a dipole in two directions but requires more than twice the length. However, they can be pruned to absolute SWR perfection with excellent bandwidth and it's a rare dipole that can do that.

'73
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WB2WIK
Member

Posts: 20636




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« Reply #5 on: February 09, 2001, 04:27:21 PM »

Two cents' worth: I like long wires, they can work just wonderfully in the right situation, e.g., when you can't get at the center of an antenna to feed it there!  A wire strung across a canyon, with a shack on one rim of the canyon, is a good example of this -- long wire, here we come!  Although just running the wire into the shack and using a tuner against ground will obviously work, other interesting feed methods include the "Zepp" method, using ladder line with one side of the ladder line grounded the the other conductor feeding the long wire, etc.  All in the antenna books, nothing new.  But one thing some folks forget is how difficult it is to end-feed a wire that is 1/2-wavelength long!  Avoid this length.  If you want your antenna to work on 40/80m, for example, avoid any multiple of 65 feet, +/- a few feet.  I would avoid any multiple of 63 to 67 feet, e.g., 126-134'is no good; 189-201' is no good; 252-268' is no good, etc.  Try 230 feet, or 280 feet, something like this.  Avoiding the 1/2-wave multiple problem can save a lot of time and trouble trying to tune an antenna having such a high feedpoint impedance that almost nothing will work to match it.  Even if you do successfully tame a 1/2-wave end-fed, there can still be RFI problems, RF "feedback" problems (hot mike syndrome, etc.), and arcing tuner problems.  Just not worth trying.  Good luck & work lots of DX!
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NB6Z
Member

Posts: 550


WWW

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« Reply #6 on: February 11, 2001, 07:29:37 PM »

I better solution with 66 feet of horizontal room and a AT-11 tuner is the NB6Zep! Check it out at http://www.teleport.com/~nb6z/nb6zep.htm
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