The demo version of EZNEC, with its limit of 20 segments, won't be too
accurate for this array. At a minimum, you should use 10 segments per
half-wavelength, so a 3-element array needs 30 segments just for the
three vee elements, and preferably more for best accuracy. In addition,
since the tower (top loaded by HF yagis) is between the vee elements, it
really needs to be included in the model. The presence of the tower
MIGHT be a non-issue, but it's really hard to tell without modeling it.
Of course, one of the main benefits of modeling is to allow you to
adjust things like the length of the transmission lines to tweak the
array for your particular installation. What was optimum for the
designs that you mention may not be the best at your site (tower a
little too short, etc.).
Whether or not floating the vees will attract more lightning, there's no
question that if the elements charge to a high enough voltage, you can
get noisy, if not damaging, arcing at inconvenient locations. Perhaps
using 100k ohm resistors from both conductors of the element feedlines
to ground is a workable approach, since you need to avoid shorting the
line inputs. Also, if the shields of the feedlines are DC grounded for
lightning, you may need to choke them for RF to avoid pattern
distortion, if the design does not already do so. For completeness, the
shields of the feedlines (with or without chokes) should be included in
the model as a "physical" conductor (with near unity velocity factor),
since the transmission line models in EZNEC are "virtual" or
mathematical models of the transmission line differential-mode behavior
only.
73, Terry N6RY
On 2009-11-17 7:48 PM, K1to@aol.com wrote:
> Design criteria:
> - 3 inverted vee elements, standard #14 or #12 insulated wire fed in the
> middle
> - Ground the unused 2 elements. A friend advises that especially here in
> FL that "floating" the unused elements will allow them to build up a charge,
> thus attracting lightning, so I should ground them instead.
> - Mount on an 87' tower that also has 5 HF Yagis on it. Guys are broken
> up with insulators. The downside of this is that the tower is a few feet
> short for allowing the vees to have the ideal included angle of 90 deg.
> - Use RG8X Mini for the feedlines to the switch box, along with the K3LR
> method of beads over a 14" section of thin coax at each feedpoint.
>
> I downloaded the demo version of EZNEC last night and enjoyed learning it
> tremendously. Wow, Roy -- what a full-functioned app! Anyway, the
> learning curve for entering the wires was not bad at all and I soon had the 3
> vees
> in their sideways positions around the center, all fed in the middle.
>
> But as soon as I got to the Holy Grail problem of how long to make the
> feedlines, I hit my roadblock and don't know how to proceed.
>
> Here's what I've found published to date:
> - K3LR/WA3FET/K3LC design uses 4 sideways vees, floats the unused ones
> (does that matter?), and uses 153.45 degrees of feedline on each element.
> Their ELNEC design was quite precise and thorough. Circa 1992. Full
> article
> in Antenna Compendium Vol 4. See p.11 in particular.
> - K1WA 5L half-wave sloper array uses 135 deg of feedline and opens the
> unused lines. This design goes back to the '70s and was published in many
> ARRL Antenna Books.
> - W7LR built a 4L 40 array and also concluded that using 135 deg was
> correct. See Feb 1995 QST - Tech Correspondence.
>
> Side note - RG8X VF is listed as 0.75 and 0.78 in various places. This is
> trivial, since it just gives a starting place from which to trim more
> precisely.
>
> 73, Dan, K1TO
>
>
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