PART 2: NEC 2 VS "THE REAL WORLD"

I originally wrote:
>
> I recently designed a 3 element wire (#12) beam for 40 meters, Using
> K6STI's NECWIRES implementation NEC2. Segementation used in the NEC
> model was on the order of 100 segements/half wave and further
> segmentation did not change the results dramatically. Great pains were
> then taken to cut the antenna to the precise lengths in the NEC-2 model.
> Upon final construction the antenna appears to perform very close to the
> model when compared to a reference dipole and vertical. By "performs
> very close" I mean that the gian, front to back and 2:1 SWR bandwidths
> are all very close to what the model predicts. The only problem I have
> is that the antenna is resonant at 6900 khz instead of the "design
> frequency" of 7050 khz.
>
> 1)Is this 2% error typical and predictable for all (non -loop) very thin
> wire elements in NEC-2?
>
> 2)Should I reduce all the element lengths by 2% to get to a design that
> really does resonate at 7050 khz? (Or I guess I could raise the design
> frequency by 2% and do my optimiztions there?).
>
> Comments, personal experience, etc apprecited.
>

Duhhhhh.

I should know better! (I do know better ! Advancing age is my only excuse).

Several folks responded by asking if I was using insulated wire. The answer is YES as it happens
I usually use bare wire but for several "mechanical" reasons I dipped into a surplus 5000' role
of of insulated/ stranded #12 for this particular antenna. Anyway guess I'll model the thing 2%
higher and then recut. It has been well documented in the past that the insulted wire will lower
the frequency of any given design. When I get a chance I will run a controlled set of experiments
 with some dipoles insulated vs bare wire It may take several months but I will publish the
results here when I get done.

Thanks to all who responded.

Dave
N0DH/7
dit dit
Received on Mon Jun 17 1996 - 14:16:00 EDT