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NEC2 - Correcting for element taper on a loaded element?
Hi All,
I tried this post on the towertalk reflector. I should have tried it first as I find more of you NEC guy hang out here (why is that anyway!??!?). I was modeling a Cushcraft 402CD loaded 40m yagi. I use Nec-Win Plus and MultiNEC. I enabled stepped diameter correction in both programs and noticed that in both programs a taper correction didn't occur. The output from NEC-Win Plus during the taper correction seemed unhappy because the element resonated around 11Mhz, but the simulation frequency range was 7 Mhz. How can I calculate a taper correction for a loaded element? -Scott, WU2X |
#2
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NEC2 - Correcting for element taper on a loaded element?
On Mon, 27 Apr 2009 12:05:37 -0700 (PDT), Scott M
wrote: How can I calculate a taper correction for a loaded element? Hi Scott, It sounds like you have more problems than taper if your 40M antenna is resonating at 11 MHz. The clue appears to be in the word "loaded element." 73's Richard Clark, KB7QHC |
#3
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NEC2 - Correcting for element taper on a loaded element?
I meant without the loads (inductors) in place. Regardless if they
are in the model or not, the model isn't being converted to a constant diameter. -Scott On Apr 27, 4:54*pm, Richard Clark wrote: On Mon, 27 Apr 2009 12:05:37 -0700 (PDT), Scott M It sounds like you have more problems than taper if your 40M antenna is resonating at 11 MHz. * |
#4
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NEC2 - Correcting for element taper on a loaded element?
On Mon, 27 Apr 2009 14:20:05 -0700 (PDT), Scott M
wrote: I meant without the loads (inductors) in place. Hi Scott, Then that detail is not essential. Regardless if they are in the model or not, the model isn't being converted to a constant diameter. -Scott This would surprise me if it did. Tapering involves the segments of one wire being converted into wires of diminishing length. 73's Richard Clark, KB7QHC |
#5
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NEC2 - Correcting for element taper on a loaded element?
On Mon, 27 Apr 2009 12:05:37 -0700 (PDT), Scott M
wrote: I enabled stepped diameter correction in both programs and noticed that in both programs a taper correction didn't occur. Hi Scott, I went back to your original posting after pondering how your question: How can I calculate a taper correction for a loaded element? wanders through several topics - such as above. Taper and stepped diameter are not the same thing, although tangentially related and asking for "how to calculate" jumps the tracks entirely. How about stripping out the extraneous details and simply describing what your problem is? 73's Richard Clark, KB7QHC |
#6
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NEC2 - Correcting for element taper on a loaded element?
Richard Clark wrote:
On Mon, 27 Apr 2009 14:20:05 -0700 (PDT), Scott M wrote: I meant without the loads (inductors) in place. Hi Scott, Then that detail is not essential. Regardless if they are in the model or not, the model isn't being converted to a constant diameter. -Scott This would surprise me if it did. Tapering involves the segments of one wire being converted into wires of diminishing length. 73's Richard Clark, KB7QHC "Tapering" is used to mean two different things. One is segment length tapering, as Richard describes. It was essential in MININEC-based programs to get accurate results from wires connecting at an angle, and it's only occasionally useful with NEC based programs in other situations. EZNEC has an automated segment length tapering feature left over from its MININEC-based predecessor ELNEC. More detail can be found in the EZNEC manual index under "Segment Length Tapering". The demo version includes the full manual, and this feature. The other meaning of "tapering", which is what the thread is about, is diameter tapering, exemplified by a Yagi element made from telescoping tubing. This tapering has a very significant effect on element length. NEC-2 produces a small error whenever wires of different diameter are connected, and this error is enough to seriously affect the pattern of a Yagi made with diameter-tapered elements. An element of a single diameter can be substituted which gives the same result as the diameter-tapered element, but the method isn't intuitively obvious. The length of the equivalent is different from the original, and the diameter isn't an average of the diameters. The method was developed by Dave Leeson W6QHS and described in his book "Physical Design of Yagi Antennas" (ARRL). EZNEC and some other NEC-2 based programs incorporate Leeson's method and automatically make the substitution when appropriate. The method assumes a sinusoidal current distribution, so EZNEC won't make the substitution if the element contains traps or loading coils, or for other reasons isn't close to natural resonance. But EZNEC also has a method of showing the user whether the substitution is being made, what the substituted wire length and diameter are, and if it's not being made, the reason why. More information is in the EZNEC manual, indexed under "Stepped Diameter Correction". It sounds like the OP's program normally does a similar substitution but doesn't tell the user the reason when it doesn't. The software developer should be able to explain the conditions under which the substitution is applied. Roy Lewallen, W7EL |
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