Would anyone care to sanity check my model and see if I did this right?

Yeah, it looks good, but you should adjust the AGT to 1.0 (0db). Taking the correction into account, your numbers seem to be about right on.

I have some 1/16 brass rod for the active elements.

If you look at F1, Help > Reference data > Wire conductivity, brass is pretty far down the list. But on a cost benefit ratio, its better to use what you have, heh.

Here are some useful info and notes by Autofils, added onto by me :

For 4nec2, there are tests that you can perform, to ensure a reliable model:

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1. From the Geometry (F3) window, Validate tab :

a) Run geometry check 'Q' and make corrections if needed.

b) Run segment checks '9' and make corrections if needed.

2. Run the Average gain Test (AGT) and adjust the wire diameter and/or the number of segments of the Voltage source tag to achieve an AGT result as close to AGT=1.0 (-0dB) as possible, at the middle and ends of the desired frequency range. For the new UHF range, the middle is 584mhz, ends are 470 and 698 mhz. Its also useful to have a printout of wire radius sizes in inches(millimeters) handy.

3. Run a convergence test, where the gain or F/B ratio has a relatively small change when you increase the number of segments in your model.

To find out more on AGT and convergence, type "AGT" or "Convergence" in the 4nec2 General Help index window.(F1)

Basic and "safe" segmentation rules for nec2 modeling:

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Each element of the antenna that you model, is sub-divided into smaller sections called segments. The "safe" rules for how you determine the segments are:

1. Use at least 10 segments per half wavelength of wire at the highest operating frequency.

(Ten segments per full wavelength is sometimes used to specify the longest allowable segment length -- 0.1 -- but doubling the segment count yields more accurate results for a larger variety of geometric wire assemblies.)

2. Use a segment-length-to-diameter ratio of at least 4:1.

(Although the absolute limit is sometimes given as a segment-length-to-radius ratio of 1:1, the much larger recommended ratio tends to prevent problems in complex geometries with angular junctions of wires.)

3. If an element is composed of more than one wire, the length of segments on each wire in the assembly should be as equal as possible.

4. To the degree possible, for parallel wires, let the segment junctions align as closely as possible.

(This rule is absolutely essential when wires are closely spaced, as in a folded dipole, and thus makes good sense as a general practice in all modeling.)

5. The Voltage source tag wire (the wire referred to in the "EX" card) should have a minimum of 3 segments. Using 1 segment can produce unreliable results.

More notes:

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1. Model just array-only first and explore it's gain characteristic for the freq range you are designing.

2. Try some variations in the array itself, looking for what variations provide the most raw gain.

Keep an eye on swr, as well as max raw gain. You should consider a swr of 4 as worst case for your design and have an objective of less than 2, and as close to 1 as you can get.

3. Once you have a good array design , now add the reflectors and run optimization runs to get the overall best design.

4. Set Auto-Segmentation to (per this forums consensus) of around 21 to avoid misreadings of gain when Auto-Segmentation is set to 13 - 15.

It's really that simple!!

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Since its not built directly into the program, Ill explain how to calculate Net Gain with 4Nec2.

Here is a method to calculate Net Gain:

NetGain = RawGain+10*log(Feed-pointGain)

where Feed-pointGain = 4*Zr*Zo/((Zr+Zo)^2+Zi^2)

The [10*log(base10)]" converts to decibels, and Feed-point Gain is less than 1, since there is a loss.

Zo = characteristic Impedance for the transmission line connected to the antenna (For use with 300 ohm twin lead or a 4:1 balun, Zo = 300)

Zr = real part of antenna's complex impedance at a specific frequency

Zi = imaginary part of antenna's complex impedance at a specific frequency

RawGain = Gain output given by 4nec2 at the specific frequency

The Raw Gain, Real Impedance, and Imaginary Impedance data can be obtained when a Frequency Sweep is calculated. The 4nec2 program then outputs a Imp./SWR/Gain (F5) Line Chart. Under the "Plot" option is the option to plot Forw-gain, R-in (real), X-in (imag). It will give an error message, ie wGnuPlot.exe not found. Ignore it and it will output/display a text file with the needed data. Cut and paste the relevant data into a spreadsheet of your choice for your Net Gain calculations.

A quick way to look at the SWR figures to subtract from Raw Gain :

Code:

```
SWR Mismatch Loss dBi
----- -----------------
1.0 0
1.5 .18
2.0 .51
2.5 .88
3.0 1.25
3.5 1.60
4.0 1.94
4.5 2.25
5.0 2.55
6.0 3.08
```