TheRadioBoard

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73 De KD8DEY

Wow - wish I knew enough to drive it.

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I spent a little bit of time learning how to drive 4NEC2. I attempted to use it to model my 12 turn box loop antenna and even to try and model spiral air coils. But I'm not 100% confident the results are valid. I am currently using some other free software to try and model my inductors and coils.


Here is my loop antenna project with the antenna pattern at 500kHz




Sprial air coil.


I can try to write a brief tutorial if there is enough interest.

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Überdyne: An extraordinary small force.
Kevin

At least not on my account, Kev - it's way over my head.

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I'm definitely interested!

Can 4NEC2 read EZNEC files properly? I tried importing an EZNEC file some time ago and I got some errors that I didn't understand when I tried to simulate the antenna.

I'm interested in modeling the efficiency of small antennas (e.g. this design is xx% efficient compared to a half-wave dipole), including inductor loss, wire loss, ground loss, etc. Can 4NEC2 output a single number like "xx% efficient"?

Also can 4NEC2 be used to calculate RF exposure in the near field? I think this can be done in EZNEC.

I haven't looked at EZNEC files, but there should be a way to do it. 4NEC2 uses NEC2 text input files, so it should be possible to just look at a EZNEC file and see if it is the same format.

I learned a little bit about the NEC2 input file format from a book called
Small Antenna Design
by Douglas B. Miron

The program can generate all sorts of results including efficiency, near field, far field pattern, impedance, etc.

There are ground models and ways to simulate antennas over a ground model, but I have not attempted to try and use the ground models.

NEC2 does take into consideration the conductor loss, but it does not take into account skin effect or proximity effect losses.

4NEC2 does have a simple tool for impedance matching that allows you to set the Q of the inductors and capacitors in the match. The tool will calculate the matching network values from the antenna impedance generated by the NEC2 results.

Here is a simple 1meter dipole example at 299MHz in free space






Here is the near E field plot (you can also plot the near magnetic field too)

_________________
Überdyne: An extraordinary small force.
Kevin

OK, so I imported N3OX's short capacity-hatted dipole EZNEC file (supplied by N3OX) into 4nec2 and made some tweaks so 4nec2 would simulate it (there were some connected wires with radius differences of greater than a factor of 10, which 4nec2 doesn't seem to be able to handle). Radiator is 4m long and 30mm diameter, and capacity hat spokes are 3mm diameter.





The last image seems to indicate that loading inductance, with series loss resistance, is already in the model. Is that right? If not, what could be the meaning of the last three lines that contain series inductance values? And how should I go about adding loading inductances, if I need to that manually?

Now, for the output: Does this simulation result look at all reasonable? I calculated the NEC output data for the far field, and also did a run of "Start Optimizer", for which I'm not sure what it actually did.





Pretend I know nothing about interpreting the output results, which isn't so far from the truth... Did I run the simulation correctly? What do I want to look at here? For example, I'm not sure if the structure as-is is even resonant at 7 MHz (I made some changes to the original file, and also there may have been some errors during EZNEC import) - where could I see or deduce the resonant frequency? Also, can the radiation efficiency of 71% be believed as-is? You can make a 1cm dipole efficient if you can drive thousands of amps through it, but obviously that is impractical. How can the simulation results be properly interpreted?

EDIT: Well, reality is starting to set in. Simulating the antenna over real ground of type "moderate ground" (and not in free space) suddenly causes the radiation efficiency to plummet to around 30% at a height of 5m at 7 MHz. Low-angle takeoff is also, as to be expected, poor. The fact that I'm seeing these results though seems to indicate that I'm using the simulator approximately correctly. And 30% efficiency might still be enough to make local contacts.

I find it a usefull program, I have MMANA and this one on my PC. I designed my 144.300Mhz beam with it. (13 element, 8,5meters length)

After building it it needed some small adjustments to get it perfect.

73
Frank

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www.pa2mrx.nl