Modeling Software for OTA Antenna Design & Creation

[ericball]

I have 4nec2 installed and I've been playing around with it and a parameterized SBGH model. I'd like to throw it at the genetic optimizer, but I'm wary of gain/SWR versus net gain. (I'm assuming the gain values aren't net gain.) What's the translation? Or should I just assume that as long as the SWR is close to the un-optimized version that everything is okay?

[Autofils]

Originally Posted by ericball

I have 4nec2 installed and I've been playing around with it and a parameterized SBGH model. I'd like to throw it at the genetic optimizer, but I'm wary of gain/SWR versus net gain. (I'm assuming the gain values aren't net gain.) What's the translation? Or should I just assume that as long as the SWR is close to the un-optimized version that everything is okay?

Ericball,

I sent you a PM, but I didn't get a reply, so I guess you haven't visited the forum for a while.

Here is a method to calculate Net Gain:

NetGain = RawGain+10*log(Feed-pointGain)
where Feed-point Gain = 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 ( Zo=300 for the GH)
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

If you have any programming skills, I would dearly love to obtain an executable that can parse the 4nec2 F8 output file for the RawGain. and Antenna's complex impedance (real and imaginary parts) for each frequency that is stepped in the frequency sweep at the appropriate Phi and Theta values of 90 and 0 degrees.

[Autofils]

Basic and "safe" segmentation rules for nec2 modeling:
===========================================

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.)

Model Reliability Checks
====================

There are two tests that you can perform on your model, to ensure a reliable model:

1. 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.

2. 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 "Find-Window"

[300ohm]

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.)

Fine, but I still dont understand the "why" of massive segmentation of a simple wire in the program. Its one of things that gives me "brain freezes" about the program.

I'm afraid I don't understand what you mean by a cheat sheet ?

You know, like something you buy in a bookstore about shortcuts to Microsoft Word, Excel etc, all the main functions laid out in a page or two.

[Autofils]

Originally Posted by 300ohm

Fine, but I still dont understand the "why" of massive segmentation of a simple wire in the program. Its one of things that gives me "brain freezes" about the program.


You know, like something you buy in a bookstore about shortcuts to Microsoft Word, Excel etc, all the main functions laid out in a page or two.

300ohm,

Here is a partial quote from the nec2 wiki page....

The Numerical Electromagnetics Code (NEC), credited to Gerald Burke, is an algorithm and generic computer application, originally written in FORTRAN. Developed in the 1970s, it is a popular antenna modeling method for wire and surface antennas.
NEC models can include wires buried in a homogeneous ground, insulated wires and impedance loads. The code is based on the method of moments solution of the electric field integral equation for thin wires and the magnetic field integral equation for closed, conducting surfaces. The algorithm has no theoretical limit and can be applied to very large arrays or for detailed modeling of very small antenna systems.

So this method of moments of the electric field and magnetic fields is a complex mathematical integration process that calculates values for every small segment and then integrates that over the complete antenna structure.

If you visualize drawing a circle by using only straight lines, then the smaller the length of your straight line segments, the closer you get to defining a circle. By taking this to very, very small lengths, you approach a perfect circle. This is in essence what segmentation is all about with nec2.

In fact, if you read up on the "Convergence Test" in 4nec2's General help, you'll see that this test is very similar to drawing the circle with smaller line segments. You'll get a more accurate model, by using more segments, but the CPU execution time increases ( It is roughly proportional to the (Number of segs)^2. So by running the convergence test, you can make an appropriate trade-off for accuracy vs cpu execution time. If you stick to the safe segmentation rules discussed in an earlier post, convergence tests are probably not that important to run.

Re the cheat sheet for 4nec2:
I haven't seen anything other than Arie Voor's first page in 4nec2's General help file. If other forum members have some info, please share it.

If you want more info on the nec2, Arie's web site makes the Nec2 user manual available, as well as a four part series on "Beginners Guide to NEC Modeling".
http://home.ict.nl/~arivoors/

Links to the Beginners Guide is at the bottom of the main page, and the NEC2 user guide is available from the NEC2 support files link.

[300ohm]

OK, that makes sense. The program allows for modeling curved antennas too.

[BobFlyer]

Anyone know where I can find a model for the above antenna, tried KQ6QV, its there but have error on my attempt to use the file for the cm-4221, segment errors.

Also, how do you enable the option to post attachments, for some reason, my profile says, "May not use Attachments"

Thanks BobFlyer

[classicsat]

As for attachments, you find hosting offsite, and can link to them from here.

[DogT]

Can someone help me with how to make the cards so I can put variables in the optimizer? I just started with 4nec2X last week and have been reading all I can understand, but need some help in understanding the inputs to the cards so I can make variables to adjust the spacing, length, etc on antennas I would like to model. I have the Modified-Hoverman.nec and have made a C-Pole antenna I would like to model too for 6 meters and don't understand how to modify the cards in order to make the variables to optimize the length, spacing, etc.

If there are other things that someone would like for me to do studies on, I would be glad to help if I can understand it. I recently got an AMD 3.2GHz Athlon dual core processor with 2GB ram, so this thing runs really fast.

DogT

[Schmerm]

Would an example help? Here's my current SGBH file that uses symbols for everything.

Basically you use the SY card to define one or more symbols, and then when creating structures you substitute hard numbers with your symbols. You can even do arithmetic with symbols as you'll see below.

Once you start using SY cards in your NEC file they will show up in the optimizer.

CM
CE
SY wr=1.295e-3,feedgap=30e-3,rtwo=1.414214
SY rodlen=142e-3
SY refw=48.26e-2,refh=101.6e-2,refd=10.5e-2
GW 1 5 0 0 0 0 -180e-3/rtwo 180e-3/rtwo wr
GW 2 5 0 -180e-3/rtwo 180e-3/rtwo 0 0 180e-3*rtwo wr
GW 3 5 0 0 180e-3*rtwo 0 -180e-3/rtwo 3*180e-3/rtwo wr
GW 4 5 0 -180e-3/rtwo 3*180e-3/rtwo 0 -180e-3/rtwo-rodlen 3*180e-3/rtwo wr
GM 0 0 0 0 0 0 -feedgap/2 0
GX 4 011
GW 17 3 0 -feedgap/2 0 0 feedgap/2 0 wr
SM 8 14 -refd -refw/2 -refh/2 -refd refw/2 -refh/2
SC 0 0 -refd refw/2 refh/2
GE 0
GN -1
EK
EX 0 17 2 00 1. 0.
FR 0 35 0 0 450 10
EN

edit: I just noticed that some of the lengths are different than the official SGBH design. I think this is because I was playing around with it to see how sensitive the design was to changes like that.

[DogT]

Schmerm,

Thanks for the example. Autofils sent me one too. I was trying to make things too hard. I also thought the variables were limited to len, hgh, etc. I am getting the idea now. Where is the help files on using this feature?

DogT

[Schmerm]

http://www.nec2.org/ is probably the definitive source. Check "part III: NEC user's guide" for a thorough description of every one of the available cards in the NEC language.

If you mean how to use the optimizer (which is a 4nec2-specific feature), then the 4nec2 help files are probably the best place to look.

[ericball]

My SBGH 4nec2 model with surface patch split reflector. This is optimized for GTA ch14-67 reception, although I haven't built it yet. (Need to get the coax in place before I replace my indoor test version.)

CM Single Bay Grey-Hovermann with split reflector
CM 4nec2 model created & optimized by Eric Ball
CE
SY lambda = 0.37758	'794 MHz
SY end=0.135	'end wire length
SY main=0.150, m2 = main / sqr(2)	'main wire length
SY nend = int ( end / lambda * 40 + 0.5 )	'number of segments in end wires
SY nmain = int ( main / lambda * 40 + 0.5 )	'number of segments in main wires
SY gap = 1in, g2 = gap/2	'feedpoint/reflector gap 1"
SY rwidth=34.5in, rw2 = rwidth / 2	'total reflector width 34.5"
SY rheight=48in, rh2 = rheight / 2	'total reflector height 48"
SY nrw = int ( (rw2-g2) / lambda * 5 ) + 1	'number of patches in reflector half width
SY nrh = int ( rheight / lambda * 5 ) + 1	'number of patches in reflector half height
SY rdepth=4in	'reflector offset
SY rgap=0.5in, rg2 = rgap/2	'reflector gap
GW	1	nend	0	end+m2+g2	3*m2	0	m2+g2	3*m2	0.003175
GW	2	nmain	0	m2+g2	3*m2	0	g2	2*m2	0.003175
GW	3	nmain	0	g2	2*m2	0	m2+g2	m2	0.003175
GW	4	nmain	0	m2+g2	m2	0	g2	0	0.003175
GW	5	nmain	0	g2	0	0	m2+g2	-m2	0.003175
GW	6	nmain	0	m2+g2	-m2	0	g2	-2*m2	0.003175
GW	7	nmain	0	g2	-2*m2	0	m2+g2	-3*m2	0.003175
GW	8	nend	0	m2+g2	-3*m2	0	end+m2+g2	-3*m2	0.003175
GW	9	nend	0	-end-m2-g2	-3*m2	0	-m2-g2	-3*m2	0.003175
GW	10	nmain	0	-m2-g2	-3*m2	0	-g2	-2*m2	0.003175
GW	11	nmain	0	-g2	-2*m2	0	-m2-g2	-m2	0.003175
GW	12	nmain	0	-m2-g2	-m2	0	-g2	0	0.003175
GW	13	nmain	0	-g2	0	0	-m2-g2	m2	0.003175
GW	14	nmain	0	-m2-g2	m2	0	-g2	2*m2	0.003175
GW	15	nmain	0	-g2	2*m2	0	-m2-g2	3*m2	0.003175
GW	16	nend	0	-m2-g2	3*m2	0	-end-m2-g2	3*m2	0.003175
GW	100	3	0	-g2	0	0	g2	0	0.003175
SM	nrw	nrh	-rdepth	-rw2	-rh2	-rdepth	-rg2	-rh2
SC	0	0	-rdepth	-rg2	rh2
SM	nrw	nrh	-rdepth	rg2	-rh2	-rdepth	rw2	-rh2
SC	0	0	-rdepth	rw2	rh2
GE	0
LD	5	1	0	0	58000000
LD	5	2	0	0	58000000
LD	5	3	0	0	58000000
LD	5	4	0	0	58000000
LD	5	5	0	0	58000000
LD	5	6	0	0	58000000
LD	5	7	0	0	58000000
LD	5	8	0	0	58000000
LD	5	9	0	0	58000000
LD	5	10	0	0	58000000
LD	5	11	0	0	58000000
LD	5	12	0	0	58000000
LD	5	13	0	0	58000000
LD	5	14	0	0	58000000
LD	5	15	0	0	58000000
LD	5	16	0	0	58000000
GN	-1
EK
EX	0	100	2	0	1	0		
FR	0	55	0	0	464	6
EN

You can do a lot in the SY tags, including calculating number of segments.

[300ohm]

I have noticed that 4nec2x DOES take advantage of both processor cores in a dual core cpu in windows xp. Has anyone checked processor usage with a quad core system ?

[ericball]

In my experience 4nec2's optimization some limitations.

First you need to understand how 4nec2 calculates it's figure of merit from the properties (e.g. gain, SWR). If you specify more than one property, it is important to scale the properties, i.e. an improvement of ? ohms is the same as 1dB. If you use the frequency sweep option, the comparison is done on the average figure of merit across the whole band. (Which doesn't always produce a flat response.) Unfortunately, the properties don't map directly to reception (net) gain.

Second you may need to tweak your variables. For example, take an SBGH with reflecting rods. The driven element has three variables: length of the horizontal wires, length of each of the angled wires, and the distance between the two sides. The reflecting rods add in more variables: distance between the drive element and the reflectors, gap between reflector halves, length of each reflector rods, and vertical placement of each reflector rod.

Whew! The problem comes in when you try to optimize multiple quasi-dependent variables simultaneously. For example, if you are optimizing the driven element, you probably want the vertical placement of the reflector rods to remain relatively the same. So for that you want your variable to be a percentage of the length of the angled wires instead of an absolute distance. (In the same way you should use a variable to calculate the number of segements per wire instead of using a fixed value.)

Finally, the optimizer is based on the assumption the figure of merit is a smooth function with only one maxima. This is kind of like a landscape with only one hill - as long as you always increase in altitude, you will eventually reach the top of the hill. Unfortunately, antenna response curves are much more complex. You may be on top of one hill, but that doesn't mean there isn't a bigger hill somewhere else. (Or, if you overshoot the top of the hill you're on that you won't land on a different hill.)

This is not to say 4nec2's Optimizer isn't usefull. Assuming you can create a reasonable figure of merit, it will happily tweak your design to the micrometer. But if you just feed in your design and click "Optimize" without a little foresight there's a chance you will end up with an even worse antenna.