**Web Link for Arie Voors 4nec2 modeling software program**

The link to Arie Voors 4nec2 modeling software is:

http://home.ict.nl/~arivoors/

1 - 20 of 26 Posts

The link to Arie Voors 4nec2 modeling software is:

http://home.ict.nl/~arivoors/

To go along with the NerdClub objective of "best of the best" hdtv antenna design, some of you might want to give

fractal antenna design a close look.

Most fractal antenna design work is being done at very high frequencies, but there are likely lots of UHF-TV possibilities that could be investigated with nec2 modeling analysis !!

Here are a couple of links about fractal antennas, to get you started...

http://en.wikipedia.org/wiki/Fractal_antenna#Log_periodic_antennas_and_fractals

http://www.hbci.com/~wenonah/cfa/fractal.htm

...autofils

I did a search for CM-1111 and all I could find was a reference to use model 3610, which was called a Crossfire model.

I eventually found a link that described the CrossFire line.

Here is the link: http://www.solidsignal.com/channelmaster_antenna_chart.asp

If the model 3671 is similar to the 1111, man these are really huge !!

Length is 173" or almost 14.5 ft; turning radius of 95.7"

Width is 110', due to Ch2; Height is 35.4" due to UHF corner-reflector.

Now there is a real-man's DIY and I thought the DBGH was large.

You can see the Raw and Net gain for the CM-3671B model at hdtvprimer.com web site "Comparing some commercially available antennas" here: http://www.hdtvprimer.com/ANTENNAS/comparing.html

UHF is "O" and VHF is "H".

Stampeder,Autofils, the Crossfires seem to me to be Quantum-style VHF yagi antennas with UHF corner-reflectors in place of the 1111's directors. I think another change was that the Quantums are golden-anodized, while the newer Crossfires are gray. Anyone ever seen a golden CM Crossfire?

When I blueprint my CM1111 we can hopefully check the spacings and measurements on the Crossfire's biggest model to see if that's what Channel Master did with such an amazing yagi design.

Do you have any idea of the date of manufacture of the VHF CM-1111?

Was it before there were a lot of UHF OTA stations, hence the Crossfire CM-3671 was updated for the full VHF-UHF bandwidth?

It will be very interesting to see the actual dimensions and design detail of your CM-1111.

Does it have any capacitance-hats, that is described in this link for the CM-3671B ? (Photo shows 4)

http://www.hdtvprimer.com/ANTENNAS/CM3671.html

I took Ken Nist's data for the CM-3671B and re-plotted it to reduce the clutter as follows:

UHF - CM-3671B vs CM-4221 and CM-4228

VHF - CM-3671B vs VU-190XR; YA-1713 and 40" Rabbit-Ears

You can view the the CM3671a.EZ file in 4nec2.

rochmndx,FYI, I own a .... DIY APS-13 FM antenna clone...

I can see that you are into really big antennas.

I found some info for the APS-13 FM antenna and it's big - 200" boom or 16.7 ft

Do you have the dimensions of your clone? I'd be interested in having a closer look at this Lpda-Yagi with 6 driven elements....thanks

* Frequency Range: 88.1 - 107.9 MHz

* Impedance: 300 Ohms

* Average Gain: 10 dBd

* Avg. F/B Ratio: 30 dB

* Boom Length: 200"

* Turning Radius: 127.5"

* Elements: 13 (6 driven)

I've seen a few postings asking "how to get started" with antenna modeling.

For those that want to explore NEC2 modeling, here are a few links.

You will need a basic understanding about antennas and be prepared to spend some time learning how to model.

If that's you...then here is a simple guide on how to get started.

Visit Arie Voors site. http://home.ict.nl/~arivoors/

Download the 4nec2 program and "NEC Beginners Guide- (a 4 part pdf set)"

You'll find the 4nec2 program's General help file accessible via F1 key to be very complete.

If you get stuck, you'll find most answers in the General help file.

Start with the ReadMe-First file and then once you are setup with a running 4nec2 program,

then read the Get_started file and follow through the examples.

Explore 4nec2 features. The 4nec2 package has many, many antenna examples that you can examine.

Once our NerdClub gets a core-base, you will find this forum to be a source of modeling projects, complete with the .nec files.

Here are some ARRL links.

http://www2.arrl.org/cce/sample-lesson/

http://www.arrl.org/tis/info/HTML/antenna-modeling/index.html

And finally there is a wealth of more advanced info on the web pages of L. B. Cebik [W4RNL] that deals with Radio Amateurs and Modeling.

http://www.cebik.com/amod/modeling.html

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?

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.

===========================================

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

(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

(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.)

====================

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,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.

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

DTV Student,....

Your comments and suggestions are always appreciated.

Thank you,

DTV Student

That's quite interesting modeling work that you have done and it can really help others that have started dabbling with 4nec2.

Here are a couple of suggestions, that would help others that have just started on the 4nec2 learning curve.

1. I know that you indicated that your model files are on a yahoo group, but could you give a url link to that yahoo group?

You could also easily just paste a single nec file in your post and if your model uses SY cards, then you only need to post that file. That would allow others to run the model and learn from your work.

2. Re you Gain plots:

Are you reporting Raw Gain ( as spit out by the nec2 engine) or have you corrected for SWR and are thus showing Net Gain?

3. Re SWR:

I didn't see any results ( plots or a summary) of SWR variations over the bandwidth that you are sweeping.

Typically a good antenna design will have a SWR >3; really good is >2.

Anything >4 really needs to be improved, as the SWR mismatch loss starts getting large.

[Net Gain = RawGain - Mismatch Loss]

SWR ..........Mismatch Loss (dB)

===========================

2 ............... 0.512

3 ............... 1.25

4 ............... 1.94

5 ............... 2.55

10 .............. 4.81

Keep up the good work.

DTV Student,Hi Folks,

I forgot to ask about the Characteristic Impedance or Zo term. Autofils said, "Zo = characteristic Impedance for the transmission line connected to the antenna ( Zo=300 for the GH)".

DTV Student

Here is a very quick note about antenna impedance and Zo of the connecting down-lead.

The NEC2 engine calculates the RawGain and the antenna's complex impedance at a single frequency, where the antenna's impedance (Za) is given by two parts, the real and imaginary values.

( The real part is resistive; the imaginary part can be either capacitance or inductance)

Za = Zr + jZi or Za = Zr - jZi.

The magnitude of the antenna's impedance is SqRt (Zr^2 + Zi^2)

When you do a sweep like 450 to 700 Mhz, step 10; the NEC2 engine calculates the Raw gain and antenna Zr and Zi for each of the frequencies defined in the sweep. ( Take a look at the F8 output and you see this).

So the

If you model a folded dipole, you'll see an antenna impedance of 300 ohms at the resonant frequency.

If you model a simple "straight" dipole with two elements, you see an antenna impedance of 72 - 75 ohms at the resonant frequency.

The transmission line is used to interconnect the antenna to your receiver.

Standard transmission lines used for TV antennas are either the old standard 300 ohm twin-line or 75 ohm coax. If the antenna's impedance is 300ohm, then you must connect a 4:1 balun at the antenna feed-point, if you are using 75 ohm coax down-lead ( 300 to 75 ohm).

In the case of the GH, the antenna's impedance is typically 300 ohm, but it does vary over the UHF band... look at the Z plot.

You want to match the antenna's impedance to the characteristic impedance (Zo) of the transmission line to simply minimize SWR losses.

Now 4nec2 allows you to specify the characteristic impedance of the transmission line from the main window, via "Char-Impedance" under the settings tab.

Once you have run a sweep, you can change that value and re-examine the SWR plot with the new value when you re-click on the SWR plot....give it a try !!

The task of calculating Net gain, was greatly simplified when 300ohm noted that the table values of all the parameters ( RawGain, Zr and Zi) are available under plot (see http://www.digitalhome.ca/forum/showpost.php?p=781674&postcount=208 ), so all you have to do is copy and paste into a spreadsheet....easy as pie !!!

I recently posted a nec file for a simple 75 ohm dipole. This is the very basic minimal antenna .

It's probably very useful to those that have just started on the 4nec2 learning curve.

I have a couple of exercises that you can try, using this file to become more familiar with 4nec2.

If anyone finds this useful, I have a lot of other exercises in mind.

If, on the other hand, no one wants to bother with such trivial stuff, that's fine by me.

I'm simply trying to help new 4nec2 dabblers, so use this as you see fit !

The 75ohm Dipole model nec file (attached), uses SY cards with the following variables:

Feed-gap spacing and Overall Dipole length (L)

The wire radius is 0.6mm

The wire radius of the Vsource (Tag1) was adjusted for AGT result of 1.0(-0dB) with wire radius of 0.58mm

All segmentation checks are error and warning free.

This design has a wire radius of 0.6mm, which is close to #16 gauge wire.

(#16 is 1.2903mm diameter per 4nec2's reference data on AWG wire table)

If you run a sweep 600 to 660Mhz;step 1 using the default values (y=0.02m and L=0.254m)

you will see at the resonant frequency of 607Mhz:

- min SWR of 1.03857

- RawGain of 2.12dBi

Can you increase the segmentation using #10 gauge wire, and remain free of segmentation warnings ?

[#10 gauge has a wire diameter of 2.5883mm]

As you attempt this, you may encounter segmentation warnings.

- Do you understand what the warnings mean?

- Can you change your model to correct and eliminate the warnings?

Please post your #10 gauge 75 ohm nec file that is free of errors and warnings and has an AGT result of 1.0(0dB).

Could you also make some comments about anything you learned in doing this exercise.

That would be a great help to others that have started up the learning curve.

===================== NEC File ========================

CM 75ohm Simple Dipole by autofils 04Sept08

CM Two variables:

CM y = 0.02m is the feed-gap spacing

CM l = 0.254m is the overall length of the dipole

CE

SY l=0.254 'l = 254mm = 605MHz 75ohm-Dipole (Mid point of Ch36)

SY y=0.02

GW 1 5 0 -y/2 0 0 y/2 0 0.00058

GW 2 10 0 -(l/2-y/2) 0 0 -y/2 0 0.0006

GW 3 10 0 l/2-y/2 0 0 y/2 0 0.0006

GE 0

GN -1

EK

EX 0 1 3 0 1 0

FR 0 0 0 0 607 0

EN

=====================================================

DTV Student,Hi Folks,

There are several really great things about working through a modeling exercise with simple antenna design like the dipole:

1) The general properties of a dipole antenna are well known.

2) Very little time is invested setting up the antenna's geometry file.

3) If one’s results agree with the well known characteristics, one builds confidence in the modeling software.

4) If one’s results do not agree with the well known characteristics, perhaps there are modeling constraints which need to be resolved, remembered, and applied in the future to more complex designs.

.........

Uncle! I have had enough for a little while.

Your comments and suggestions are always appreciated,

DTV Student

I totally agree with your 4 reasons above for using the simple 75ohm dipole model, as teaching tool for 4nec2.

So many folks start right into a complicated model, and end up with lots of frustration, because they aren't understanding some of the basics. The 75ohm model, allows you to test out known antenna theory using 4nec2, and if the results are way off from known theory, then probably there is something wrong with your model or you're missing some other factors.

Excellent work!! and your commentary about the steps you took in looking at this example are very good with lots of detail, which will be a great aid to others that what to have a go at this first challenge! .... thanks

ps. I'm not sure why you get a slightly different raw gain number. The difference is very slight, but there should be agreement if the models are exactly the same, so I'll take a look into this.

Would you be interested in a few more 75ohm dipole challenges ?

btw: How many segements are in your core? [ You will see this in the nec2 engine's dos window, when it starts the calculations for the frequency sweep ]

Antenna impedance and the effect of the characteristic-impedance of the transmission that connects to the feed-point of the 75ohm dipole....

... and .... can you avoid the Raw-gain trap ??

We will use the same nec file as before:

===================== NEC File ========================

CM 75ohm Simple Dipole by autofils 04Sept08

CM Two variables:

CM y = 0.02m is the feed-gap spacing

CM l = 0.254m is the overall length of the dipole

CE

SY l=0.254 'l = 254mm = 605MHz 75ohm-Dipole (Mid point of Ch36)

SY y=0.02

GW 1 5 0 -y/2 0 0 y/2 0 0.00058

GW 2 10 0 -(l/2-y/2) 0 0 -y/2 0 0.0006

GW 3 10 0 l/2-y/2 0 0 y/2 0 0.0006

GE 0

GN -1

EK

EX 0 1 3 0 1 0

FR 0 0 0 0 607 0

EN

=====================================================

Before, when you ran a sweep 600 to 660Mhz, step one, I mentioned that you should set the Char-impedance (under settings tab) to 75 ohms, and note the Raw gain and SWR at the resonant frequency of 607Mhz.

( I assume you that you agree that the resonant freq is 607Mhz... yes? )

1. Can 4nec2 actually tells us the 75ohm dipole's impedance at the resonant frequency?

Hint: What value does the Z plot show at 607 Mhz?

2. Can you also find the real and imaginary values for the dipole impedance at the resonant freq 607MHz?

Hint: Look for the R and X values at 607Mhz.

3. Can you write the equation for the dipole's complex impedance at 607Mhz?

Hint: it will be of the form Za = R + jX... should the imaginary part be positive or negative ?

If you calculate SqRt (R^2 + X^2) what value do you get?

Is this the same as the value in the Z plot at 607Mhz?

If yes, why?.... If not, why?

3b. What is the dipole's complex impedance at 600Mhz ?

4. Now what happens if you set the char-impedance under "settings" tab to be equal to the value you found for the dipole's impedance at the resonant freq of 607Mhz?

For the resonant freq of 607Mhz....

- What happened to the SWR value? Why?

- What happened to the Raw gain value? Why?

- What happened to the dipole's impedance? Why?

5. Now set the char-impedance to 300ohms...

Again, For the resonant freq of 607Mhz....

- What happened to the SWR value? Why?

- What happened to the Raw gain value? Why?

- What happened to the dipole's impedance? Why?

Do you see a pattern ? If so what is it and what did you learn ?

6. What about frequencies above and below resonance ?

Run a sweep at 500 to 700, step10

- What is the SWR at 500Mhz?

- What is the raw gain at 500Mhz?

Now run a sweep at 100 to 1000, step 20

-What is the Raw gain at 1000Mhz? Is that correct ?

Hey it looks like the raw gain is increasing...wow...wonder what it is between 1000 and 2000 Mhz?

Do a sweep 1000 to 2000,step 20 and find the max raw gain !!!

"Holy batcave, batman"... it looks like 4.91dBi at 1560 Mhz...now that's one hot dipole...right ??

Nope !! Sorry Robin... the best a dipole can do is only 2.12 dBi

Can you explain why it really isn't that nice juicy 4.91dBi at 1560Mhz ??

If you can, then you won't fall into the raw-gain trap !!

===================== End of Challenge #2 ====================

Ready for challenge #3 ? .... Coming soon !!

If you are using a free-space environment, you might have a problem !!(from 300ohm)

What is the correct way to excite a "whip" type antenna ?

In other words, where do you place the load source on a single wire antenna ?

Regardless of where you put it, what else do you connect it to ?

...The whip is only one wire ...right ?....and the V or current source needs two connections, does it not.....

.....so where is the 2nd connection available in free-space to use to connect the Vsource ?

Perhaps you should consider one of the 4 ground environments that 4nec2 supports.

Of course I am assuming that you have some equivalent circuit for the device that the whip attaches to.

That way, you can choose the appropriate ground environment.

The Vsource would connect from your single wire to ground in whatever ground environment you selected.

300ohm,Try copying and then rotating a group of wires, and then try attaching the rotated wires to a gap in a single wire. Its very tricky the way the program handles that, heh. It works, but its tricky.

Rather than just tease us with this feat of yours, why not provide a short tutorial? That way everyone would really benefit

300ohm,My plan is for it to replace the telescoping antenna on my 5 inch portable am/fm/vhf/uhf tvs. I have no idea what the impedence should be. But I know I cant connect 300 ohm line to it in any effective way, and I have tried many ways, heh. On the inside of the tvs, its just one lousy wire from the telescoping antenna to the pcb, which then goes to the tiny tuner. The tuners on those sets are really quite sensitive, considering the measly telescoping antenna they use.

A lot of these portable TV's have a connector on the back for an external antenna. I have an old Ken-Tech and it has a mini-earphone, two conductor jack with a shorting contact. The shorting contact is used as a switch, to switch from the whip to the external antenna, when a plug is inserted. So guess where the external antenna is connected ??

One end is connected to the tuner and the other end is connected to chassis grd, which is battery negative.

The reason the tuner appears so sensitive is the minimal agc action designed into the tuner.

btw: I just made a 75ohm adapter to plug into the TV's external antenna jack.

This makes installing and aiming the GH4 on the roof nice and easy!!

( you loose your analog TV Feb09, we still have analog until Aug11)

I suspect the wire you are talking about actually connects to the whip antenna.Mine have the same mini-earphone jack for the external antenna. But the wiring of them from that jack on the inside only connects to the one wire on the antenna, heh. There is no second wire and when an external antenna is plugged in, the telescoping antenna isnt disconnected, the external antenna just becomes an extention of it. Weird, huh ?

You have to look at the cct board side of the external mini-jack connector.

You'll see three separate traces connecting to the mini-jack. Here's my best Asci art ......

1 0-------------------- ....( connects to the whip antenna )

2 0------------------^ ....( connects to the tuner input )

3 0-----------======= .....( connects to battery grd )

When there is no plug inserted, 1 and 2 are shorted and the whip antenna is connected to the tuner.

When a min-plug is inserted, 1and 2 are open cct and the external antenna connects between 2 and 3.

Check to see if the grd side of the jack mechanically makes connection to the cct board copper trace which is battery grd.

That's how mine was made.

If not, then you just have a capacitance effect to battery grd.

At TV frequencies that stray capacitance doesn't have to be very large.

Just draw an equivalent circuit of the tuner input.

I simply took a female F-connector and wired it to a mini-plug (pins 2 and 3 per asci above) with a short piece of speaker wire of around 1-2".How specifically did you make it ?

Get a laptop and a USB ASTC tuner stick and you're in business.But I figure sooner or later, Ill be able to pick up a digital portable. It looks like no one is making dirt cheap crt's anymore, so the prices will be higher unless lcd's drop. After looking at my tuner in the Zenith CECB, I have faith that digital tuners will eventually be as cheap as analog ones, heh.

And while you're at it, get a set of in-line attenuators and you would have a total portable antenna measuring system as well as a powerful computer !!

I'd get one, if I could afford it ....but that's another story........

1 - 20 of 26 Posts

Join the discussion

Canadian TV, Computing and Home Theatre Forums

A forum community dedicated to Canadian TV, computing and home theatre owners and enthusiasts. Come join the discussion about home audio/video, displays, troubleshooting, styles, projects, DIY’s, product reviews, accessories, classifieds, and more!

Full Forum Listing
Recommended Communities

Join now to ask and comment!