NEC modeling

[Deep Thought]

You may (or may not) have seen the nifty front-page picture of me taking a monitor point reading at KLFF, Arroyo Grande, CA in this week's (11/19/08) Radio World. If you did, you may have also seen my comments regarding the alleged massive cost savings the proponents of this change have been hawking at the major benefit.

I still say the main reason AM proofs cost so much these days is consultant greed, but that's not why I started this topic. I have the tools necessary to model arrays and have done it when faced with cranky (no pun intended) arrays in order to find a decent starting point for tuning. However, the public domain tools are clunky and nonintuitive and the specialized broadcast-oriented pre- and post-processors for NEC-2 and MiniNEC are expensive.

So, I have been searching around for something a little more reasonable and found a what looks like an outstanding resource. It's called Basic NEC with Broadcast Applications by J. L. Smith, published by Focal Press and the SBE. For less than a hundred bucks you get the book, a modified NEC-2 which behaves better for broadcast uses and front and back ends which translate the cryptic NEC-2 input and output files into something a tired, overworked engineer can read. I am about halfway though the 300 page book and I am very impressed. If you are at all interested in this you should investigate the book. The SBE is allegedly giving a $10 discount for members but you can get the same price with free shipping at Amazon.

And to prove that my money is where my big mouth is, I'll share whatever models I come up with among the members here and try to help explain any of the head-scratchers. I am still learning the nuances so we can work on this together.

http://www.amazon.com/Basic-Broadcast-Applications-J-L-Smith/dp/0240810732/ref=sr_1_1?ie=UTF8&s=books&qid=1227334405&sr=8-1

[K4WRF]

Looking forward to that exchange. Have Purchased J.L. Smiths book and so far, so good. I have not been able to try any of the examples due to “retirement” work load. For me, found that reading through it straight the first time then going back with a detailed read worked. Requires a lot of referring back and forth between text and several appendixes, it also works better for me to make notes as I go. Very detailed instructions on input output cards. I advised one former co-worker to take a look at material developed by the late L.B. Cebik www.cebik.com and get a feel for mini nec concept, though Cebik treatise based around amateur antennas, gets one thinking in a Cartesian coordinate world. No Clift Notes versions for this, the Devil is in the details. I agree that the time savings would be in the cumbersome and lengthy initial tune-up process used now. Do you know if the new FCC rules allow for array synthesis by MoM or is the current method going to be used? I also wonder how many marginal arrays are tuned up on what Smith refers to as “local minimums”?

w/

[Deep Thought]

The new rules allow modeling in lieu of a traditional antenna proof under fairly restrictive conditions (series fed, uniform cross section towers, no other components on the tower (i.e. no FM antenna transmission line 'bazooka sections') or top loading,, etc.). Ironically, these simple arrays will get the least benefit since they are the easiest to tune and proof. Any antennas not meeting the requirements still require old-school proofing.

I've been reading the book straight through too with the benefit of having done some modeling before reading it. Smith diverges pretty severely from the Hatfield and Dawson methodology of adjusting the tower heights to match real-world measurements, and in fact is fairly critical. Considering that the book has only been out for a bout six months that has to be a targeted slap. Interesting stuff...

[davedybas]

I've been playing with a "freebie" NEC program called 4NEC2. It takes a little bit of time to learn but if I can learn it then anyone can. I can't really say just how accurate it is or how it compares to the big buck modelling programs. For me, modelling existing DA's gives me results that compare with those of the original consultants design.

If you want to play around and not spend a dime here is the link: http://home.ict.nl/~arivoors/

Dave Dybas

[K9EZ]

Great topic! Long time listener, first time caller...

I have worked with NEC2, but have not studied as much as I would like as to the details. I have been able to models the nine tower array of WISN and with surprising success.

But when I get into models of 'different' designs, I seem to run into issues. Particularly when I am trying to model grounds.

Any advice you more experienced NECers might have would be great. Looks like I am going to have to pick this book up.

[davedybas]

Modelling grounds...what I've found that works for the 4NEC2 program I use is that the standard 120 wire ground radial system must not actually be on the ground. You must locate it above the "Z" plane by some small amount. I use "0.1" inches as my height above "0.0" in the Z plane. Depending on your program, you can specify the type of ground, be it marshy soil, dry sand, etc that lies under your ground radials.

I understand the the programs based on NEC-4 do not have this problem and accept radials below the ground. But, of course, you pay for the license to use the NEC-4 core...which can cost more than a few pesos.

Dave Dybas

[Deep Thought]

The ground issue is indeed a limitation of NEC-2 but for broadcast use it's not that much of a problem unless you are attempting to guesstimate the effects of different, limited ground systems. You can get around that somewhat by making sure your ground plane is at least 0.1% of the radial segment length above 'ground'. For a 60 meter ground radial that's only 0.06 meters (~2.5 inches) but it depends on how you've set up your units of measure. 0.1 inches would violate the rule unless you are modeling a 40 meter vertical.

And yeah, the NEC-4 core handles buried ground radials but Lawrence Livermore Labs wants $300 for non-commerical ($1100 for commercial) use. While the additional accuracy is nice I don't know if it is absolutely required for what we do...

4NEC2 looks great, by the way.

[K9EZ]

I wanted to model an elevated ground system. Guess I will see if the company will cough up the extra money.

[davedybas]

You should be able to model an elevated ground system with any of the "freebie" NEC2 programs.

Dave Dybas

[R. Fry]

Below is the elevation pattern I got for a model of a monopole with elevated radials, using NEC-Win Plus...

[K9EZ]

You should be able to model an elevated ground system with any of the "freebie" NEC2 programs.

Dave Dybas

But I would like to compare that to a standard 120 radial system.

[R. Fry]

But I would like to compare that to a standard 120 radial system.

The h-plane gain in the NEC model I posted is the same as a perfect 1/4-wave monopole (zero ohm r-f ground loss) over a perfect ground plane.

Brown, Lewis & Epstein showed in 1937 experiments that a 1/4-wave monopole using 113 radials each 0.41-lambda long produced a field at 3/10 mile that was within a few percent of the theoretical maximum for the radiated power. Their test site was in N. Jersey, with poor conductivity.

Rich

[Deep Thought]

You should be able to model an elevated ground system with any of the "freebie" NEC2 programs.
Dave Dybas

But I would like to compare that to a standard 120 radial system.

As Dave said, any of the public domain/shareware/freeware programs will do that quite nicely. There is a NEC-2 card which will automagically do the copy/rotate/paste for you as well. What the NEC-2 based programs won't do is model that same ground system at or below the surface.

Adding to what Rich said, the 'perfect ground' assumption is valid for most AM broadcast towers since the radial system we use has zero resistance and only a tiny reactance at the tower connection point. "Real" ground doesn't come into play until you try to model the far-field intensity, which introduces variables you can't really model properly with NEC, nor was it designed to.

[R. Fry]

Elevated radials, cont'd...

Below is a page of various NEC data for a 1/4-wave monopole with four elevated radials.

This model has no wire connection to earth ground whatsoever, although it is modeled over a perfect earth. Peak gain is 5.15 dBi, which is the theoretical peak gain of a perfect 1/4-wave monopole driven against a perfect ground plane. Input Z for the model is about 31.2 -j 9.9 ohms, which is a 1.7 SWR in a 50 ohm environment.

The configuration appears to have some promise.

RF

[K9EZ]

How would this compare to the 120 radials? Also would there be any difference with 6 elevated radials?