4nec2 & Other Antenna Design Modeling Software

394535 Views 1313 Replies 97 Participants Last post by rabbit73, 4 mo ago

stampeder

Discussion Starter · Dec 19, 2007

This thread is for discussion of OTA Antenna Modeling Software such as 4NEC2, ezNEC, NEC II, and other such tools.

It is also for sharing ideas and concepts involved in OTA Antenna design.

Discussion of the Gray-Hoverman antenna design will remain in its own thread:

http://www.digitalhome.ca/forum/showthread.php?t=81982

561 - 580 of 1314 Posts

Dalek Prime

· #561 · Sep 15, 2010

Hey Walter, perhaps allowing some of the variables in the file to be determined as shell switches, or during runtime?

PS. I'm moving, so I won't be here much until I set up internet at new home. Ttyl.

nikiml

· #562 · Sep 15, 2010

polarization

How does the nec engine take in consideration the wave polarization?
I tried rotating my structure in 4nec2 and got exactly the same predictions?

what am I missing?

300ohm

· #563 · Sep 15, 2010

got exactly the same predictions?

On the single channel Far Field calculation, look closely at the polar graphs and I think youll see a change of direction with the same gain.

For a Frequency Sweep, you specify the Theta and Phi. Dead straight ahead is 90 degrees.

nikiml

· #564 · Sep 15, 2010

I am not sure what you call polar graphs? The patern graphs perhaps?
rotating the antenna around X by 90 degrees just swaps the horizontal and the vertical pattern. But does not change the gain.

Anyway I read that if a horizontally "polarized" antenna is used to recieve verticaly polarized wave the antenna gain would suffer by about 20dB.

So I need to now how the antenna will perform with horizontaly polarized wave and that should be different (gain-wize) from its performance with verticaly polarized wave.

300ohm

· #565 · Sep 15, 2010

Anyway I read that if a horizontally "polarized" antenna is used to recieve verticaly polarized wave the antenna gain would suffer by about 20dB.

Im not sure where you read that, but a lot of antennas can be used for vertical or horizontal polarized signals by simply turning them, with no change in gain.

Perhaps that statement means if you dont turn a horizontally polarized antenna to receive a vertically polarized wave ?

nikiml

· #566 · Sep 15, 2010

Perhaps that statement means if you dont turn a horizontally polarized antenna to receive a vertically polarized wave ?

yes. that is what it means. my concern basicaly boils down to how to be sure that the forward gain nec is showing is true for horizontaly polarized transmission? Or vertical if desired.

300ohm

· #567 · Sep 15, 2010

Good question, how much loss from a horizontally polarized antenna to a incoming vertically polarized signal ?

On a typical bi-quad with its round balloon front pattern, pretty much none, no matter what polarization.
But on a LPDA or Yagi, it should be quite a bit.

nikiml

· #568 · Sep 15, 2010

I thought the pattern is all about aiming...

jbkeh

· #569 · Sep 16, 2010

But does not change the gain.

Which "gain"? "Total" shouldn't change, but I'd expect "Horizontal" and "Vertical" values should 'swap' with a quarter turn.

300ohm

· #570 · Sep 18, 2010

I thought the pattern is all about aiming...

Yep. But I think our minds are getting messed up about the incoming angles. (one reason why I think Arie Voors 3D patterns are a godsend)

Cheezebooger

· #571 · Sep 27, 2010

2

How'm I doin'?

I downloaded a DB4 file of yours and plotted it

Then I made a design of my own and plotted it.

Am I on the right track?

How do you determine the impedance at the feed point?

Why is the balun a wire? won't that short out the signals unless it carries some sort of impedance factor?

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stampeder

Discussion Starter · #572 · Sep 27, 2010

Cheezebooger said:
Am I on the right track?

It depends on your goals. If you just want to learn antenna modeling, it's a start. From your balun question I suggest that you read through the OTA FAQ, and Post #5 in particular to make sure that we have common terms of understanding.

If you want to build the best 4-bay bowtie reflector antenna for your needs just go over to the Bow Tie TV Antenna Designs (mclapp's M4 featured) thread and build the most suitable M4 variant.

300ohm

· #573 · Sep 27, 2010

How do you determine the impedance at the feed point?

The impedance is set in the Settings, ie 300 ohms most common for TV antennas so you can use a common 4:1 balun with 75 ohm coax. (but could be set to anything you want)

The SWR (standing wave ratio) shows how far from the set impedance you are. 1:1 is a perfect match.
As you can see above, impedance and the mismatch from the set impedance varies with frequency.

Why is the balun a wire? won't that short out the signals unless it carries some sort of impedance factor?

Its the antennas excitation point and its not an actual physical wire, but just treated as one in the models for the computations.

holl_ands

· #574 · Sep 28, 2010

2

The "Nominal Impedance" of an antenna is a compromise across all operating frequencies.
The "Impedance" Chart depicts frequency dependence of REAL and IMAGINARY components.
To find "best" impedance for an antenna, change (JUST) "Char-Impedance" under the 4nec2
"SETTINGS" tab until you get the "best" SWR curve. [Click on SWR chart to see each change.]
Try 50, 75, 150, 300 & 600-ohms (etc) and "eye-ball" an answer....

Sometimes this is the impedance with smallest-maximum within the operating band,
but can also be a joint compromise with the Raw Gain curve so that the
Net Gain isn't adversely affected--loss due to higher SWR can be tolerated better
on those channels that have higher Raw Gain.....such as Hi-VHF 2-Bay below...

1. Results for the above UHF DB-4 are in the ball-park for typical 4-Bays (NO Reflector):
http://photos.imageevent.com/holl_a... NO Reflectors - UHF Raw_Net Gain and SWR.jpg

2. Your above Hi-VHF 2-Bay with Triple-Element-Triangles has excessive SWR on
higher channels, which probably indicates that you sized it for maximum Raw Gain,
rather than Net Gain. Since it is slightly "too big"....adjust & try again....

3. Here's a Hi-VHF 2-Bay with standard Four-Whisker Elements after I optimized it:
http://imageevent.com/holl_ands/multibay/hivhf2bay
Note typical rising Raw Gain curve and SWR under 3.5 (Reflector will lower it):

4. Six-Whisker Element Bowtie (open at ends) was better than Four-Whiskers:
http://imageevent.com/holl_ands/dipoles/hivhfbowtie
But I did not compare when used in Multi-Bay Bowties.....
Nor have I (yet) analyzed the above Triple-Element-Triangle Bowtie...

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Cheezebooger

· #575 · Oct 1, 2010

300ohm said:
The impedance is set in the Settings, ie 300 ohms most common for TV antennas so you can use a common 4:1 balun with 75 ohm coax. (but could be set to anything you want)

The SWR (standing wave ratio) shows how far from the set impedance you are. 1:1 is a perfect match.
As you can see above, impedance and the mismatch from the set impedance varies with frequency.

Its the antennas excitation point and its not an actual physical wire, but just treated as one in the models for the computations.

How do I determine 1:1 SWR? By the numbers or matching the curves?

I designed a 4 whisker 1 bay 6dbi with nearly flat curves but I don't know what to look for except for gain.

300ohm

· #576 · Oct 1, 2010

How do I determine 1:1 SWR? By the numbers or matching the curves?

OK, take a look at holl_ands SWR chart above. On the left hand side of the chart going up, there are numbers 1, 1.5, 2, 2.5, 3, 3.5, 4.
They mean to 1, ie 1:1, 1.5:1, 2:1, 2.5:1, 3:1, 3.5:1, 4:1.
(by clicking on the line or point in 4nec2, youll get the exact number)

If you do a single frequency Far Field Calculation, the SWR number will be in the Main dialog box. The Raw Gain will be in the Pattern window.

but I don't know what to look for except for gain.

Basically you look for high gain and low SWR and the radiation patterns (horizontal, vertical and with 4nec2 you have 3D ones too). And Front to Back ratios if your local environment requires it.

Theres a lot of info that 4nec2 kicks out, but keep in mind that a lot of it applies to transmissions and ground types and ItsHF data.

Since were only looking for RECEIVING antenna performance in a FREE AIR SPACE environment, it makes life a lot simpler, heh.

Free Air Space is the only fair way to do comparisons between antennas.

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Cheezebooger

· #577 · Oct 1, 2010

300 ohm:

How's this?

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300ohm

· #578 · Oct 1, 2010

Single bay VHF-Hi antenna. Post the NEC file.

To post the NEC file from 4nec2,

From Main Window, > Settings > tick off Notepad Edit.

Then Edit > Input File.

Copy and Paste the text to the post.

Tips:
By using the Code Tags (#), it will keep the text aligned.

Large NEC files will exceed the character per post limit of this forum. To post those, youll need to use a online storage site. I use Wuala, which is free for up to 1 GB of online storage.

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Cheezebooger

· #579 · Oct 2, 2010

CM Hi-VHF 4-Whisker AWG14 Elements.
CM FeedSep= 1.5-in (AWG10)
GW 1 1 0 -0.75 0 0 0.75 0 5.e-3
GW 6 21 0 35 5 0 0.75 0 0.04040404
GW 9 21 0 -0.75 0 0 -35 5 0.04040404
GW 10 21 0 35 -5 0 0.75 0 0.04040404
GW 11 21 0 -0.75 0 0 -35 -5 0.04040404
GS 0 0 0.0254 ' All in in.
GE 0
EK
LD 5 0 0 0 3.e7 0
EX 0 1 1 0 1 0
GN -1
FR 0 1 0 0 174 0
RP 0 1 73 1510 90 0 1 5 0 0

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300ohm

· #580 · Oct 2, 2010

Pretty good, youre catching on.

The raw gain is overstated by around .73 dbi. You need to correct the AGT results, so that AGT = 1.0 (0 db).

AGT is explained in the Help file, Main window, Help > GENERAL HELP > AGT > Display.

To increase the AGT value, increase the wire radius of the excitation point, the wire referred to in the EX card (ie the balun connection point).
To decrease the AGT value, vice versa.

Tip: It helps to have an AWG wire chart print out showing wire diameter and radius handy.

Side note: The reason the AGT value has to adjusted is due to the mathematics of the NEC engine, ie the simultaneous solution of a gazillion unknowns.

561 - 580 of 1314 Posts