antenna-theory.com

[sdow]

hi
Actually i work on circular antenna 5.8ghz with 4nec2 software . But when i plot the 3d graphic the axial ratio is every time low ( 0.03) . I not no if the software can calculate axial ration for circular antennas . This software is new for me .

Thanks

[hagster]

I don't think 4nec2 can plot the axial ratio. Or, at least I can't find the option.

The data is logged in the output file though (press F.

Here is an example for a perfect crossed dipole I modelled, it shows the AR at Phi 0°, Theta 0° to be exactly 1 and degrading as it goes off boresight exactly as expected. I have tested other less perfect CP antennas also and get realistic results.

[sdow]

Hi

when you go in 3d plot also select magnitude , total gain plot

and above you can select axial ratio ....do you have the same results . Can you take screen shot of your axial plot thanks

[sdow]

can you help me why my axia ratio is very low also with crossed dipole

thanks

[hagster]

I will run the model later, but for now just check that you are exciting each dipole with the correct phase. I.e they need to be 90 degrees apart.

[hagster]

Ok, so there were a few problems with your model.

1) the feed was across adjacent arms of the X (in an L configuration). Thus the two dipoles were 180° out of phase not 90°.
2) The excitation sources were identical. I have changed them to be 90° apart.
3) The X axis and Y axis arms were not of equal length. (thus one radiated better than the other)
4) I have increase the density of the mesh and reduced the number of segments in the dipole arms to be something more reasonable.

It now gives a axial ratio of 1.

[sdow]

thank

But can you explain me ...i not understand good the sources how they works . this is a image of one crossed dipole ...
http://fpvlab.com/forums/attachment.php?attachmentid=7015&d=1329693218

in this picture are one source ...how i can design it with one source

[hagster]

I see what they are doing now. One arm is slightly longer and thus slightly inductive. The other arm is shorter and hence slightly capacitive. This should cause the current to lag the voltage in the long arm and lead in the other to create the circular polorisation.

This is how lots of GPS patch antennas work (they are slightly off square). To get it to work you need to get the resonant length just right and get the exact right length difference between the two. I sugest you use the optimiser to find the best values. I think you are also better withl a single feed point like the one shown in the link not the two you have.

I dont know your level of expertese, but this is probably not the simplist antenna for a newbie with limited measurement tools to build. The CP is also inherently narrow band using this method so its probably not great for FPV.

[sdow]

for build the antenna for me is not a problem ...i have cnc machine and laser machine i can do it with very high precision .For me the problem is 4NEC2 is new for me . I will try to make it with 1 source and send you a PM . I have also my network analyzer.

Many thanks

[sdow]

Hi

One more question if i work with only one source ...what i need to do in 4nec2 that one L is connected in the middle of coaxial cable and other L is connected to gnd of cable .

Thanks

[hagster]

[WinnySud]

Hi...i am a new user here. As per my knowledge you should check these: feed was across adjacent arms of the X. Thus the two dipoles were 180° out of phase not 90°.The excitation sources were identical. I have changed them to be 90° apart.The X axis and Y axis arms were not of equal length. I have increase the density of the mesh and reduced the number of segments in the dipole arms to be something more reasonable.

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