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Mishka

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Posts posted by Mishka

  1. Posted

    On 12/17/2020 at 1:35 PM, Mishka said:

    For curiosity I've built the cocoaNEC generated dual-band LPDA...

    Measurements:

      Hide contents

    EpZbmO_XMAEGcBp?format=jpg

    lpda-smith.png

    Aieee! I used diameter value in the place of radius in my #cocoaNEC code. Stupid me! Realized this when worked on another antenna.

    After changing the NEC model to actual wire gauge (S = 2.5 sq.mm, measured D = 1.66 mm, R = 0.83 mm, but NEC code was optimized for R = 1.7 mm) it became exceptionally accurate! At 2400 MHz, both real (see VNA data inside the spoiler) and modeled values differ by just 0.01 for VSWR and 0.3dB for return loss! This also explains impedance mismatch.

    ErU1ZBTW8AYvNB5?format=pngErU1vNVXYAMudLs?format=png

    Going to rebuild the antenna now.

    1 person likes this

  3. Posted · Edited by Mishka

    1 hour ago, Admin said:

    ,,, and here is the situation with this log-per...

    I think there is still work to be done on this antenna ... !!!

    Wow! Nice!

    Please note, in the NEC model it's fed by a balanced source, and for my assembly I've rotated the SMA connector by 90 degrees (unbalanced feeding).

    Well, as for ultra-wide band (2.4 - 7GHz) it looks not too bad. It seems all LPDAs are sort of compromise between bandwidth and quality. At least the model and implementation proves the gain is at about 7dB. Assuming a nice and big reflector which may add up to 20dB, should we consider the Kroks KNA+27 antenna in the subject may virtually go up to +27dBi?

  6. Posted · Edited by Mishka

    For curiosity I've built the cocoaNEC generated dual-band LPDA. Well, the good news is that the VSWR and return loss are close to what was predicted by the NEC. Please note, the antenna was calculated for 2.4GHz and 5GHz WiFi ranges only and should not be expected to work outside of the bands. Also, my VNA can't see above 4.4GHz so unortunately I was unable to test it at 5 to 6 GHz range.

    On the other side, antenna is out of impedance (which is sort of okay for LPDA). This may happen due to improper connection or other assembly issues. Such, for simpler assembly I've soldered a PCB edge SMA connector to the boom - this way the connector will fix the boom distance, but it appears to introduce phase shift between ground and the feed point. Also, the antenna assumes balanced input and this may also put it out of impedance. I'm thinking about adding the impedance matching stub like it was done by Andrew McNeil in the video above. Any thoughts on how to bring it to 50Ω will be highly appreciated.

     EpZbVDBWMAAw3UD?format=jpg&name=small

    Measurements:

    Spoiler

    EpZbmO_XMAEGcBp?format=jpg

    lpda-smith.png

    1 person likes this

  8. Posted

    On 11/21/2020 at 10:17 AM, Admin said:

    ,,, watching this video, I considered it necessary to try to design a Bester antenna made of FR4 fiberglass....

    ,,, so I present below the simulation designed and sketches for making the antenna....

     

     

    57.jpg

    How do you think, would make it two sided with via stitching improve it in any way?

  10. Posted · Edited by Mishka

    3 hours ago, Mishka said:

    Here we go. LPDA 1700-2700 MHz, it's made of 2.5 sq.mm copper wire and has 6 dipoles in total.

     

    OOPS!

    On the antenna drawing the distance between wires in the boom is 2.6 * 2 = 5.2 mm!

    The NEC model is symmetrical, so 2.6 is the distance from the axis (zero) to each wire.

    KNA27-dim.png

  11. Posted

    7 hours ago, Admin said:

    ,,, dimensions, please ... !!!
    ,,, or, I may need to install the NEC program again...!!!

    Here we go. LPDA 1700-2700 MHz, it's made of 2.5 sq.mm copper wire and has 6 dipoles in total. All dimension in mm. Antenna is [loosely] matched to 50 Ohm, but looking at the Smith chart it seem behaves closer to a dipole. Antenna fed with a symmetrical source.

    Of course, a tin-plated version will have different dimensions. As I mentioned earlier, to simulate it there's must be function for a wire mesh. Not too complicated to write though, but I'll be happy if you guys could do it in the CST.

    KNA27-dim.png

    Simulation data:

    Spoiler

    Smith chart:

    KNA27-smith.png

    VSWR:

    KNA27-vswr.png

    Return loss:

    KNA27-RL.png

    Azimuth (horizontal - solid, vertical - dashed):

    KNA27-azimuth.png

    Elevation (horizonal - solid, vertical - dashed):

    KNA27-elevation.png

     

    1 person likes this

  13. Posted

    Hey, the KNA-27 is built of six dipoles, not five as I counted initially!

    But there's more! When I entered its parameters - 1700-2700 MHz bandwidth, 5 dipoles - into my NEC script, it resulted in a model where the third dipole is clearly missing. Interesting.

    lpda-kna.png

    Of course, the model is optimized (Smith chart inside the spoiler):

    Spoiler

    lpda-kna-smith.png

    I'll re-run for 6 dipoles now.

  15. Posted

    1 hour ago, clanon said:

    CST is GUI intuitive...fast and efficient...

    (no need for code or commands)

    Although the NEC itself is pretty old piece of software, the cocoaNEC has its C-alike programming language so it's possible to calculate parameter values using a custom optimization algorithm. This is especially useful when there is no strong antenna theory behind the model and there are a lot of parameters to tweak so a simple sweep won't work well. As another example, someone could mention fractal antennas where the antenna itself can be generated.

    Therefore I'm expecting the CST (or any other modern toolkit) can automatically perform many parameters adjustment in order to minimize VSWR, return loss, etc. Can it?

    P.S. I'm on a Mac, and it seems any modern antenna software requires Windows nowadays. I'll be happy to know if the CST can run gradient descent or something similar over a set of parameters, or if it has a scripting interface.

  16. Posted

    On 11/16/2020 at 9:46 AM, Admin said:

    Mishka,try to design a log-periodic  antenna as above (Kroks KNA+ 800-2700 MHz)

    I believe the antenna is made of a single plate, and then bent near the center. Perhaps, the feed points located at a horizontal and a vertical boom sides.

    Well, I must admit it's not an easy task to model it in the cocoaNEC. The NEC has support for patches, but the surface of every element must be described in 3D with hundreds, if not thousands of those. I could describe the model with a wired mesh. The mesh will have at least two times elements less and is simper to program. For this particular antenna with five dipoles per side, there are about 20 parameters to optimize - five dipole length & width, plus element to element boom length & width. And there's also the reflector.

    Does CST, or HFSS, or FEKO, or anything else besides the cocoaNEC support a programming language, or maybe have a built-in gradient descent or something like that to do the job?

  17. Posted

    To design something more or less practical, I've put a model for dual-band LPDA for 2.4GHz and 5GHz WiFi into the cocoaNEC. The antenna was designed to be buildable from a copper wire S = 2.5 sq.mm, d = 1.7 mm. All other dimensions including distance between feeding rods (the boom) were optimized for VSWR using stochastic gradient descent. The antenna is matched to 50 Ohms symmetrical input.

    Both the NEC cards deck and the cocoaNEC SGD code are available. Please note though, to build this particular antenna I've added 2.6-4.9 GHz band-stop "virtual filter" to prevent SGD from optimizing for the non-interested bands. As a bottom line, it was calculated for 2.2-2.6GHz and 4.9-6 GHz. Considering the WiFi bands only, max VSWR is below 1.4 at 5875 MHz and 2485 MHz.

    Simulation data:

    Spoiler

    EmXzgoiXMAAKXzB?format=pngEmX0VpaXUAUkL4Z?format=pngEmX0ZrPW4AEFBuP?format=png

    Azimuth (horizontal + vertical polarization):

    Em5tFL7XYAEPP29?format=png

    Elevation (horizontal + vertical polarization):

    Em5tJJdXYAE5uzP?format=png

    1 person likes this

  19. Posted

    To continue on the LPDAs, there are a number of interesting PCB antennas by Kent Britain WASVJB: https://www.wa5vjb.com/products1.html

    I tried to simulate that one at 850-6500 MHz band range with CocoaNEC (no Windows here, sorry) using the method described at https://hamwaves.com/lpda/en/index.html, and eventually reached similar parameters, but my antenna is definitely larger. For 850 MHz the λ/2=17.64 cm, so I'm surprised how WASVJB was able to fit it into the 13.4 cm PCB.

    For the simulation I've set tau = 0.85, sigma = 0.12. Element to element transmission line impedance is 75 Ohm. All elements are of constant width 0.16 mm.

    Exact location of the elements is as follows (all dimensions in meters):

    Spoiler 
    
 WIRE      OFFSET     LENGTH
    1      0.0000     0.0882  <- quarter wave of 850 MHz, must have two of those
    2      0.0423     0.0749
    3      0.0783     0.0637
    4      0.1089     0.0541
    5      0.1349     0.0460
    6      0.1570     0.0391
    7      0.1757     0.0333
    8      0.1917     0.0283
    9      0.2053     0.0240
   10      0.2168     0.0204
   11      0.2266     0.0174
   12      0.2349     0.0148
   13      0.2420     0.0125
   14      0.2480     0.0107
   15      0.2532     0.0091

     

    Simulation data:

    Spoiler

    EkzbPy8WAAMv7Yc?format=pngEkzbDn2WAAc330v?format=pngEkzcw2kXUAAdGGT?format=pngEkzc6y_XUAExxv_?format=png

     

    1 person likes this

  21. Posted

    1 hour ago, Admin said:

    ,,, not,,, small radius of the ellipse => 49/2 = 24.5mm and large radius of the ellipse => 95/2 = 47.5mm

                 the center of the ellipse = 110mm from the 0y axis


     I thought my picture above was pretty clear...!!

    I'm sorry, but this doesn't converge into anything practical :-(

    Help needed.

     

    covina-2.png

  25. Posted

    3 minutes ago, Admin said:

    ,,, this do you like ,,, ?? parabolic offset 800mm

    [image removed]

    Yes, thank you very much! Really appreciate it!

    I see the simulation data is for 2150 MHz. I'd be grateful if you could do it for 2442 MHz. Well, I don't think it will change dramatically because the batwing is wide band.