openEMS model of my new 1420 MHz cantenna feed (patterns, VSWR & standing wave animations)

[Ayushman Tripathi]

Hi, here's an openEMS model of my 1420 MHz cantenna feed with patterns, VSWR, and in-can fields. I also made a new animation of the standing wave showing why the probe sits where it does. I'm still improving my model.

[Ayushman Tripathi]

(these GIFs are compressed to fit the upload limit and I'll share the full-quality versions soon)

[Ayushman Tripathi]

Here are the high-quality versions of these:

https://www.icloud.com/iclouddrive/05fbbMTPgdNvm_Sj1NQXOHuFg#Cantenna

[Ayushman Tripathi]

Apologies, that first link needed sign-in. Here are direct download ones:

https://www.icloud.com/iclouddrive/08e7LiE9o-GiXvpmf7qsDXG2w#cantenna2%5Fwhy%5Fprobe

https://www.icloud.com/iclouddrive/036c0REMzBqvMCoR2UKf3hkfw#cantenna2%5Fstandingwave

https://www.icloud.com/iclouddrive/0275TPqLPDyQs68LyOSwnVuRQ#cantenna2%5Frx

[fasleitung3]

Hi Ayushman,

The calculated pattern is a bit surprising to me. Please find attached a measured pattern of a cantenna at 1420 MHz which is much more well behaved. Please ignore the blue arrows in the drawing, these were the directions of the rim of the dish we were using this cantenna for.

What is the length of the waveguide which you used in the simulation?

Best regards,

Wolfgang

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[Andrew Thornett]

Looking at the top right GIF with the waves, does that mean that the monopole probe could be put further up tube where the next peak wobbles left and right to catch the peak of that instead?

Andy

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From: sara...@googlegroups.com <sara...@googlegroups.com> on behalf of Ayushman Tripathi <ayushmantr...@gmail.com>
Sent: Saturday, 06 June 2026 04:21:53
To: Society of Amateur Radio Astronomers <sara...@googlegroups.com>
Subject: [SARA] Re: openEMS model of my new 1420 MHz cantenna feed (patterns, VSWR & standing wave animations)

 

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[Andrew Thornett]

I do like all these animations.

Maybe this one answers the question I sent you couple minutes ago after I saw your previous email.

By putting probe quarter wavelength from bottom, does this mean that it captures both signal coming in from outside and reflected signal from bottom lf can, and that they interfere constructively at this point? Is that correct or am I interpreting it all incorrectly?

Andy

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From: 'fasleitung3' via Society of Amateur Radio Astronomers <sara...@googlegroups.com>
Sent: Saturday, 06 June 2026 08:27:40
To: sara...@googlegroups.com <sara...@googlegroups.com>
Subject: Re: [SARA] openEMS model of my new 1420 MHz cantenna feed (patterns, VSWR & standing wave animations)

 

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[Ayushman Tripathi]

Hi Wolfgang,

Thanks, yes, I'm trying to create an ideal-dimension reference cantenna model to get a simulated pattern I can compare against your measured one. The length of the cantenna that I'm using for this model (which is the same as the new one I made) is 320 mm with some non-ideal dimensions/probe placement, so that overlength may be part of it. It can also be a bug in my model, I'll double-check everything and share my findings.

Still getting the hang of openEMS, so I really appreciate the feedback!

Thanks.

[Ayushman Tripathi]

Hi Andrew,

Yes, exactly! The wave reflects off the shorting back wall, which forces a field null (zero) right at the wall. A quarter-wavelength (guide wavelength) away from the wall, the incoming and reflected waves line up in phase and add together, giving an antinode (field peak). The probe sits at that antinode to grab the strongest field.

Good thinking regarding the next peak. I was wondering the same thing last week. From what I read, in principle, yes: the next peak is ~3Lg/4 up. But the standard cantenna length is about 0.75 Lg (= 3Lg/4) so that peak is near the open mouth where the open aperture disturbs the standing-wave pattern, making it less predictable than the first peak near the back wall. The Lg/4 peak near the back sits in the cleanest and most predictable part of the standing-wave pattern, so it's the better, standard spot.

For longer cantennas, this would be interesting to test. Has anyone experimented with placing the probe near the 3Lg/4 peak in a longer waveguide and compared the results?

Thanks.

[Ayushman Tripathi]

Note: Lg here is the guide wavelength (the wavelength inside the waveguide), which is longer than the free-space wavelength.

https://www.changpuak.ch/electronics/cantenna.php

[Robert Meade]

It appears E and Had plane fields were plotted, not the power magnitude pattern which comes from pointing vector S proportional to E cross H. Did I miss a power pattern shown at some point below? 

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[Ayushman Tripathi]

Hi Robert,

"E-plane"/"H-plane" are just labels for the two cut-planes, not separate field components, both are the power pattern. In the far field, E and H are perpendicular and in phase, with H = E/η₀, so the Poynting vector reduces to:

S = ½ Re(E × H*) = |E|² / (2η₀)

So power density ∝ |E_far|². We compute directivity directly from that:

D(θ,φ) = ( |E(θ,φ)|² / |E|²ₘₐₓ ) · D_max (plotted in dBi)

where:

• S = Poynting vector — power flow per unit area (W/m²)

• E = electric field (V/m); H = magnetic field (A/m)

• η₀ = impedance of free space ≈ 377 Ω

• Re = real part; × = cross product; * = complex conjugate

• θ (theta) = polar angle measured from the antenna's +z axis (boresight in this model)

• φ (phi) = azimuth angle (around the axis)

• D(θ,φ) = directivity (radiation concentration vs an isotropic source); D_max = its peak value

• |E|²ₘₐₓ = peak of |E|² over all directions; dBi = dB relative to isotropic

So the polar plots are the power (directivity) pattern, and E×H is fully captured by |E|²/η₀.

I'm still refining the model and fixing a couple of issues Wolfgang flagged.

Thanks!

[Stephen Arbogast]

Well.... you  guys  inspired me  to open  my  Jackson - Classical  Electrodynamics  -  Second Edition..  for  a review.

And .... I took  the plunge...  installed  Ubuntu  26.04 on  a separate  ssd drive  and  successfully  installed  ezRA   along  with  with Ted  Cline's mods for  Air Spy.  So far all is  good   ...   I am   collecting  data  now  with my Air Spy  SDR.

Stephen

[Stephen Arbogast]

Here  is a  screen shot  of  ezCol  for  AirSpy....    

[Stephen Arbogast]

I have   been  accused of  being  too much  of  an  Engineer..  I  take that as   a compliment.. I  prefer   Physics...     We solve Helmholtz  equations  with   boundary  conditions ....   for the  modes   in  wave guides.. some  examples..

solutions  of Hemholtz equation  in wave guides

On Friday, June 5, 2026 at 8:51:54 PM UTC-6 Ayushman Tripathi wrote:

[Stephen Arbogast]

Modes  in wave guides come from  the  Helmholtz  equation.....   eigen values.....  https://en.wikipedia.org/wiki/Helmholtz_equation

[Ayushman Tripathi]

Hi Stephen, Nice.
Thanks for sharing.

[Ayushman Tripathi]

Hi Wolfgang,

I redid the cantenna pattern for an ideal D = 150 mm can (single-mode TE-11 at 1420 MHz) after fixing a few bugs in my simulation, you can find it attached. This one's computed from the aperture physics directly, the standard circular-waveguide aperture formula for the main beam plus rim-edge diffraction for the back/side lobes.

The E-plane is a bit narrower than the H-plane.

Thanks.

On Saturday, June 6, 2026 at 3:27:55 AM UTC-4 fasleitung3 wrote: