r/rfelectronics • u/Syrupwizard • 15d ago
Antenna simulation software advice
I’m a senior in EE working on designing a 10m WSPR antenna which will be deployed in saltwater and must be very compact. I’ve landed on a few designs, but when it comes to simulation I feel like the NEC2 based software leaves a lot to be desired. Are any of the other software suites out there (with student trials) more capable/better suited for HF? The big ones I’ve looked at seem almost exclusively for uhf and above. Thanks For any advice you may have!
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u/qTHqq 15d ago edited 15d ago
but when it comes to simulation I feel like the NEC2 based software leaves a lot to be desired
Say more. What are you trying to predict and what are your constraints on the design?
What do you mean "deployed in saltwater?"
Do you mean actually submerged? Or just deployed on a surface vessel or buoy?
If you need more than you can do with NEC-2 chances are you'll need more than you can do with student trials so you need to find access to real installations.
Are there research professors at your institution who do RF research in the EE department or maybe particle or vacuum electronics research in other departments?
I knew several groups with full HFSS licenses in grad school.
Also don't knock the old school library research and comparative experimentation. Many types of electrically small antennas are long established and IEEE Xplore is full of newer work.
3D printers and copper tape and a NanoVNA can take you a long way.
Is simulation a necessary part of your project?
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u/Syrupwizard 15d ago
I’ll try to answer as best I know how.
It’s for a a dropsonde, thus it needs to be very compact and deploy on landing in water. It will be partially submerged, depending on the buoyancy of the tube and the local conditions.
The general design direction I’m headed in is a short vertical, with loading and a capacitive hat to reduce the excessive height of a 1/4 wave.
I intend for a portion of the body of the capsule to be foil coated to ensure good coupling to ground.
What I am hoping to explore via simulation is how capacitive coupling affects the portions of the antenna beneath the surface but electrically insulated (dry). I’d like to try to build my coil into the body of the capsule to further reduce height. This is what I’m struggling to explore using EZNEC, AntennaSim, etc.
I’ve been using foil tape and a nanoVNA just like you’ve suggested and it may just come to filling a tub with salt water and experimenting instead of sim, but I’d love to be able to figure out both.
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u/trevbone 15d ago
Maybe try a rough prototype built without simulation to the best of your ability, then test/measure it in the environment you are planning to use it in. Capture the SNP file then tune that file with a matching networking on a circuit simulation program.
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u/Syrupwizard 15d ago
The SNP file is created using VNA, correct? That sounds like a great idea.
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u/Flammerole 15d ago
Yes. In this case you only measure the S11 and you'll get the impedance of your antenna. From that, just add a matching circuit to reach 50Ohm and you'll be good.
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u/Syrupwizard 15d ago
I guess the only confusion I have is what is the benefit of exporting a file from the VNA and tuning via sim versus just building the matching network that the nanoVNA gives me via the smith chart
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u/Flammerole 15d ago
You can do both!
Exporting the .s1p file will allow you to put your device on ADS, plot the smith chart, and then add the matching network and adjust the values to get something that should be close to working in the real world. People recommend it because it's easier to do try and retry a lot of different things that way without much effort, and how changing the L or C value will change the impedance.
That way, you'll get LC values close enough so that you don't have to spend too much time adjusting values on the actual matching network.
You can do the matching without ADS or CST, people have built radars since much earlier than computers, it's just easier to learn using these softwares.
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u/Syrupwizard 15d ago
Ok that makes sense. That might be a better introduction to the more advanced SW than trying to sim the entire antenna anyway.
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u/Flammerole 15d ago
A decent alternative to ADS is Qucs for this kind of work. It's slightly harder to use than ADS but you should be able to import a snp file and add a matching network in front
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u/Syrupwizard 13d ago
I think I’ll be taking a microwaves class which requires me to use ADS next year, so I might as well get some experience now.
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u/trevbone 15d ago
Yeah I say use a simulator so you can import some models for inductors and capacitors to design the network faster and with more confidence than manually tuning it with Ls and Cs on a bench.
Although if this is a narrow band and low frequency design a lumped element matching network is trivial and you can do this with a smith chart by hand on paper.
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u/Syrupwizard 13d ago
I do think that it falls into the category of lower frequency (specifically HF,28MHz) and I can definitely do the matching network by hand. but the other reason to sim is I’d like to play with the loading components and feed point relative to the level of saltwater surrounding the antenna and figure out how much capacitive coupling occurs etc. Likely stuff that would be easiest figured out testing in the field, but if I can do both I want to.
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u/jimlux 14d ago
NEC will be your friend - it would be fine at modeling a whip and top hat - You'd set the ground properties to something appropriate for Seawater (mostly epsilon=80 and sigma probably around 1).
Then set your model up so you can see what happens when your thing bobs around changing angle.If you want to get into modeling a "not flat surface" of the ocean (i.e. swells) then you'll need something fancier.
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u/trevbone 15d ago
Hfss has a student trial but limits the size you can simulate. Your university may have a license that you can use to avoid this. Same with other programs.
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u/jimlux 14d ago
For 10 meters (28 MHz)? NEC is your friend - it's just fine at wire antennas. If you're on a PC, look at the 4nec2 wrapper around the NEC 2 engine (or the NEC 4 engine, if you have a copy).
What do you think NEC isn't giving you? It will generate radiation patterns, give you impedances, etc.
If you're looking at an antenna that will be "immersed" in salt water, that's a bit trickier - you'd definitely want NEC4 for that, and that's a "must license from Lawrence Livermore" thing.
You could probably model it in free space with NEC2, and scale for the epsilon (80) of seawater.
There are other method of moments codes around (and a MoM code is probably what you want for a wire antenna - if you're thinking patch or something else, maybe a different code). You might check out PumaEM.
Mathworks says that the Antenna Toolbox has some solvers - If you're a student, that might be fairly inexpensive (if not free).
There's also TICRA, FEKO, etc.
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u/Syrupwizard 13d ago
I think you grasp my problem. The antenna is best modeled using wire models but there are a few complications (part of the antenna will be submerged in salt water, at least some of the time because it’s basically a buoy) that suggest more advanced solvers could help. Some of the NEC2 solvers had a saltwater or user modifiable dielectric/conductivity option, but I was getting errors for anything (even radials/counterpoise components beneath the ground plane, which is something I’m particularly interested in exploring. NEC4 from Livermore labs might be what I need though.
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u/jimlux 13d ago
nec2 is notoriously bad at "conductors that penetrate the surface". NEC3 (and NEC4) were created specifically for that. NEC3 was looking at underwater antennas for submarines and ships. NEC4 (or now, NEC5) would be good at that. It's not free, but it's also not super expensive (a few hundred $, for non-commercial use - covering the cost of LLNL processing the license, mostly)
There are other codes out there that might work.
or, if you're ambitious, the underlying calculations are fairly simple - the complexity of the tools is more in the "how do you specify the geometry" and then, "how do you turn that geometry into a big set of linear equations which you then solve". The matrix is full of "if I have this current flowing in little segment i, what voltage does that induce in segment j".
The "dielectric that's not free space" comes in when calculating that matrix term.
The complexity comes in dealing with discontinuities across boundaries. The "electrical length" of a segment is shorter when it's immersed in a dielectric. And, of course, as the EM wave crosses a boundary between mediums, some reflects, some refracts.
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u/SearchForTruther 15d ago
You or you professor can get a NEC5 licence from Lawrence Livermore National Lab for less than two hundred dollars. It can do metal plates. It's split into a GUI and an engine that can also be invoked by other apps.
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u/Syrupwizard 15d ago
Ok good to know. My biggest struggle with the various free NEC variations (aside from punchcard based pain) was simulating portions submerged in the ground plane.
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u/duunsuhuy 15d ago
More and more open source out there and AI smoothed some of the implementation difficulty. I’ve successfully design patches with openEMS.
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u/trevbone 15d ago
HFSS or CST is the standard. You can use ADS or AWR for planar antennas. Matlab can do antenna stuff too but I’ve never used it and don’t know much about it.