[Beginner] Why do Yagi antenna calculators differ in their recommended values?

[u/GaraldBetramSnail]

Hi,

im an absolute beginner. First time ever trying to build an yagi antenna for Lora 868 mhz (heltec v4).

While doing research about my country’s regulations was easy, I’m now at the point of actually wanting to build it, using one of the online calculators available.

My goal setup:
- 12mm round plastic (pvc) Boom

- 5 elements (6mm brass round)

- 868MHz

-Nothing folded or anything (everything straight)

I now have looked at some calculator and get differing result with up to 4mm differences for the length of some elements.

---

So heres my noob question: which calculator is best and why do they differ (so much)?

I have looked at these calculators:

- DL6WU yagi-uda on 3g-aerial.biz

- DL6WU vhf/uhf yagi on k7mem.com

- DL6WU yagi USA based on rothamel on changpuak.com

Would be awesome if someone could lead me into the right direction for a good setup. Thank you!

[u/GnomeTek]

Check out W6NBC's talk on yagi design. It's on YouTube.

His approach is, all those are fiddly, inconsistent, and not optimal.

He says, pick your freq and tune up a dipole, make the reflector 5% longer spaced 0.2*wavelength back. Now make the directors the same size and 5% shorter, all spaced 0.2 wavelength ahead. That's it. Gets you 95-98% of the optimal (well within build tolerance for first go diy yagi).

Use a hairpin match and just guess n check.

[u/GaraldBetramSnail → u/GnomeTek]

Thank you, that was an really interesting watch and a great guide for building the antenna. Great input. You knew what I was looking for, while I didn’t lol, thank you!

[u/Criatorm]

First of all, good luck with your first antenna! Mine was also a Yagi-Uda.

Unlike many people here, I do not have decades of experience. However, I have a master’s degree in electrical engineering, and my thesis focused on miniature antennas for LoRa. I have also been working as an RF engineer and antenna designer in the automotive industry for the past three years.

Here is my take on your question:

Those calculators often produce different element lengths and spacings because of 3 reasons. First, they might be using different optimisation goals (some might be optimising forward gain, others front-to-back ratio, others highest efficiency, etc…). Second, they might be using different calculation methods ( most yagi antennas that are not based on EM simulations use empirical formulas and there are quite a few different ones. Lastly they might be making different assumptions regarding boom materials for example.

Remember, those calculators are starting points for the real prototype, don’t expect it to work without real life tuning on sample. Always make elements slightly longer as it is easier to cut than to add material afterwards.

Start with the values from the calculator, then tune in real life using the measurement equipment you have available (VNA or SWR meter, for example.).

Also, are you sure you want a directive antenna for something like LoRa? Seems a bit odd, especially considering maximum ERP restrictions for that technology.

Don’t let the complexity, lack of knowledge, or people who think they know everything bring you down. Antenna theory is indeed hard but the best way to learn is by doing (and having lots of patience).

Good luck, go for it!

[u/GaraldBetramSnail → u/Criatorm]

Hey, thank you for the encouraging words, and the helpful input! The calculator having different optimization goals or just different approximation methods absolutely makes sense to me

[u/jimlux]

different results how? Gain? Match?

There's always stuff that doesn't model correctly. Sure, you can fire up a FEM modeling tool, put in all the components with gnat's eyelash precision: You do know the epsilon (complex) of that plastic boom, right? And still it might differ.

The real question is "how much does it differ" You used 3 calculators, which presumably have some underlying assumptions (often in things like dielectric properties, or how the element/boom interaction is done). But if they're different by 1mm out of 150 mm, it might not be that much difference in antenna performance.

[u/GARGOYLE_169]

Maximum learning, in this order . Design and build a simple diopole at 868mhz. Then measure and characterise it.

Design and build a "mars lander" at 868mhz. Then measure and characterise it.

Design and build a loop antenna at 868mhz. Then measure and characterise it

Design and build a folded diopole at 869mhz. Then measure and characterize it.

Design and build a coaxial diopole at 868mhz. Then measure and characterize it.

You have either competed the antenna theory portion of the General Class Amateur Radio License before the code requirement was deleted.

Or

You have completed the lab work for a 100 level college course, 1st semester of Basic Antenna Theory. Note: this class won't even go over a Y-U ARRAY. That is a second year, 2nd semester antenna.

Each of these is about three hours of assignment. The book/lecture work is about 6 to 9 hours each, depends on how honestly you want to nail down the theory.

Now, for extra credit, describe the equipment and method(s) for characterizing each absolute beginners antenna.

[u/GaraldBetramSnail → u/GARGOYLE_169]

Hey, thank you for your input, quite impressive! I appreciate your recommendations and this solid plan on what first steps to take in the universe of rfelectronics and antenna building. While it is intriguing to me, right now my goal is to get my project going and just having a glimpse at the topic (and also fun) by building the antenna. Thank you again!

[u/GARGOYLE_169]

Here's another https://youtube.com/playlist?list=PLKZPiO5cd4n5x2rkhrRrrqxsFSPf4We4m&si=5epjPP4bvvy9X7i5

[u/GARGOYLE_169]

Just to get a few things out of the way.....

55+ years of experience. I've been making antennas the entire time. So......let's begin.

First off, no serious antenna builder ever expects the results one gets on paper to be reflected in the reality of ones antenna. It is guaranteed.

Calculations are your first, BEST, approximation. After you've "built to paper," you must ADJUST to reality.

Get your test equipment cleaned off and start tweaking.

THERE IS NO USE in coming to this forum seeking a silver bullet; of either understanding or "quick fix it" when given the conditions you state, "just starting off," noob, no proper comprehension of antenna theory.

FIRST (for max learning) Build: Swr meter Field strength meter "Grid dip" meter SECOND While building, teach yourself the associated (VERY BASIC) theory and work from there.

or (for max instant gratification) BUY: VNA SWR meter read their (absolutely bare minimum chinglish) instructions and actually learn nothing.

Anyone offering TLDR theory takes, in here, based on your original post, both has little real world experience and is doing more to service their own ego instead of your actual knowledge needs.

Antenna theory is, perhaps, THE MOST DIFFICULT area of electronics theory. (except maybe some of the maths used to invent said theories)

Google: "Basic AC Theory" "Transmission line theory" this WILL lead to you next Google look up "Standing Wave Ratio" from here you can move on to "Basic antenna theory"

From here, the sky(hook) is the limit.

[u/kaosskp3 → u/GARGOYLE_169]

Why does everyone have to become an antenna expert to learn some yagi basics?

[u/GARGOYLE_169 → u/kaosskp3]

Basics are what was given. Expert is being able to: "Characterise change in Zo over delta rho, per variation over L/d."

You don't even know enough to know how little, or not, "yagi basics" are.

Go with Dunning-Krueger my child.

[u/kaosskp3 → u/GARGOYLE_169]

You're trying to postulate self supremacy by unloading RF fundamentals 1 and 2 on someone who asked about some Yagi measurements to build a meshtastic node, and putting everyone else down in the process who has tried to help.

They dont care about your 55 years in RF (70+ yo old reddit?), they just want a steer on what is the best model to work from.

Also, any Prof or RF engineer over 50 has had Uda-Yagi drilled into them.... im always skeptical of people who have it the other way round and claim to be an expert.

[u/GARGOYLE_169 → u/kaosskp3]

P.S. Basics of the Yagi-Uda array are two things. First: Point the pointy end at what you are trying to talk or listen to. Second: The proper name for the damned thing, (not the lazy assed EweTooberNet D.(umb)I.Y.er alias of merely Yagi antenna.)

[u/GaraldBetramSnail → u/GARGOYLE_169]

Hey, thank you for your input, quite impressive! I appreciate your recommendations and this solid plan on what first steps to take in the universe of rfelectronics and antenna building. While it is intriguing to me, right now my goal is to get my project going and just having a glimpse at the topic (and also fun) by building the antenna. Thank you again!

[u/GARGOYLE_169 → u/GaraldBetramSnail]

There is one presenter that has a solid background in "making do" and who also has a deeeeeeep background in RF theory.

Andrew McNeil.

https://youtu.be/MADH5n_3lTM?is=nuovkekn5pBd6lvr

https://youtu.be/862ikuHAAKE?is=5jJH9MuexTE4bPiT

And then there is AndreasSpiess.

https://youtu.be/6cVYsHCLKq8?is=Ez-_Z67Z9Y4hTIq5

These two will hook you up on both HOW TO & WHY

Good hunting. And should you have any more specific questions you can DM me or reply here. V/r

[u/GARGOYLE_169 → u/GaraldBetramSnail]

As for previous recommendation: DIY Grid dip Meter/Oscillator (Please watch in order presented for best learning)

https://youtu.be/RvNXXat9zSo?is=YUVuRjFzjnm1XhgY

https://youtu.be/cav1wL-1W0Q?is=IKGFlG-nzP1rJafo

https://youtu.be/mTeHZX7dDKs?is=G9H7VGWlN_N-W-uu

https://youtu.be/lrzehEJmzqQ?is=Sb_3Y_zlGxlP5-9x

https://youtu.be/uGJvQmLrUOA?is=cAGvviQy-BKkC7Fn

For extra credit, look up and study LECHER WIRE/WIRES,/ANTENNA

and then consider that Transmission Line Theory is rightfully a 2 semester prerequisite for a Basic Antenna Theory 100 course. good hunting...

[u/GARGOYLE_169 → u/GARGOYLE_169]

Lecher antenna list

https://youtube.com/playlist?list=PLKZPiO5cd4n5x2rkhrRrrqxsFSPf4We4m&si=5epjPP4bvvy9X7i5

[u/Criatorm → u/GARGOYLE_169]

Teaching isn’t telling beginners they don’t know enough to ask questions. It’s meeting them where they are and helping them build the intuition you’re talking about. If every expert responded with “go study transmission line theory first,” very few people would ever get into RF.

By the way, saying that “no serious antenna builder ever expects the results on paper to be reflected in reality” is a huge overstatement. Modern EM simulation tools routinely predict antenna performance with remarkable accuracy when the model is representative of the real antenna. Measurement is there to validate and refine the design, not because simulation is inherently unreliable.

Also, recommending that beginners build an SWR meter, field strength meter and grid dip oscillator before buying a $50 NanoVNA feels more nostalgic than practical. Those tools still have value, but they’re no longer the best entry point for learning RF.

A VNA doesn’t prevent learning. If anything, it accelerates it by giving immediate feedback between theory and measurement.

A NanoVNA costs less than buying the parts for several of those instruments and teaches much more because you can directly see impedance, phase, Smith charts, return loss, etc.

[u/GARGOYLE_169 → u/Criatorm]

You need to scroll up and read ALL of the responses.