4NEC2 is an old but valuable tool allowing you to play with antenna design…
Here is a clip from a video I’m working on and story https://www.thethingsnetwork.org/labs/story/play-with-antenna-design
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Hey, that’s really interesting, even as an antenna design numpty like me!
I’m looking forward to seeing how the rest of this series goes. It looks like powerful software but your presentation of driving it makes it look accessible.
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4NEC2 works like a charm. I use it a lot for modeling and testing antennes.
Who wants to give it a try and play with the 868 Mhz GP design, you can download the NEC of the 868Mhz design file on :
http://www.bolkesteijn.nl/blog/index.php?page=lora-gp-antenna
(scroll down to : LoRA GP antenna - simulation )
There is also a few links to the " A Beginner’s Guide to Modeling with NEC" pdf’s.
P.s. nice to see that people like to read my LAB and that they get inspired by it. 
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It is idiosyncratic
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and very very neat ! (in the old fashioned sense of the word)
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Proper antenna design is a must! Great tool and great link… as important as good antenna is the correct impedance matching so energy is transferred from the source to the load… unfortunately this still require a VNA to do it properly.
Whilst a VNA is very useful, you can design, build and tune antennas without one, radio amateurs have been doing it for years, using only an SWR meter for instance.
You can even dispense with the SWR meter and test your antennas simply by measuring the radiated field with an RSSI meter, which is an easy build.
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Yes, spot on! I released this cut-down, video as I became fed up with the production delay and the growing pile of half finished ideas mounting on my desk. The FULL version explains VSWR, S11 etc etc in what I hope is an easily digestible form (without being condescending). Those cheap AAI N1201SA devices bring a lot of these concepts within the budget of most. https://img.banggood.com/file/products/20160922213704N1201SA%20-%20USER%20MANUAL.pdf
I’ve been borrowing a VNA to tune L Networks. Although I can measure the VSWR/S11 with my spectrum analyzer it doesn’t tell me the impedance…
I understand at lower frequencies a variable inductor and variable capacitor can be used to tune antennas (HAM Radio), but those are large and not very precise parts…over 500MHz I think things are more complicated to do this way (AFIK).
If you could expand your answer or point to some link describing it I would appreciate… this would save me 3 hours drive to use a VNA 
I cannot expand the answer into a general tutorial on how to tune or match antennas, its a big subject.
In general you will be following some published design and the instructions for that ought to provide details on how to tune or adjust that antenna, how this is done varies a lot, depending on the type of antenna.
At the frequency required you need to adjust the antenna for minimum SWR, this should be below 2.0, 1.5 and lower is good.