A good antenna is crucial for the radio to function properly.
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A NanoVNA is very helpful to test if an antenna is resonant at the specified frequency (otherwise it's useless).
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You can find good clones for about 50€/$ (beware of really cheap/bad clones).
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+ A NanoVNA is very helpful to test if an antenna is resonant at the specified frequency (otherwise it's useless).
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The Return Loss should be >= 10dB at the used frequency, which means that at least 90% of the power is going out of the antenna, and only 10% is reflected back to the radio.
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RL back out VSWR +--------------------------- + 0 dB 100% 0% infinity + 1 dB 80% 20% 17 + 2 dB 63% 37% 9 + 3 dB 50% 50% 6 + 5 dB 32% 68% 3.5 + 6 dB 25% 75% 3 + 8 dB 16% 84% 2.3 +10 dB 10% 90% 2 +15 dB 3% 97% 1.4 +20 dB 1% 99% 1.2 + +RL ..... Return Loss +back ... reflected back to radio +out .... going out of antenna +VSWR ... Voltage Standing Wave Ratio+
+ You can find good NanoVNA clones for about 50€/$ (beware of really cheap/bad clones). +
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+ + The screenshot shows a very good Return Loss of 22dB. +
++ The antenna needs to be in vertical position while testing, so it's best practice to build a little stand for it, so one can test without touching the circuit, which would inevitably falsify the measurements. +
It's highly recommended to only trust specialized shops (mouser, RAK store) for antennas, because there is simply too much mass produced garbage out there (talking from experience).
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+ If an antenna is totally off when I measure it, I like to dissect it.
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Here you can see the inside of one good example, and the worst offenders so far (at the outside all share the exact same length of around 20cm and the exact same shape, see cut off plastic part on the left):
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