2.4G antenna length

[ShowMaster]

This is for any 2.4G system.
I'm pretty antenna savvy but can't answer this question I was asked, only generalize.
The question or questions asked a lot are
Why are the antenna lengths (exposed coax tip) so different between manufactures and ebay replacement antennas.

Another big one is why does it seem that all Frsky RX antennas are the wrong 1/4 wave length and shortening them seems to make them better?
I'm assuming that Frsky knows what their doing since they did design a very good RX to start with. Coax velocity factor would explain why one brand vs another would have a slightly different 1/4 wave length exposed at the coax end for the antenna.
The claims on the forums that are being made the the Frsky antenna is too long and making it shorter improves the range confuses me. Is Frsky that "out of tune" in their antenna design or are the posted claimed about it wrong?

For most us it'll never be an issue since the stock antenna range in my case, has been solid control and good RSSI reports as far as I can see my plane.
Any thoughts on this subject with some facts to back them up are welcome. I also plan to ask Frsky this question.
SM


ShowMaster
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[thomas9x]

Why are the antenna lengths (exposed coax tip) so different between manufactures and ebay replacement antennas.

Surprisingly, many "import" antenna suppliers are careless and/or clueless. It's shameful.

Another big one is why does it seem that all Frsky RX antennas are the wrong 1/4 wave length ...

Not that I've seen the issue on my FrSky Rx antennas, but for sure the length of an antenna can suffer from sloppy factory workmanship or bad engineering. But perhaps you mean the math does not match the measured length? The popular math formulas for element length are essentially estimates and in practice should not be considered the exact value. Materials, construction, and installation also affect the length.

Keep in mind that the FrSky coaxial Rx antenna is a simple monopole. Unfortunately the required counterpoise/groundplane is missing in our R/C application because our model aircraft are not sufficiently large metal structures. So each antenna installation will behave differently and experiencing optimum performance with the stock antenna would be similar to winning the lottery. I'm not saying they don't work -- I'm saying the antenna system is not expected to be optimized due to its simplicity and the varied installations. So if tuning the installed antenna's length helps, it does not mean the factory supplied antenna deserves all the blame.

Any thoughts on this subject with some facts to back them up are welcome.

To provide some practical data I measured two different FrSky antennas for you (new in package, so factory fresh examples). One was about 150mm overall length and the other was about 400mm. Both had driven elements that were about 32mm long.

FrSky antenna, 150mm long

FrSky antenna, 400mm long

... and shortening them seems to make them better?

The freq range the antenna must provide is about 2.40GHz to 2.48GHz. Casual review of the measurements would hint that shortening my two antennas would be the wrong thing to do since their center freq is already slightly high. Furthermore, in a common installation the antennas are against the wood/foam surfaces so there would be slight reduction in the resonant freq (up to -30MHz is typical). So I would conclude my two examples have reasonable lengths.

These simple 2.4GHz coaxial monopoles are not known for high performance. They are best described as cheap and practical. For sure, in our intended "visual sight" R/C application they do just fine. If someone needed to fly much further then they have to give up the convenience of the stock antenna.