How long 2.4GHz antenna should be? The experiment with an antenna meter

Support my work / pawelspychalski
NS1201SA VSWR meter: bit.ly/3m23hmx
Antennas for the win! Hell yeah... At least in a few places in internets you can find something like "FrSky shipped wrong antennas (for XM+, XSR, R-XSR, you name it), they should be 31.25mm while they are shorter!!"
It kind of makes sense, after all, with a speed of light, "correct" quarter wavelength for 2.4GHz is 31.25mm. Too bad it is not true, and all those "pseudo-experts" (damn, I think I'm one of them now) tell the only small piece of truth. Equation "c/f/4" is correct but only assuming antenna is made from the void. And we all know that this is not the case.
In this part, it is time for some practical exercises. Let's take a VNA antenna analyzer and first make a "proper" full quarter length monopole without including Velocity Factor into the equation. And then let's include Velocity Factor. Guess which one will be better...
Oh, I can also show you what happens when you mount antennas touching quadcopter arms!
In a wire, things like dielectric constant, Velocity Factor (Propagation Factor) starts to do their things and as a result, those 31.25 for 2.4GHz antenna length is only a myth.
On top of that, the 2.4GHz band does not mean this is exactly 2.4GHz. Frequency hopping adds something from itself!
This video shows why and what goes where.
en.wikipedia.org/wiki/Velocit...
en.wikipedia.org/wiki/Relativ...

[@aSpyIntheHaus]

What a great video. I will be going through all my models and fixing my antennas. (removing bends, adjusting placement and removing heat shrink that I used to protect them)

[@benoitpiavaux7850]

Great video. I love the experimental part as it simply shows your points!

[@WimukthiBandara]

Beautifully done Pawel.

[@xdroner1009]

I like this kind of vids ! Thx for info based on science not empty words

[@Dandle01]

I like your video. Thanks for making it.

[@nocknock4832]

love your explainations

[@marksherman630]

Yes, more!

[@snoopy_fpv119]

Thank you for this information. I have various no name replacement antennas with ufl plugs and also without plugs for direct soldering for my various receivers. With these antennas, the active elements are longer than those built into the receiver. If I have understood correctly, can I simply shorten these active elements to the length of the original antenna? What influence does the diameter of the active element have?

[@zenman8269]

Great video! Would be great to know what size to cut replacement antennas for our receivers. So far no one has been able to give a scientifically satisfactory answer.

[@FPVUniversity]

I can not give you that answer because it all depends and the antenna materials, how receiver is constructed and so on. Assuming receiver itself is "neutral", then 90% of the c/f seems to be universal value. So around 28mm. But it really all depends

[@cobra4829]

interesujące. jestem ZA

[@ibnurachmat8444]

great! but most of wifi users prefer to use sectorlal/patch or even biquad antena, what do you think?

[@FPVUniversity]

Who are 'most'? Most users have no idea what antenna is ;)

[@ibnurachmat8444]

@@FPVUniversity really? but perhaps they have idea that antenna is the part of the router as the "door" so the router can transmit and receiv the signal itself.

[@arturskrzypczyk9250]

Yes we wan't more of this, maybe some explenation about how helical or cloverleaf antennas works ?

[@FPVUniversity]

I wish I knew ;)

[@EnglishTurbines]

If its a 5.8G antenna, then the short answer is invariably badly.....because they are nearly always dimensionally inaccurate.

[@tylerwilliams786]

So what's good length of antenna for the frysky rxsr 27.6mm? Great video

[@FPVUniversity]

somewhere between 27 and 29mm yes

[@tylerwilliams786]

@@FPVUniversity thnk u

[@gezakopter1153]

Hello Pawel! Does the non active cable length matter? I wanna to make some pigtails on 2.4G and also on 5.8g and I am wondering how long I need to make them. Thanx!

[@FPVUniversity]

yes it does. Laymans terms: lemgth should be a multiple of half wavelength in coax. So: multiply of Veloticy_factor * C / f

[@gezakopter1153]

Thanks for the swift reply! It kinda bothers me how could I find out the velocity factor of a chinese coax.

[@FPVUniversity]

if it has a name like RG174 RG178 RG316 then should be easily findable. If not, then usually between 65-75%. Use 70% when in doubt

[@gezakopter1153]

Hmm, I am planning ordering the below. They seem to be crimped on the standard silver antenna coax: www.banggood.com/RP-SMA-Female-To-U_FL-IPX-1_13-Pigtail-Cable-20cm-p-923077.html?rmmds=mywishlist&cur_warehouse=CN

[@FPVUniversity]

well, you have to assume that someone computed it correctly :)

[@IceCreams62]

Why do you use 'Arrivederci' :-) ??? Are you in Italy ??

[@FPVUniversity]

Nope. At the moment Germany ;)

[@507fasfpv2]

Fook yeah!!

[@nortonixx]

w praktyce i tak wszyscy pchają anteny w termokurcze, a wtedy współczynnik skrócenia się zmienia i z dobrej anteny robi się gówno (a rozstrojona może wejść w pasmo). dodatkowo przeciwwaga z carbonu też nie pomaga:-(

[@Siamect]

So conclusion is still that FrSky have shipped wrong antennas, it's just that the bad stuff is the x4r-sb and not on the xsr etc... Is it really true that the transfer of the signal into space is best when the antenna is perfectly tuned? Say that we have a singularity sized Vector Impedance Analyzer... the antenna itself would present a larger part than the ground plane of the analyzer and so the whole equation would be thrown of the planet. This happens in reality too, although it presents a less significant part of the problem. And it happens in the aircraft too. So obviously each installation would benefit slightly from being individually tuned based on actual radiated signal measurements... All for those who cares about details.

[@FPVUniversity]

Once again it's not that simple. X4R-SB might have some extra capacitance in the feed line and that requires longer active part of the antenna.

[@andrewallen6406]

I mount my antenna on zip ties would they effect it

[@FPVUniversity]

Their effect is very little and it is fine to use them

[@sebastiannobody1142]

many people use the zip ties or plastic tubes to mount the antennas at their copters. What would be a reasonable approach for velocity for the coax cable including the teflon insulation and this plastic material?

[@FPVUniversity]

those things affects VF in a very very little way. It's fine to use them

[@LeonVuksic]

Now modify a XSR antenna and show us DVR RSSI difference :)

[@FPVUniversity]

I'm not saying that 27 is correct. I'm only saying that saying "31.25 FTW" is not correct ;)

[@yokonunz]

What if you don't hold the carbon spar with hands?

[@FPVUniversity]

effect would be smaller

[@yokonunz]

Paweł Spychalski Yes :) worth a try in the next video

[@yokonunz]

Paweł Spychalski Yes :) worth a try in the next video