How long 2.4GHz antenna should be?

Support my work / pawelspychalski
It's a common believe that the "perfect" antenna length is "wavelength by four". It is, however, not true. Or at least, not entirely true. This video explains this phenomenon and explains what the real antenna length for Express LRS or other radio should be.
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 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...
And yes, there is a second part: • How long 2.4GHz antenn...

[@scottyh72]

Love your style. Subscribed.

[@gsegallis]

I suppose that is one theory. I'm not an antenna expert, but another theory might be that the goal is to maximize power from amplifier to antenna, which occurs when the antenna is resonant (meaning zero reactance). That is typically around 0.48X the wavelength (meaning each leg is 0.24X the wavelength), which would be about 29.9 mm. This is for a very thin conductor. You get more bandwidth as the thickness increases and that 0.48X might be more like 0.45X for a typical antenna, which would be ~ 28 mm per leg. And then the feedwire from amplifier to antenna further affects the resonant length, as does any inductance in the feed. To get zero reactance the antenna might be shorter still.

[@SanJacinto23]

Also the factory can tune the reciever internals to pretty much any frequency they want so we really have no idea what the antennas length on any given receiver should be, they could all be different

[@FPVUniversity]

Yes!

[@AndrewNewton]

You are a very funny chap, love your work.

[@JoshWeaverRC]

Andrew Newton. He is a funny fellow. Great to not take himself too seriously. I first saw a video of his through his review on RDQ for the Matek F405 Wing. Can't wait to see some videos on that FC.

[@sofyanhardiyono9152]

how about using bowtie antenna since the frequency is hoping moving from 23xx mhz to 24xx mhz..so the antenna can accept and have resonance on each frequency hoping..

[@ze2004]

can we reduce the protected part of the antenna or will reduce the performance? I have radiolink antennas and they are 27 mm but i have sent an email to the company and they said that i should buy a new antenna and not shorten it in the protected part. But the things is, they do have different size from the protected part. Maybe im wrong. "We do not recommend to make any modifications on both antenna and coaxial cable. If you damage the antenna, the control distance will be damaged. Both of them are important. But if the top part antenna is damaged, there must be problem of signal transmission. Hope it will be clear to you."

[@fares3651]

5:00 you actually broke your favourite pen at 4:21 :)

[@mikesmith1290]

I measured my new r xsr antennas, one side was 31, the other was 33. Since I broke one in a horrible crash, not sure what length to cut it too. Maybe I'll just go with the standard 31, and tune from there. I had an XSR from last year, the length was somewhere in the mid 20's, and after watching the rotor riot antenna test, I cut mine to 31, and the rssi and range did improve. Before I wasn't able to get over 85 rssi, now im able to get in the mid to high 90's. Just my experience with this.

[@gorsky3]

That's cool and interesting. But lot of people including myself experienced range increase after "fixing" antenna length on XSR receiver. Could you please explain this ?

[@FPVUniversity]

I'm not saying that FrSky did not fcked a batch of XSR. I'm saying that 31.25 is not the answer.

[@gorsky3]

So, it turns out, that best way to figure out correct lenght, is to increase it to somewhere about 32mm. And cutting by ~1mm each time observing rssi :)

[@TheRoybert]

@@FPVUniversity This is out of my wheelhouse. I'm all digital. I would love a lesson in the specs of that however. DJI goggles and the antenna capacity, frequency, range, fixes, upgrades, etc.

[@LAHover]

Well right or wrong, stripping my R-XSRs & XSR receivers to 31~mm dramatically improves the range (yes I have repeatedly tested it)... in fact my XM+ receivers (31mm out of the box) have always out ranged the others... So if 31 is not the magic number than what the heck is going on?

[@djvaliro]

same here .. blah, blah ... i have tested this in the field before modding antennas to 31.2 i was getting only 400m range .. after mod over 1.6km so talking is useless.

[@BBFPV]

Pawel fights with the calc 😅😅

[@rjredd]

awesome, kind of knew there would be a reason why they made them to the lenght they did.

[@Leos-World]

HAHA.... this presentation made me smile and was informative.... polish style ;-) Thumbs-up from me :-)

[@josephdunbar8697]

LOL IKR!!! Loved it

[@nixxda270]

and I'd push it again if I could! after actually watching the video. Thanks for finaly enlightening me!-) BUT, 31.25mm always kind of worked for me... On an more urgent note, what would your suggestion be for the smallest sized 868Mhz Antenna? Folded Dipole made of 0.1 spring steel rod?? Goal is Crossfire on a 3" quad! I know its a bit stupid but I've somehow managed to kill all my Frsky TX modules over the years...

[@FPVUniversity]

Just 80mm of copper wire somewhere. On a 3" you will not go far and Crossfire even on lowest power will cover that. LoRa modulation over there is fantastic

[@nixxda270]

Gona try! Do's it have to be Copper? I always liked using spring steel back in the 35Mhz days! (well it was actually 40Mhz where I live!)

[@FPVUniversity]

with the power output we have here, it makes no difference. Whatever suits you more

[@DJLNR.]

Nice antenna class.. Also great side lesson on how to make swearing sound beautiful...

[@alasdair4161]

Your spot on Pawel, I make many of my own antennas, and I always use brass elements where possible, but it needs to be way longer than copper to get perfect swr, the material used including alloys of copper and coatings, like nickel or silver make a substantial difference to correctly tuned length. I also use the same Vector analyser, my favourite tool.

[@GianfrancoVadurro]

Di u know if a hackrf one, may be used as a VNA? Or if there are any software to apply that make it simulate it?

[@FPVUniversity]

Not directly but I do not know the details

[@kavindugilshan]

Does the velocity factor change with length of the coax ?? ( if the coax VF is 69% , does this value change how long coax i use ?? ) Great tutorial!! 👍

[@FPVUniversity]

No, velocity factor is constant for the conductor. So VF on 1m and 5m will be the same: 69% for example

[@kavindugilshan]

@@FPVUniversity thank you for reply . 😊 Another small question . When i make an antenna with copper . It should be 95% , and then i connect to coax that has 69% . Should i calculate original wave length minus 5% and then that value again 69% ?? Or i can calculate 69% + 5% = 74% ??

[@nixxda270]

the intro alone made me push the thumbs up!-) and "first" btw..

[@freebeenergy]

by me at S8R reciever were the antenas 35mm. Now I buyed new. The new are 34mm. But the rf signal is still critical to.

[@krzysztofx9426]

Good work:) Somebody show this to Joshua Bardwell, so he stops cutting his antennas :)

[@minkorrh]

Looks at average formulaic antenna length, does percentage calculation in his head. This is why we watch and learn.

[@TheRoybert]

"So, WTF." Best commentary.

[@josephdunbar8697]

"what the hell???" "what the fuck???" LOLOLOLOLOLOLOLOLOLOLOLOL

[@josephdunbar8697]

"i've Broke My Favorite Pen!"

[@peregrineflow7354]

Bravo.

[@thegreen4me]

an r-xsr still has a much shorter active element than even 28.2mm, and from my own real world testing, stripping off a few mm of shielding more than doubled the range.

[@Com4nd3r.]

How much did you strip of?

[@youeladamas7840]

you did your homework i had to learn from ham guys

[@chrisnoren777]

😀. 👍

[@MohammadAdib]

I love when you say wtf 😂😂😂😂

[@FPVUniversity]

Thank you :)

[@callmevil]

your calculations has too much classroom assumptions built in to it... basically what you said at 11:22 is the truth of the matter with regards to an antenna’s final length. It varies depending on how the receiver is designed. Even then, there is again the tx power to be considered.. lol

[@senseisecurityschool9337]

"What you're saying is bullshit. You have no idea what you're talking about." Not true! What they're saying is correct - for step 1. They are properly calculating a quarter wavelength. They just didn't know to then multiply by the velocity factor. They did exactly right for step 1 (calculate 1/4 wavelength). It's not bullshit, it's correct. THEN step 2 is to apply the velocity factor of the medium.

[@arturskrzypczyk9250]

I hate cliffhangers ...

[@FPVUniversity]

Sorry, but almost 20 minutes was just too long... I will give a link to a second part in a week or so

[@FPVUniversity]

kzbin.info/www/bejne/a5OypJt9gcxrrck

[@zoran9a3hpdiy]

300/frequency in Mhz= give you lenght in meters Simple math.

[@JasonRennieWTF]

I think you might be talking nonsense. For copper, the radio wave travels along the surface, rather than through the copper, due to the skin effect. Unlike with coaxial cable, where it does go through the copper.

[@FPVUniversity]

lowpowerlab.com/guide/rf-best-practices/velocity-factor/#:~:text=The%20term%20we%20use%20to%20indicate%20how%20much,is%20around%200.95%20%2895%25%20the%20speed%20of%20light%29.

[@JasonRennieWTF]

@@FPVUniversity I think that's not true. Perhaps the enamel is acting as a dielectric and reduces the velocity factor. Plastic coatings definitely do this, but a copper wire in free space has a velocity factor of almost 1 and in air its about 0.9999. Or perhaps I'm mistaken

[@FPVUniversity]

I think the best way for you to accept that is to get a VNA and measure by yourself how long antenna should be. Especially the one with any insulation on

[@guidelineuk]

😂 subscribed

[@macelius]

So..WTF?..WTF? ..the fuck lol

[@jackevans2386]

10:09 29.07 and NOT 29.7 You make fuck mistake ! Your calculator read 29.07

[@HaveFaithInGod]

I think you may know something but sorry I just can't get past your vulgar language. You actually limit your following. Think about it.. who will leave or not subscribe because you are NOT a crude talker? All of us want to learn and get answers but not all of us will tolerate bad language. Maybe it's a cultural thing and although you have learned English, maybe you learned it from the wrong people. I am a minority I'm sure but we exercise our freedom of choice. You chose one way I have chosen another so I don't have to put on ear buds when my kids are watching KZbin with me.