Pawel posted an interesting video today regarding the length of the antennas supplied with the R9MM. It seems that FrSky are supplying FCC 900MHz tuned antennas with EU-LBT labelled receivers which should be operating on the 868MHz frequency under EU law.  That certainly seems to be the case with the EU-LBT labelled one that I bought direct from an RC Hobby supplier in the UK. The exposed active part on mine measures 68mm while the ground shielding part measures 73mm.

Pawel is trying to gather some stats on the length of the active and ground parts of "EU" supplied R9MM receiver antennas, so if anyone has one that is labelled EU-LBT, can they please add measurements of theirs to the comments in Pawel's video.

Interesting the lengths on yours.  68mm is way off of the 82mm the active element should be.  And if the antenna is a true dipole on the R9 system the ground plane (passive element) should be the same length as the AE.  68mm is roughly the length for 1.1ghz...I'm surprised your TX would even talk to the RX with that disparity.  Maybe it is picking up a harmonic or something, very weird. 

Seems FrSky has some quality control iissues.  But so does Crossfire.  There have been variances as much as 3-4 mm on the antennas I've gotten in my kits from TBS.  Thats why I build my own.  Maybe you FrSky guys should build your own as well.

I didn't watch the video last night as I was in bed when I read your post, Snow.  But this morning I watched it and read his narrative below the video.  He says "seems like my R9 Mini antenna has 70mm long active element and that tunes her nicely for 920MHz. But tunes very very badly for 868MHz."  

This is wrong, as a perfectly tuned 1/4 wave dipole for 920Mhz would be 78.74mm.  70mm would be a very poor antenna and he should know that.  So not sure what this guy is doing, but clearly based on what you posted Snow, that your antennas are the wrong length, and this guy's antennas are the wrong length,  FrSky is not providing a good out of the box antenna for their RX.

(30-Nov-2018, 01:54 PM)Krohsis Wrote: I didn't watch the video last night as I was in bed when I read your post, Snow.  But this morning I watched it and read his narrative below the video.  He says "seems like my R9 Mini antenna has 70mm long active element and that tunes her nicely for 920MHz. But tunes very very badly for 868MHz."  

This is wrong, as a perfectly tuned 1/4 wave dipole for 920Mhz would be 78.74mm.  70mm would be a very poor antenna and he should know that.  So not sure what this guy is doing, but clearly based on what you posted Snow, that your antennas are the wrong length, and this guy's antennas are the wrong length,  FrSky is not providing a good out of the box antenna for their RX.

OK. You'll have to forgive me because I know virtually nothing about radio frequency stuff or the makeup of antennas unless and until I spend some time researching the subject in more detail, but at 3:15 into the video he mentions something about velocity factor which seemingly has an effect on the overall calculation, and he appears to be guesstimating that value when applying it so whatever calculation he is running. So maybe that is where he came to the conclusion that 70mm works for a frequency of 920MHz. I don't know.

What is blatantly clear looking through the comments under the video is that FrSky are shipping antennas with their EU-LBT labelled receivers that fall well short of the required length, and from what you're saying, it even looks like the antennas being supplied with the FCC receiver are of the incorrect length as well.

I think I probably need educating a little in this subject. I will of course do some of my own research but I may as well start with a basic question here because you seem to know what you're talking about and I read that great tutorial you put up on building your own 915MHz and 868MHz antennas, but I didn't see any calculation in there. What formula is used to calculate the length of the active part of an antenna that needs to be exposed, and how do you calculate and apply a relative velocity factor?

Pawel also discusses the velocity factor here:


Do not just blindly trust what the calculator tells you.

I'm just using TBS stuff. Immortal T for the receiver and TBS Diamond on the Taranis. Seems to work well although I haven't done any range testing

Velocity factor is an issue....slight, 5% or less.  Also SWR is an issue......again slight on a receiving antenna, a bigger deal on a TX antenna.  His numbers are off.....any way you cut it.

Looks like I need to educate myself more on antennas

(30-Nov-2018, 02:37 PM)SnowLeopardFPV Wrote:  because you seem to know what you're talking 

I don't pretend to be an EXPERT, but I know a little about antenna theory.  One thing some guys get wrapped up in is that TX antenna theory is the same for RX, and it isn't.  I had a huge debate on the long range page with a self described expert on the issue on Standing Wave Ratio, (SWR).  What he couldn't get past was the differences in the effect of SWR in a TX circuit vs an RX circuit.  

So here is a snapshot of my run out to 6.5km, 21,336 feet.  This is with my 10 inch with a homemade RX antenna, like in my tutorial.  My LQ was running 97-99 in a interference dense environment.  So yes, I know a little bit about building antennas, a tiny bit....

(30-Nov-2018, 02:37 PM)SnowLeopardFPV Wrote:  What formula is used to calculate the length of the active part of an antenna that needs to be exposed, and how do you calculate and apply a relative velocity factor?

LOL...for me to regurgitate what is out there on the topic is silly.  Besides, knowing you, you will research outside what I post anyway.  I would suggest that after you google it, take a look at the Ham Radio forums.  Lots of knowledge there on building antennas of all types and frequencies, and done so for the beginner to understand.  Http://qrz.com is one of my favorite forums.  There is more too antenna/RF theory than Velocity Factor and SWR.  So if you really want to be well schooled on antennas, plan to do lots of reading.  Antenna building is a big hobby for hams.  I built 3 antennas about 35 years ago when I got my license.   I haven't done anything with antennas in the ham world since.  Frankly much of it is boring to me.  But go forward and learn lots....maybe you can come back and school us all.

Perhaps when you become a super star antenna builder you can help me with my 80 mile out system for my wing.....

(30-Nov-2018, 11:04 PM)Krohsis Wrote: Perhaps when you become a super star antenna builder you can help me with my 80 mile out system for my wing.....

Hmmm... LTE?

(30-Nov-2018, 11:20 PM)KonradS Wrote: Hmmm... LTE?

Hey, if it works, I'm in!! 

I talked to an old time ham friend of mine who has built more antennas than anyone I know, hundreds if not into the thousands.  He knows his stuff.  He did say that I shouldn't be so completely dismissive of velocity factor as it is a player in antenna design.  He gave me some ideas on how to improve antenna design for our quads in the 800-900 Mhz bands.  

I plan to build one to his specs in the future and test it.  I will report back after testing, and have some data for all to digest. 

He did give me a link to an antenna calculator that he considers one of the better ones online, as according to him there are many poor calculators out there.   He said if I built all my antennas with this calculator, I wouldn't be disappointed.  He feels it isn't as precise as doing the math ones self, but adequate for most builders.  The link to the calculator is below.  

When using the calculator you want the numbers for a 1/4 wave antenna.  Our quad antennas are 1/4 wave for size reasons, i.e. 1/4 wave is smaller than a 1/2 wave or a full wave.  You certainly can make a 1/2 or full wave antenna if you choose, ( I might try a 1/2 wave or 5/8 wave on a quad in the future ) and performance gets better as the antenna gets bigger.  But I doubt you want to fly around with some huge whip antenna that a trucker and his CB would be proud of.   

Obviously you will need to convert inches to mm if you prefer that unit of measurement.  I like mm as it tends to be easier to be more accurate.

Antenna length calculator

I know this has stirred up a whole load of debate/controversy but for completeness, below is Pawel's follow up to his original video.

I also did some research on the N1201SA VNA meter that Pawel is using in his video to prove his findings and it turns out that these meters are pretty accurate devices. See the second video below by a Swiss guy named Andreas Speiss which demonstrates in detail how to use the device and pitches it against some professional lab level calibrated equipment. What I can make out from stuff I've read, the calculation method can be somewhat hit & miss depending on the type of calculation used and it can take some know-how to understand what you're doing. Knowing how good these VNA devices appear to be, it seems to me a must have tool for anyone making their own antennas who cares about absolute accuracy in tuning, because each individual antenna can be custom tuned to the perfect length for a required frequency and application without having to worry about the quality of the wire of the type/thickness of any insulation or tubing it may be housed in.

I think now is probably a good time for me to go and duck behind the parapet 

Lol....you're a funny dude, Snow.  You do entertain me.....too bad you're so far away.  We could have a beer and chat.

When I start having range problems, I might have to get me one of those must have VNA devices. 

So far I'm out to 5 miles with 99 LQ at 500mW, doesn't look like I need one yet.  And the wing guys are out 12+ miles with similar LQ, in the mid 90s.  Battery life and video signal are more of an issue than RX for me with a quad.

I look forward to your first long range with your homemade antenna.  I expect you to set new records.