If you accidentally damage your 2.4GHz receiver antenna in a crash or due to spinning propellers, don’t worry! You can easily fix it without the need for a new receiver or replacement antenna. Moreover, it’s important to understand the considerations regarding the antenna’s length.
Further Reading: Learn about the basics of radio transmitter and receivers.
Table of Contents
Understanding the Construction of Receiver Antennas
Two types of antennas are commonly used in radio receivers for FPV drones: monopole and dipole. I have an article explaining the different types of antennas used in FPV: https://oscarliang.com/best-fpv-antenna/#types.
Here is a monopole antenna.
And this is a dipole antenna (also referred to as T Antennas).
This article will focus on repairing monopole antennas.
A typical monopole antenna on an receiver is essentially made from a coaxial cable. From the outside to the inside, the first layer is a plastic skin for physical protection. The next layer is grounded metal shielding that prevents noise from getting in and the signal from getting out.
Removing the shielding reveals a thin insulated wire, which is the active element of your receiver antenna. The length of this active element determines the resonance frequency of the antenna.
Calculating Antenna Length
The ideal length for the active element of a 2.4GHz antenna is 31.23mm. (To find this, you can search in Google: “c / 2.4GHz / 4”).
The length of the active element of a monopole antenna is determined by the wavelength of the radio wave. The resonant frequency occurs at a quarter of the signal wavelength, where the inductance and capacitance in the antenna are equal and cancel each other out. At this point, the signal of this frequency is received most strongly.
A shorter antenna will have more capacitance, while a longer antenna will have more inductance. The added capacitive or inductive reactance will result in degraded antenna performance.
Cutting Antenna and Removing Shielding
The process for making or repairing a 2.4GHz receiver antenna is similar to what was described in our DIY 5.8Ghz monopole antenna tutorial.
- Take a clean cut at the end of a broken antenna.
- Measure 31.23mm from the end, and carefully cut and remove the surface plastic layer and the shielding, leaving only the center wire exposed.
- The total length of the antenna wire is not critical, as long as there is only 31.23mm of the wire without shielding exposed. It’s acceptable for the exposed wire to be slightly longer or shorter; your antenna will still function, but the closer to 31.23mm, the better the range.
The Length of the Shielded Wire Matters!
During some tests with my LiteVNA, I discovered that the length of the shielded wires plays a significant role in achieving the optimal SWR value around 2.4GHz.
A brand new 2.4GHz antenna comes with an active element 31.2mm long and a shielded part approximately 119mm long. Its resonance frequency (where SWR is lowest) is at 2.4GHz, which is exactly what you’d expect.
However, as soon as I began to shorten the shielded part while maintaining the element’s length at 31.2mm, the resonance started to shift higher. Eventually, when the shielded wire was reduced to about 59.5mm (half of the original shielded wire length), the resonance frequency shifted back to 2.4GHz.
The takeaway here is to avoid modifying the original 2.4GHz antennas; use them as they come when purchased. If you must shorten them or remove the shielding for repairs, keep the length of the shielded part around 59.5mm to maintain optimal performance.
Receivers with Longer/Shorter Antennas
The 1/4 wavelength of 2.4GHz is 31.23mm. However, some 2.4GHz receivers come with antennas of varying lengths. For example, we measured the antennas on several different FrSky RXs as follows:
- R-XSR – 23.5mm
- X4R-SB – 33.25mm
- XSR – 26mm
- XM+ – 23mm
As mentioned, the length of the antenna affects the capacitance and inductance, thereby shifting the resonant frequency. It’s also possible to adjust the capacitance and inductance by adding an inductor or capacitor at the root, theoretically allowing us to tune the antenna to any desired length. This could explain why some receivers come with longer or shorter antennas.
Therefore, my recommendation is to follow the length of the original antenna that comes with the receiver, as receivers are not always optimized for the 1/4 wavelength! Only consider experimenting with different antenna lengths if you are experiencing significantly poor range.
Heatshrink Tubes Change Antenna Frequency
Placing antennas inside heatshrink or plastic tubes for protection will “detune” the antenna by shifting its resonance frequency. This might slightly reduce your signal strength, but it’s unlikely to cause issues unless you’re flying long-range. In that case, you should try to avoid covering your antenna.
Keep Active Element Straight
Ensuring the active element (the exposed wire) remains straight is crucial. Any bending can diminish its performance.
Replacement RX Antennas
Without the equipment to test antennas, you cannot be certain if a repaired antenna still performs as expected. Therefore, I always advise purchasing new antennas to replace the damaged ones to prevent any surprises.
Note that in the latest batch of FrSky receivers, they’ve begun using a smaller version of the IPEX connector, named “IPEX 4th Generation.” Exercise extreme caution when shopping for replacement antennas for your FrSky receivers to avoid compatibility issues.
Old IPEX Antennas (3mm)
- AliExpress: https://s.click.aliexpress.com/e/_DFiJQd3
- Amazon: https://amzn.to/3J2KDmj
New IPEX 4 Antennas (2mm)
- Amazon: https://amzn.to/3U3T3Ar
- AliExpress: https://s.click.aliexpress.com/e/_DkGsl5f
Edit History
- Feb 2015 – Article created.
- Jan 2018 – Updated article with sections on “why some receivers have longer or shorter antennas than 1/4 wavelength” and “does the length of the shielded wire matter?”.
- Mar 2018 – Added a warning about the new IPEX connector for the latest FrSky receivers.
- Apr 2024 – Included findings from LiteVNA.
69 comments
Oscar – i purchased 5 TBS Tracer cables – I spoke to GetFpv to purchase these cables – they beleive these cables will snap-on to the frsky r-xsr receiver instead of soldering – if i have to solder, do you know where i can solder these cables to – front or back? Thanks – Richard Litt – i can be reached at richardlitt6@gmail.com
Not quite sure what you mean by TBS Tracer cables, could you clarify? Do you mean antennas? Or the harness that carry the power, ground and signals?
If you mean antennas, I am not sure as I have never owned any Tracer receivers. But if the GetFPV support vouch for it then maybe it’s worth a shot.
Thank you for this article. I’m trying to increase the range on a close x450 and interested if I could increase it by replacing rx and tx antenna with something more like the “tbs immortal T” to get more range out of than the factory config. Thanks for all of your articles they have been very helpful over the years.
Hi,
Highly documented and help me for my Saab car antenna issues
Thano you very much for this invaluable information.
Hi,
I have moved the article to:
rcgroups.com/forums/showthread.php?4131525-RC-Receiver-Antennas-How-They-Work-replace#post49197671
It may answer some of the questions with regard to “Does the length of the coax cable transmission line matter.
With regards to:
Hi, I wrote the following article on RC receiver antennas. You may find it interesting? It uses Smith Charts to describe the RC antenna
Hi, I wrote the following article on RC receiver antennas. You may find it interesting? It uses Smith Charts to describe the RC antenna.
https://www.dropbox.com/s/dzazqoe1x902a49/RC%20Antenna%20Article%20How%20do%20they%20work.pdf?dl=0
Well, an old page but I just did this to fix my banged up Moblite7 antenna and it worked a treat! (At least in benchtop tests, will have to wait for the wind to die down to test at range.) Thanks for this and all your other great pages.
Regarding the “exact length” debate I figure between the curve in the wire and the blurry hand-cut edge of the sheath the idea of getting the length accurate to 2dp is pretty fanciful anyway.
Cheers and keep up the good work.
Hello Oscar, if the exposed antenna is longer than 31.23mm this to will diminish reception? I was told yes by someone else buy wanted confirmation. Tyia!
Hi can you joint a cut of wire ?
If it’s just a piece of wire, yes. But if it’s a shielded wire, then no.
To test if a sleeve is suitable for placement over an antenna element, microwave it (with a separate cup of water) and see if the sleeve material remains cool.
g’day Oscar
Just wondering about Rx aerial lengths / quantity
I have an multiplex RX-5 with a single aerial (that is on a futaba 60cm extension) which works great!
Now I want to swap to an FrSky RX6R (because I want to use opentx based transmitter)
Can I just connect, the extension onto one of the aerial terminals and leave the other blank? As the extension is the only way to get the aerial outside of the carbon hull?
Thanks again for your infosite!
I have a mini remote controller and there is a cable antenna soldered (only main wire and insulator) on it and no MMCX port; is it possible to connect a MMCX antenna directly on the card without port ? How ? Can I solder only the main wire of MMCX antenna to my card and let the ground without connexion ? Thanks if you can help because I really did not find any informations about it.
Always used to put my antennas in double layer of heatshrink, after with ziptie third layer.
And now I have read that it can reduce range :(((
How to live
Oscar I generally love your site, but there is a lot of bad info in this page. 31.23mm is simply the wrong length for an RX antenna! You cannot cut or extend your RX antenna without an antenna meter – that is the only way to know what a given antenna is tuned at! The 31.25mm antenna length is a MYTH because it makes several incorrect assumptions. The first is that your radio does NOT use 2.4 GHz, rather, it’s a range of frequencies between 2.4 and 2.5 GHz, so most are actually tuned to 2.45GHz, which is a different length. The second is that that 31.25mm antenna length is the 1/4 wavelength for the frequency IN A VACUUM AT THE SPEED OF LIGHT. The wave slows down quite a lot in a wire, and even more in coax, and insulation slows it down too, as does capacitance. This is a very complex topic with a lot of variables which is why if you actually measure antennas for receivers you’ll find they’re all different lengths as y ou noted! This is because of differencex in antenna material! A good intro to this topic: youtube.com/watch?v=8iBoRNyrrPM
it’s a complicated subject, exactly.
most people in this hobby come from non-academic background and it’s unlikely they would have a way to measure antennas.
as explained in this post, there are many factors that affects the length of the antenna.
31.2mm is just a general figure, or safe figure to try for 2.4GHz, if one doesn’t have any idea what antenna length it should be.
Consider shortening the antenna by ~ the factor 0,94 and using the center frequency 2450MHz.
Yes it is a complex topic, but using 0,94 times is better. Without doing it, you will be off by around 150MHz.
I am trying to bring my 1/32 scale SUB back to life from drydock of 20 years on the shelf. My old radio (75 MHz FM) no longer works, so I bought a new 2.4 GHz radio; not good for SUB,s! What I’m hoping to do is run the antenna up the periscope and keep the top of the scope from going under water. My problem is that the antenna wire is too short. Can I lengthen it some how ?
Hi!
As prop cut my FS-A8S antenna, I used replacement antennas but made a range test.
First I cut the replacement antenna element to about 26mm the original FS-A8S length, my failsafe triggered at 200meters range.
Then I put replacement 2.4Ghz antenna pigtail on it without cutting, about 33mm length (not measured perfectly) the failsafe triggered at 310meters range.
So my test resulted that we should use the full 2.4ghz sized antennas for better range, and not to cut them back.
At least with my FS-A8S this worked like this….
I also repeated the test again and got the same result.
Hi, I have just swapped over from RC aircraft to yachts using my FrySky 2.4 rx that has 2 aerials. Do I have to mount the rx in a plastic hull with the aerials protruding parallel up & out of the hull which will mean a water ingress problem. Thanks.
Fresh first experience with R-XSR an hour ago. I flew at the same place as my previous XSR and at a bit longer distance than XSR (about 300-330m) and got a failsafe. Then I flew again now just 20m in front of me and got another failsafe!? Do you ever heard of R-XSR failsafe problem or range problem? I updated it to the latest FCC171009, SBUS, S.port if I don’t missremember the version.
Check antennas are connected firmly? And they are not damaged?
Hi Oscar,
I understand that the antenna should not be run along the carbon frame. What would be the best way to run it (up down, to each side)?
Thanks
Dale
The best position would be sticking out above the top plate like a letter “V”.
I often mount them on the arms too if I don’t have space, like this.
For short range flying that’s totally fine.
Hi Oscar,
Love reading your pages.
Here is wee scrounging tip that might help someone.
If you happen to have lost an antenna with an ipex 3mm connector like I noticed mine had fallen off the satellite Rx on my 2″ brushless quad last night. Just now I had an idea “ta dahh moment” … ” I know where I’ve seen those connectors before”
Digs out a very old broken laptop ( you could get one at your local tip/ dump) and inside as remembered are not one but two nice ipex connectors with a good 10+ cm of coax going to work antenna under screen. Just ship farthest away from the WiFi module and you’ve got two new antennas to construct as per your instructions.
Whilst I was at it I got 6 x18650 batteries that all hold charge well and will make a simple fpv goggle pack .
Greeting from up north ( nr Oban)
good tip for emergency replacement :D but still easier to get a bunch of spare lol :D
Can anyone tell me both covered and uncovers length of Fly sky R6B antenna length? I need length kept my manufacturer. So it will be tuned properly. please tell me if anyone having one
Mine came off at the solder joint. Any idea how i could fix that?
hi, i own a small drone with a 2.4 ghz wifi camera. about midway through the flight the transmission suddenly stops, as if the signal weren´t strong enough. i opened the drone and found that the antenna wasn´t a true 2.4 antenna (it was longer but did not reach the 1/2 wavelength). could the antenna make the transmission range shorter when the battery is lower than normal because it is not a true 2.4 antenna?
The length of the antenna can be tuned by adjusting the reactance in the circuit, so don’t assume every 2.4Ghz receiver has to use antennas of the same length. Just stick with what you are given by the manufacturer you should be fine.
By “exposed”, I’m assuming that you can cover the 31.23 mm in plastic antenna shielding, cocktail stirrer, etc.? it doesn’t have to be open to air does it?
Yes, your can cover it with anything that is not RF shielding. Plastic should be fine.
Your calculation is based on a wavelength in free space and does not take into consideration the velocity factor for the coax you are using for the whip antenna. Therefor the length of the active part of the antenna will be between 69% to 95% of this length.
I understand the antenna exposed at the desired length part etc but at the solder point of the antenna and the shielding to ground on the fc/pcb board, that part is also exposed at around 0.5mm? . Shouldn’t that part be shielded somehow?
Hi oscarliang the best.
Except error on my part.
About the calcul speed of light. Google rounded the value to 300 000 000 instead of 299 792 458 real value “wiki”
And c/2.4ghz/4=299 792 458/2.4ghz/4 = 11.29
With 300 000 000/2.4ghz/4 = 33.25
This is practically double. What do you think
Thanks
Your website is great
Neogen974
speed of flight slows down a bit in air.
But I think that’s close enough anyway :)
Esta conta está super errada.
R= 31.22 mm
Hi, I always use FrSky receivers but I was flying a FlySky that comes with a different antenna, the one that seems to have a bullet shape and I broke one, can I attach any other antenna from my FrSky receivers? can I even use an antenna from a X8R receiver? Thank You
Yea the antenna should be interchangeable since they all operate at 2.4Ghz. just make sure the lengths of the active antenna are the same.
What about those of us that have the boda antenna ? Can you cut those out all together or could you re solder anything to them?
I know I am late to the party, but BODA can easy be fixed as long as you didn’t lose the metal piece under the heat shrink. Cut the antenna as you would with a straight wire and then solder the shielding to metal sleeve and expose the center wire out of the top. If you lost the sleeve, no worries because the antenna will still work, but the concept of the sleeve is to reduce interference. That being said, you can make a BODA antenna on any receiver if you have the proper length sleeve.
Hi,
I see that the XM+ antennas do not come at the length you calculate. They come at 21 mm. Do you have any opinion about this?
Thanks
yea on some of the Frsky RX such as the XSR, antennas come at different length because they are using 1/5 wavelength..
Quite a lot of people complain to have range issue with that and they strip the shielding back to make it 1/4 wave… i.e. 31mm.
If you have this issue you could try this.
Hi Oscar,
I am currently in the process of waterproofing my multirotor.. it is hard to do that with my Stock Futaba Rxs coaxial antennae. My thought is to use a SMA to U.FL wall panel adapter mount and using a more traditional plastic 2.4 antenna instead. Is there any reason I can’t do this or anything i need to take into consideration?
Thanks for your time.
Hi dear,
Last week i had rc airplane crash.
Till know i cut around 6cm of the reciever antenna because of crash.
The people there said that i crashed because the lenth of the main wire anttena is not long enough. But i should mention that always i had 3 cm unshield anttena our of the rc airplan rather than the shieldet part.
The question is that the lenght of the antenna wire is important or just it is important that there be 3cm unshield of antenna wire?.
Thanks.
yes only the length of the unshield part of the antenna matters, but I guess if the antenna is longer, the less probable signal gets blocked by aircraft material.
Hi Oscar,
If I’m correct, the shielding on an antenna needs to be connectet to a ground? If so:
I just received an Devo-RX601 receiver for my devo 7 controller to use in a Quad, and noticed that the antennas are really short. I need them to be longer. I have several antennas that is made for Rc 2.4GHz receivers, so I know that they are correct for the purpose. However, the antennas that followed the receiver are so short, they are not shielded (or coaxed?). Therefore the receiver has no ground solder point for the shielding. Would it work if I connected it to another ground that is somewhat connected to my battery? Let’s say the ground connecting to the receiver?
Thanks on beforehand!
Hi, yes the shielding needs to be connected to ground.
Yes ground should be common on the receiver PCB, so any ground should do.
Thanks!
Only tested it indoors so far, but seemed to work great! This site is awesome ;)
Thumbs up!
This site helped me a lot, thanks!
Hi Oscar
I have a walkera runner 250 and a 250 advanced with gps. Not had much luck with the gps version so far but the earlier model has been flying fine even after a few crashes and rebuilds. But I noticed on my earlier model both ariel wires wasn’t stripped down at all. They had the full outer plastic casing all the way to the top but I was still flying using both my devo 7 and devo 12E well over half a mile with no issues. The other day I completely cut one of the wires off after a crash all the way down to the receiver unit so there was only 1 wire left. It still flew out over 1/4 of a mile before ran out of batteries. Anyway I have bought some replacement 2.7 wire to repair it and am I correct that I can completely remove the wire from the circuit board and just re solder my new wire to the larger of the 2 connections on the circuit board as this wire doesn’t have the outer shielding so will only have 1 connection to the board instead of 2.
maybe the top end of the plastic casing has no shielding?
So I just get crashed and my exposed antenna wire is gone. So if I make that 31.23 mm shield cut on that broken antenna, its gonna make the total length of wire shorter on one side. Is that okay? Thank you.
that’s absolutely fine
Hi Oscar! I am new to the site and have an antenna question. Some back ground I am building a sailing craft and wish to keep the xr8 2.4 receiver inside the vessel’s hull for waterproofing. I want to put a 2 meter extension on the antenna so it is well out of the water. Can you explain how this could be done. Also what are the implications of occasional spray contacting the exposed antenna?
Hi Oscar … thank you for your blog.
Can I use an old 2.4 TX whip antenna (like from a Taranis Radio) without the surrounding plastic casing off course :) as an RX antenna?
I have one of those and I’m asking because from the tip of the weird bullet dome to the tip of the actual antenna core it’s measuring 2.55cm not 3.123cm like you said.
Tell you what though Oscar, i have a thought….. (Dangerous :) )
When the cable gets too short to facilitate 32mm whats to stop me unsoldering the coax completely and just adding a normal piece of single core antenna cable to the center core joint of the pcb and just trimming that back to 32mm, eliminating the outer core completely.
Would this work?
Yes that should work :)
Test it thoroughly before using it :)
Hi Oscar, great page, thanks.
I never knew the active part of the antenna was just the “open” side, I now have my walkera runner 250 back in the skys.
Top man, many thanks.
Thanks for all your posts and pages! Very helpful overall… So I cut my antenna the other day in a crash. If I understand correctly you can remove the plastic wrap as long as you leave the same amount exposed? My question is can I solder on a wire and then expose the very end of it to match the preferred length? I had a longer antenna and would like to replace it to the same length if possible. Thanks!
i think so Jake that should work, after you done that just do a range test and make sure that’s working reliably i guess
It has to be a shielded coax antenna wire with the shield contacting the receiver’s ground. Standard wire does not have a shield to ground.
So, basically the “working” part of the antenna is only the exposed wire ? Does it mean that I can hide the first part of the antenna under the frame and really just care to properly place those 31 mm of exposed wire ?
That’s correct! yes you can!
Hi,
You have so many cool info. I’m a newbie
in the RC community. My question is,
can I tin the expose wire? What will happen if its tinned?
Thank you
sorry why do you want to tin it? I don’t think it would do anything really…
Hi Oscar
You have one small mistake at last sentence. Longer antenna means lower resonant frequency ;)
In reality that´s not so critical, antenna will be OK within 1 to 2 mm tolerance.
you are absolutely right. updated now :)
thanks for pointing that out.