Fixing 2.4GHz Radio Receiver Antenna

If you accidentally damage your 2.4GHz receiver antenna in a crash or by spinning propellers, don’t worry! You can easily fix it without buying a whole new receiver or replacement antenna. And there are considerations about the length of the antenna you should know about.

Further Reading: Learn about the basics of radio transmitter and receivers.

Understand The Construction of Receiver Antenna

A typical monopole antenna on an RX is simply made from a coaxial cable (coax).

From the outside to the inside, the first layer in a coaxial cable is plastic skin for physical protection. The next layer is grounded metal shielding that prevents noise getting in and signal getting out.

By removing the shielding reveals a thin insulated wire, which is the active element of your receiver antenna.

Construction of a coax cable

The construction of a coaxial cable

Calculate Antenna Length – 31.23mm

The length of the active element of a monopole antenna is determined by the wavelength of the radio wave. The resonant frequency happens to be 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 received signal of this frequency is the strongest.

With a shorter antenna we will have more capacitance. Or a longer antenna we will have more inductance. The additional capacitive or inductive reactance will result in antenna performance degrade.

For 2.4GHz, the quarter wavelength antenna is about 31.23mm long. (Enter this in google “c / 2.4ghz / 4“)

Cutting Antenna and Stripping Shielding

The process of making or repairing 2.4Ghz receiver antenna is similar to what I 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. only leaving the centre wire exposed
  • It doesn’t matter how long the whole antenna wire is, as long as there is only 31.23mm wire without shielding. It’s okay for the exposed wire to be a tiny bit longer or shorter, your antenna will still work, but the more accurate it is the better range

Why Some Receivers Come with Longer or Shorter Antennas? – Antenna Tuning!

The 1/4 wavelength of 2.4GHz is 31.23mm, so why do some receivers have different antenna length? For example we measured the antennas on a few different Frsky RX’s to be:

  • R-XSR – 23.5mm
  • X4R-SB – 33.25mm
  • XSR – 26mm
  • XM+ – 23mm

It’s suspected that Frsky is tuning the antennas in their receivers.

As mentioned, antenna that is too long or too short will change the capacitance and inductance in the antenna and shifts the resonant frequency. But in fact, we can adjust the capacitance and inductance by adding inductor or capacitor at the root, so theoretically we can tune the antenna to any length we want!

This might be why some receivers come with longer or shorter antennas than 31mm.

Therefore, my recommendation is, follow the length of the original antenna that comes with the receiver, because receivers are not always optimized for the 1/4 wavelength! Only experiment with different antenna length if you are getting really bad range.

Heatshrink / Tubes Changes Antenna Frequency

Putting antennas in heatshrink or plastic tube for protection will “de-tune” its frequency. It might reduce your signal strength, but might not be enough to cause noticeable issues, just a thing to be aware of.

Does the Length of the Shielded Wire Matter?

One interesting theory suggests that the length of the shielded part of the antenna might also affect the performance of the receiver antenna. I am not familiar with the concept but I am told that by keeping the shielded part of the antenna to a multiple quarter wavelength, you should get better antenna performance.

When fixing your RX antenna, you normally have to shorten the shielded wire to an odd length which can mess with the 1/4 wave antenna tuning and introduce unexpected problems. It’s interesting to note that the XSR receiver has 3/4 wavelength shielded wire, but it’s not the case on other Frsky receivers. Maybe it has something to do with antenna tuning we just mentioned?

However, I have not verified this theory. The length of the shielded antenna doesn’t seem to affect range noticeably from my experience. On one of my quads I even had the shielded wire shortened down to less than 2cm, shorter than the active element and it still seems to work pretty well within 800m (not getting signal warnings in LOS). Perhaps I wasn’t pushing the range hard enough to notice the difference on a mini quad.

If you want to play safe, I think it’s best to follow the length of the original antenna by modifying a brand new replacement antenna.

Replacement RX Antennas

I always try to fix any broken RX antennas first, but if I started to get signal issues such as failsafe or little range, I just simply replace that antenna and save the headaches.

Note that in the latest batch of Frsky receivers they’ve started using a smaller version of the IPEX connector, called “IPEX 4th Generation”. Be extremely careful when shopping for replacement antennas for your Frsky receivers and don’t get the wrong one.

Old IPEX Antennas (3mm)

New IPEX 4th Antennas (2mm)

Edit History

  • Feb 2015 – Article created
  • Jan 2018 – Updated article with sections talking about “why some receivers have longer or shorter antennas than 1/4 wavelength”, and “does the length of the shielded wire matter?”.
  • Mar 2018 – Added warning about the new IPEX connector for the latest Frsky receivers

48 thoughts on “Fixing 2.4GHz Radio Receiver Antenna

  1. Oran

    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.

    Reply
  2. Dale Stephenson

    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

    Reply
    1. Oscar Post author

      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.

      Reply
  3. Toby Burnett

    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)

    Reply
  4. Shivaprasad

    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

    Reply
  5. celia cavasotto

    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?

    Reply
    1. Oscar Post author

      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.

      Reply
  6. Mark

    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?

    Reply
  7. Steve

    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.

    Reply
  8. StogFly

    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?

    Reply
  9. Neogen974

    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

    Reply
  10. Joyvel

    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

    Reply
    1. Oscar Post author

      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.

      Reply
  11. Mike

    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?

    Reply
    1. IflyHigh

      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.

      Reply
  12. Black

    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

    Reply
    1. Oscar Post author

      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.

      Reply
  13. Ben

    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.

    Reply
  14. Hamidreza

    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.

    Reply
    1. Oscar Post author

      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.

      Reply
  15. KWAD

    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!

    Reply
    1. Oscar Post author

      Hi, yes the shielding needs to be connected to ground.
      Yes ground should be common on the receiver PCB, so any ground should do.

      Reply
  16. Mark Ramsdale

    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.

    Reply
  17. Fivedragon

    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.

    Reply
  18. Stan

    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?

    Reply
  19. Rafal

    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.

    Reply
  20. Revomikey

    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?

    Reply
  21. RevoMikey

    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.

    Reply
  22. Jake

    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!

    Reply
    1. Oscar Post author

      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

      Reply
    2. Travis

      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.

      Reply
  23. Pakal

    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 ?

    Reply
  24. Charles

    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

    Reply
  25. Jiri

    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.

    Reply

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