If you accidentally damage the antenna on your 2.4GHz radio receiver (RX) in a crash, or by spinning propellers, don’t worry! You can easily fix it without buying a whole new receiver or replacement antenna. However, there are also considerations about the length of the antenna.
Further Reading: Learn about the basics of radio transmitter and receivers.
Understand The Construction of an RX Antenna
A typical monopole antenna on a receiver 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 antenna.
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 1/4 of the signal wavelength, where the inductance and capacitance in the antenna are equal and can 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 if the antenna is too long 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 antennas 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.
Does the Length of the Shielded Wire Matter?
One interesting theory suggests that the length of the shielded part of the antenna might also matter, which can affect the performance of the 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 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.
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)
- 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