Half Wavelength Antenna Mod on CrazyBee F4 (Sailfly/RedDevil/Trashcan)

by Oscar

So many people are doing this “half wavelength antenna mod”, which they claim can improve the range of the Crazybee F4 Pro FC, simply by replacing the original antenna with a longer wire. But does it actually work? I am here to find out.

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Disclaimer: the approach shown here is what has been going around the community, and people claim it might improve range. The point of this post is to confirm whether this approach work or not. 

What’s Wrong with CrazyBee’s Range?

The Crazybee F4 Pro boards are very popular and affordable as the flight controller for whoops and toothpicks. It’s used in many BNF models such as the Happymodel Sailfly, Eachine RedDevil and Eachine Trashcan.

If you have one of these models, you should know the range is not great.

The Crazybee F4 Pro has an integrated radio receiver with a very simple antenna – essentially just a piece of short wire. Because of its close proximity to the flight controller, it’s normally buried under the canopy and other electronics such as the VTX and FPV camera.

For good range, we should avoid getting the antenna blocked by carbon fibre or PCB. Range can be improved here if we can extend the antenna. So this is why people have been experimenting with the idea of using “half wavelength” antennas.

What’s “Half Wavelength” Antenna?

The stock antenna is what we call a “quarter wavelength” antenna – the length of the antenna is about 1/4 of the 2.4GHz signal wavelength. This is about 31.23mm.

I measured that piece of wire on the Crazybee F4 board, and it’s around 31.5mm, which is close enough. (the tiny difference shouldn’t affect performance much)

“Half wavelength” antenna is basically double of the quarter wavelength – at around 62.46mm. What we are going to do here is to replace the original shorter wire with the longer wire.

The original antenna is a 26AWG silicon wire, so I will be using the same 26AWG wire too for the longer antenna.

With the longer antenna, you can pop it out of the canopy so it doesn’t get buried under, I think that’s the whole idea behind it. How is the range now? Let’s find out.

Half Wavelength Antenna = Massive Fail?

Testing both antennas up close (within 5 to 10 meters), I really couldn’t tell much of a difference. But as I go further out, RSSI began to drop much faster with the “half-wavelength” antenna.

For a more direct comparison, I was able to get RSSI using “range mode” on the Taranis. Here are the measurements taken from the same distance between the TX and RX for both antennas. Conditions are the same in the two tests.

The Half Wavelength antenna shows a lower RSSI than that of the Quarter Wavelength.

So yea, you can make the antenna longer and pop it out of the canopy with the “half wavelength” antenna mod, but at the same time it reduces your maximum range…

To me it seems like a waste of time as the original quarter-wavelength antenna actually performs better given optimal positioning.

Other Ways to Improve Crazybee’s Range

You can bind the Crazybee (Frsky SPI receiver) to your Taranis in either Frsky_X (D16) or Frsky_D (D8) modes. Frsky_X mode gives you Telemetry capability, and Frsky_D doesn’t. However binding the Crazybee F4 in Frsky_X mode has been known to cause “FC lock-up”,  so the general consensus is to bind in Frsky_D mode whenever possible.

It doesn’t actually have much to do with range, mostly just reliability. Here is how to bind the receiver in Frsky_D if you haven’t already:

  • In Betaflight Configurator, go to “Configuration”, under “Receiver”, change “SPI BUS Receiver Provider” to “Frsky_D”.
  • On your Taranis’s “Model Setup”, scroll down to “Internal RF”, Change “Mode” to D8.
  • Now proceed with binding

Another way to improve range is using an external receiver with dual antennas, such as the XM+ or R-XSR. Both receivers should provide 1Km or more range assuming the antennas are positioned optimally. Still not enough? Consider the R9MM (requires R9M Module).

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iz3man 4th December 2021 - 10:31 am

I experimented a lot now by leaving some 35mm unexposed and then cutting down one mm at a time. I found that a transmitter that used a 1/4 wave antenna by default should have the same antenna after modification. 1/2 wave doesn’t work as good!

Nidge. 8th September 2020 - 7:30 pm

Concerning 1/4 wave versus 1/2 wave.
A simple 1/4 radiator will present a 50ohm impedance to the rx/tx, most solid state RF devices want to see as close to 50ohm as possible. However a 1/2 wave radiator has a completely different feedpoint impedance and some kind of matching network is needed. This is done on antennas used in communication systems by fixing a matching transformer between the rx/tx and the antenna.

Stephen 27th January 2020 - 2:50 pm

Is there an antenna-ground pad which could be used to attach a longer (and therefore better-placed) coax antenna on this board? There’s a pad next to antenna on the crazybee v3 but not sure if it’s ground, doesn’t feature in any documentation.

Oscar 2nd February 2020 - 3:48 pm

I guess you can just connect it to the closest ground on the board. But it probalby wouldn’t work as well as a dedicated ground from the receiver circuitry.

Franz P. Val 20th August 2019 - 9:25 am

There is a way to improve the poor performance of this SPI receiver. I know that 1/2 wavelength doesn’t make sense, but there isn’t a good HF design on this board… So I tried different lengths (in 1mm steps) with this setup:
CrazyBee F4 pro (mounted in normal orientation on a frame, placed in 2m above ground and walking away with a X7S in range check mode. So I noticed a huge improvement in range at ~80-81mm length.
FrSky_X (D16) mode: This mode works fine, if you disable the telemetry button in BF – FC crashes gone and you still get RSSI on the radio.

Mike 20th August 2019 - 4:26 am

Any details on that groundstation?

Oscar 29th August 2019 - 11:11 pm

I am still working on the 3D printed case design, not optimised yet. Once done i will make a post about it.

Ian Murphy 19th August 2019 - 2:16 pm

What about using a piece of co-ax cable with a quarter wavelength of shield removed? That’s what the normal receivers use and would extend the antenna into a better position.