Best Antenna Positioning for 900MHz RC Systems – R9M/Crossfire

Long range radio systems using 900MHz including the Frsky R9M and TBS Crossfire typically use a single dipole antenna on the receiver. The optimal antenna positioning can be different from traditional 2.4GHz systems with diversity monopole antennas on the receivers.

There are different ways to place your antennas for the best result depending on factors such as the type of aircraft, your flight style and space limitation. In this article we will go through some popular ways of antenna positioning and what the pro’s and con’s are.


Further Reading:


The Types of Dipole Antenna

There are two ends in a dipole antenna used in the R9M and Crossfire systems. One end is the antenna element and the other is ground.

These dipole antennas commonly come in two forms, one is well protected in T-shape rubber tubes.

These antennas are pretty tough, and can take a few prop strikes and survive, but they are a little heavier and more expensive. They are also easy to mount in a frame.

The other form exists as loose wires like this.

It’s usually a lot cheaper and lighter, and very flexible so you can mount it like “T” shape or “L” shape. But it’s easy to break and probably not the most beginner-friendly antenna to use I found :)

We will talk about how L shape and T shape stack up in terms of performance in a moment.

Important!!! Because we use two frequencies in different regions, i.e. 915MHz for the US and 868MHz for Europe, you have to get the correct antenna that is tuned to that frequency you intend to use for optimal performance.

Stay away from carbon fibre!

Before we start, bear in mind to keep antenna as away from carbon fibre as you can. Especially avoid putting the antenna element too close to carbon fibre (the parts without shielding), which can “de-tune” the antenna and have a negative impact to your reception.

Different Antenna Positioning and Range

The alignment of transmitter and receiver antennas can affect range.

As shown in case #1, you get the full range of your RC system when both of your antennas are in horizontal T-shape, and aligned in parallel. When your quad turns 90 degree, it goes into case #3, where one end of the RX antenna points at the TX, and so your range will be reduced considerably.

The worst case scenario is case #4, where you now also turn yourself 90 degree, so the ends of the two antennas are now pointing at each other. Anyway, this can be easily avoided by making sure you are always facing your aircraft :)

Mounting your RX antenna like an L shape in case #2 is a compromise. You will sacrifice some range, but it gives you more reliable signal regardless the attitude of your aircraft. It’s great for mini quad flying in short range where we do lots of flips and rolls.

To conclude, the “best way” to mount your antenna depends on the application.

Further Reading: Make sure to monitor RSSI/LQ while flying long range!

How to Mount Crossfire/R9M Receiver Antenna?

You can zip tie them to the arms, probably the easiest and most common way to install your antenna.

Image credit: beeb

Or be a little bit more elegant and use a 3D printed mount

As demonstrated in the diagram above, mounting your dipole antenna like an L shape is a compromise to range and attitude of your quad. Here is an example.

Image credit: Eric Konasty

As a bad example, try to keep both ends of the antenna as far away from carbon fibre as possible. At least not touching like this.

Please let me know if you have any questions or comment.

3 thoughts on “Best Antenna Positioning for 900MHz RC Systems – R9M/Crossfire

  1. Jon

    I mount the crossfire immortal T antennas in the vertical (y axis) plane out of the back of the quad, I have found this gives consistently the best link quality. (I have 3 x 250 and below crsf quads using the crsf nano rx) The downside of this is that it takes more effort during the build. To stiffen up the pigtail section and reduce the likelihood of prop strikes I cut a 6mm OD PU tube along the axis then hot glue and heat shrink in place. I also 3D print a support out of something light weight like HIPs or PLA that I can hot glue three antenna into; this reduces any rotation force being put onto the ufl connector and helps keep the antenna in position – the total additional weight of the pu, 3D printed part and dabs of glue is 1 to 2grams depending on the quad, insignificant on a 4 or 5” quad

    Reply

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