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.

The alignment of your TX and RX antennas affects signal strength and range a lot. And it’s important to think about the orientation of your antennas in order to achieve the best possible range.


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 anywhere and however you like. But it’s easy to break and probably not the most beginner-friendly antenna to use.

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 parallel to each other.

When your quad turns 90 degree, it goes into case #2, where one end of the RX antenna points at the TX, and your range will be reduced. This is why it’s recommended to mount your RX antenna vertically. Not that easy on a mini quad, but more doable on a wing or plane. Many people just mount the RX antenna horizontally, it’s not that bad since we fly short/mid range most of the time anyway on quads.

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

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

How to Mount Crossfire/R9M Receiver Antenna?

For “loose” dipole antennas, I use two pieces of zip tie wrapped around the arm, and secured by heatshrink tubes.

With T-Immortal style antennas, you can strap it to the arms with two zip ties, 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

Here is a bad example, you see both ends of the antennas are touching the carbon fibre. BAD!

Try to keep both ends of the antenna as far away from carbon fibre as possible, since that’s where the active element is.

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

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

  1. Dennis

    Does it matter if the antennas are rotated relative to one another while still being parallel (i.e. parallel, but not in the same plane)? Thx!

    Reply
    1. Oscar Post author

      Yes, you get some signal reduction when turning one antenna away from the other even when they are parallel.

      Reply
  2. Kenn Elliott

    What about the R9 with two flat antenna on the ends of two wires? What’s their best orientation Oscar?

    Kenn

    Reply
  3. Andreas

    Hey,
    how do I recongize if I have a 915 MHz or a 868 MHz T Antenna? They look similar and I don’t know how to discern.
    Thank you!

    Reply
    1. Oscar Post author

      From Frsky, they should be have a color heatshrink indicating the frequency. Not sure how TBS differentiate it as I no longer use their system.
      But anyway when you buy any receiver or antenna, there should be an option where you can select which frequency you want.

      Reply
  4. Jens J

    Have you had a prop-strike on that contraption? The reason I’m wondering is that it’s easy to underestimate just how destructive they can be: just the other day I had a worn out prop nut which came lose (my bad for not checking it) at takeoff – the throttle was at maybe 20% – this made the prop detach from the motor shaft, cleanly cut through both antenna tubing and antenna and take off a chunk of the XT60-connector before launching into space, never to be seen again. I’ve had prop strikes damage pretty much anything but the actual carbon fiber on other occasions. So it seems to me, the only real protection from strikes is a tube of carbon fiber or steel, neither being a very good idea… Me, I just make sure to mount things where there is as slim chance as possible of a prop ever getting to them.

    Reply
  5. 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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