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.
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.
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.
The alignment of transmitter and receiver antennas can affect range.
As shown in case #1, you get maximum range when both of your antennas are in 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 to 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 two antennas are now pointing at each other with single ends. Anyway, this can be easily avoided by making sure you are facing your aircraft at all times 🙂
Mounting your RX antenna like an L shape in case #2, you will sacrifice some range. But you will get a more reliable signal regardless the attitude of your aircraft. It’s great for mini quad where we do lots of flips and rolls.
Further Reading: Make sure to monitor RSSI/LQ while flying long range!
You can zip tie them to the arms, probably the easiest and most common way to install your antenna.
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.
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.