Long range radio systems such as TBS Crossfire and Frsky R9M typically use dipole antennas on the receiver and transmitter. The optimal antenna positioning can be different from traditional 2.4GHz systems which normally have diversity monopole antennas on the receivers.
The alignment of your TX and RX antennas plays a big part in signal strength and range. And it’s important to think about the possible orientation of the antennas during flight in order to maximize range and reliability.
Crossfire Dipole Antenna Explained
There are two ends in a dipole antenna used in Crossfire – one end is the antenna element and the other is ground. Both ends are equally important!
Crossfire dipole antennas commonly come in two forms, one is the Immortal-T antenna – a T-shape antenna in well protected rubber material. It’s heavier but much more durable and easier to mount.
The other antenna form exists in loose wires. It’s cheaper, lighter and flexible. But it’s vulnerable to “prop strike”.
Important!!! Crossfire uses different frequencies depending on the region: 915MHz for the US and 868MHz for Europe. In order to maximize your range, you should get the correct antenna that is tuned to that frequency you intend to use.
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 plays a huge role in your range.
Case #1 shows the optimal antenna alignment for maximum range – both of antennas should be parallel to each other.
When your quad rotates 90 degree, it goes into case #2, where one end of the RX antenna is at 90 degrees to the TX antenna, and your range will be reduced. To avoid this, you could mount your RX antenna vertically (as well as your TX antenna). This is not easy to do on a mini quad due to space, more doable on a wing or plane. But since we don’t often fly long range on mini quad, many just mount the RX antenna horizontally (as well as TX antenna) for convenience.
The worst case scenario is case #3, where the tips of the antennas are pointing at each other. This can be easily avoided by making sure you are always facing your aircraft (if you are mounting the antennas horizontally), or just have both of your antenna placed vertically.
Further Reading: Make sure to monitor RSSI/LQ while flying long range!
How to Mount Crossfire 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.
Or be a little bit more elegant and use a 3D printed mount
Here is a bad example, as both ends of the antennas are touching the carbon fibre.
It’s best to keep the 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.