I like to use XM+ receivers on many drones as they seem to work well and are small/light. What is the best angle and position to mount the antennas of an XM+ receiver in order to get the best signal?
I have often seen XM+ antennas mounted at a 90º from one another on the back of a quad. Is this a good configuration or are there other configurations that have better reception?
TL;DR: From purely a range consideration the 90 degree mounting is the best because of the orientations your quad might see. You'll want to get the antennas as far away from the frame as you can so that the frame can never block at least on antenna from seeing your transmitter.
When you are considering ways to mount your antennas on a miniquad however there isn't one best way to mount them. Range, quad orientation and durability all play a part in what will work best for you. That said there are two things you should know to understand about antenna placement and why the 90 setup is recommended for range.
The XM+ antennas both have the same function. The receiver simply compares their signal strength and picks which one is best. This lets you place them in different orientations so that depending on your orientation on antenna or the other will have adequate signal.
Generally the monopole antenna has it's best signal reception at a direction that's perpendicular to the antenna itself. Conversely it suffers from pretty bad reception in the top and bottom directions. This image shows the typical radiation pattern. Imagine the wire antenna standing straight up inside this donut. The red directions have good reception and the green are where it is bad.
Now, since you have two antennas you can see how rotating the second one by 90 degrees would cover the deadspots of the first antenna.
These two orientations make for good coverage in almost any orientation. However, it's not perfect, those donuts are pretty flat. Mounting both antennas in a more upright 'V' orientation could provide a lot more even coverage, but only if you don't plan on doing something like diving straight down.
With all that said, realize that those nice donut patterns can be blocked by your frame if they are placed in such a way that the frame can come inbetween you and the antenna. You can see how this would happen in Paul's example. Not to say that's a bad way to do it. I do it that way often. For typical park rages it still gives you that 90 degree orientation and will protect your antennas from prop strikes and crashes more effectively. For extended ranges though, one antenna sticking out the top and one out the back will give you the best range.
I really like the way the Joshua Bardwell does it.
He puts one on each rear arm facing forward with a zip-tie and heat-shrink over the top. It places the antennas at 90 degrees to each other and so you get good coverage from at least one antenna.
On the picture below, the yellow line is a zip-tie and red curve is the antenna from the XM+
Remember not to put too much of a tight angle on the antenna wire as it can deform the inside of the cable and have a negative effect on the signal receiving ability.
RC Antenna mounting is always a compromise between RF signal performance and durability in crashes. If you are trying to maximize range, high vertical mounting (or ideally, mounting both high, offset 60° from each other) should perform the best, but it's always hard to mount them in such a way that surviving crashes can be a struggle. For maximum durability, running them heat shrunk to a zip tie that is attached to a top plate standoff and runs entirely contained following that zip tie inside that rectangle of standoffs provides amazing durability, but RF performance isn't as great. With certain PHX3D style canopies I can run both antennas out the top of the canopy towards the rear and get a great compromise of both, but the active elements are exposed, and I tend to break the plastic protector tubes that go into canopies.
Overall, my preferred answer is very similar to Joshua Bardwell's, but I run those off the front pair of arms, bent down (something you can do with bottom-mount batteries) in most cases, and set to where the active element begins ~4mm away from the zip tie closure (otherwise they tend to kink there).
