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The frame should not be electrically connected (aka should be at floating potential)
Complies with: UL1740
This can be checked with a DMM (when the system is not powered)
This means that if a PCB we are using has electrically connected mounting holes we need to take proper steps to achieve isolation
Sometimes PCB mounting holes are electrically connected to a GND plane for thermal reasons so we should also approach thermals with caution on sensitive electronics
Conformal Coating can be used within reason
The pro of conformal coating is
waterproofing / weatherproofing
also would help avoid the ESCs catching on fire when drone gets thirsty and goes for the pond
makes it harder to accidentally short with a screw driver
this has happened in the past 💀 at competition when everyone is rushing around
The con of conformal coating is it prevents heat from leaving a PCBA
So anything that is conformal coated should receive a thin layer at most
Another con is that it can be annoying to remove in the case of reworking a PCBA
For example it would be fine to conformal coat a flight controller that runs cool
However, for example an RF transmitter we would want to approach with more caution
We could place a large heatsink on the primary chip(s) using thermal paste
Then we could consider conformal coating the other portions of the board
Cables, especially longer cables, should be within sheaths when reasonable
This offers protection against abrasion during natural flight vibrations
Exception to this rule is shorter cables that are placed further from anything that could be abrasive, specifically cables purely within the avionic compartment of our aircraft
Avoid loops in cable runs if possible
Varying conductor types should be separated when reasonable
Generally we consider four different types of cables: Digital, Analog, Power, & Coax/Rf
Digital: Characterized by signals with fast edges
Definition of a fast edge in this context and some examples to be added by an EE
Generally transcieving in 0 and 1 states asynchronously or synchronously
I.E. a GPS module’s cable connection to our flight controller is digital
An IMU is an example of a particular digital device that is sensitive to external noise.
Analog: Characterized by signals with slow edges
Definition of a slow edge in this context and some examples to be added by an EE
Generally transcieving data in varying voltage states
Though some protocols we consider analog will also transmit some digital data as well
I.E. a hobby fpv analog video camera has an analog data output
An analog video feed out of an analog camera is an example of a particular digital device that is sensitive to external noise.
Power: Characterized by constant voltage varying current designed for power transmission
Generally the voltage is constant though higher frequency noise may be present
These conductors can have significant current pulses
I.E. a connection from a battery to an ESC
Coax/Rf: Characterized by oscillating signals across the spectrum (~20kHz to ~300Ghz) intended for wireless transmissions and notably contained within a coaxial cable
A coaxial cable, specifically for our applications utilizing an SMA or RP-SMA connector, is generally used to guide sensitive RF signals between transceivers & antennas.
It is also worth noting antenna placement is critical, this is noted below in more detail
SMA & RP-SMA connector info should be found in the corresponding connectors arch doc section.
Because coaxial cables offer strong noise immunity the routing constraints of these cables are looser than others
Coax cables are very good at eliminating outside noise
For further information see: Coaxial cable
Each of these type of conductors should be physically grouped together, however, each type should be separated
I.E. All power stuff near each other, all digital near each other, but power separated from digital
This grouping and separation is physical distance, though of course there are other factors
Within PCBAs these groups may be mixed, this is fine, we will assume the PCB designer has taken the proper care to avoid issues as necessary
RF Transceiver Care
An RF transmitter should not be turned on (given input power) without a proper antenna connected as this can permanently damage the transmitter
Possible violations of this policy should be reported in Discord and transmitters should be labelled as damage (notably degraded performance) may not be immediately evident. We don’t want to blame each other, stuff happens, we just want to note it for the future! If you do not feel comfortable stating this publicly feel free to DM a lead you’re comfortable speaking to who can relay the message without naming names.
An RF dummy load should be connected instead of an antenna if we would like to power on.
Transmitters generally get warm
They require a lot of power and therefore require some cooling
They are sometimes designed to be mounted outside an airframe for ambient cooling of wind passing the frame. As we may not be doing this we need to approach this with caution.
Notes regarding conformal coating are above.
If a transmitter has a heatsink then it should be mounted to optimize for airflow.
The lower the frequency the longer the distance we can get for the same output power generally
The higher the frequency means more bandwidth generally
Antenna Selection
More information to be added eventually, see other docs in confluence I guess
Polarization
Radiation Pattern
Forward Gain
Power Ratings
Quality Factor
Other Resources
Antenna Feed Lines (coax) mounting
The shorter the better always
thinner coax is generally more lossy than larger coax
ideally don’t run sensitive signals right next to coax and feed line connectors
Antenna mounting
GPS sensors in particular are to our 900 MHz and 1.3 GHz transceivers and should be mounted away from antennas operating at these frequency ranges
GPS frequencies are fixed frequencies are1100MHz and 1500MHz
Our transceivers will hop around frequencies around their range looking for available channels so they are capable of hopping close to GPS frequencies and causing issues
For context it’s worth noting a 900MHz and 1.3GHz transceiver are capable of stepping on each others frequencies as well
For further reference see Guide: 1.2GHz -1.3GHz FPV Video System - Oscar Liang & GNSS Frequencies and Signals
Often times we will run two GPS antennas. For reasons not explained here spacing out GPS antennas from eachother will improve accuracy significantly. Recommended ~1 meter in most cases which is not archivable in most of our systems, but keep this in mind.
Radio waves do not like to change medium
Specifically they do not like to pass through materials of varying dielectric constant
Passing waves through some varying materials may be fine, but be careful!
At the frequencies WARG operates at (6 GHz and below) foam will not have a considerable impact on signal integrity
Rain will have a measurable impact on signal integrity
This means mounting antennas outside of cases/airframes and providing LOS when reasonable
Antenna polarity should be deliberate
Antennas that use diversity should be mounted not in the same plane
There are different types of diversity
The VN-300 employs “Spatial Diversity”
The RFD9000 employs “Polarization Diversity”
Antenna diversity is not the same as true diversity
Antenna polarity matching follows the below graphic
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