Antenna Theory
A vertical trying to talk to a horizontal (90 degrees between) will experience a 30 dB loss
-each 3 dB is approximately ½ signal strength so a 30 dB loss is A LOT
Gain is measured in dBi, referenced to 0 dBi isotropic antenna (perfect sphere)
Higher gain means less 3D symmetrical radiation pattern, often better range but poor beam width and or beam height, high gain antennas often limited in radiation pattern off to sides or behind antenna.
Increase of 3 dBi is an increase in range of 1.4, so increase of 6 dBi is appr. 2x range.
Even a low gain antenna has null zones, usually directly above and below the antenna.
Diversity receivers allow a combination of 2 antennae for better reception.
dBm is decibel milliwatts
0 dBm is 1 mW, 30 dBm is appr. 1 W
each 3 dB change is appr. 2x signal strength or ½ if negative change
noise floor is often around -90 dBm
GPS signals are very weak, ex. -130 dBM
With helical antennas, greater number of turns usually means higher gain
EMI
-Electromagnetic interference. Electricity flowing through a wire will induce a magnetic field around the wire which can interfere with onboard systems. Voltage ripple can also travel through wires and disturb equipment that is connected to the same circuit.
-Ground loops are when there are multiple ground paths to the same point within the same circuit. These can cause interference. Make sure there is one common ground, often at the battery negative terminal, and that there are no ground loops.
RFI
-Radio frequency interference. Radio signals can sometimes interfere with other radio signals. If they are on the same band they might interfere and even different frequencies can interfere. The most common scenario is a 1.3 GHz video link with a 2.4 GHz radio controller. Certain channels in the 1.3 band have a harmonic frequency (approximately 1.3 x2) that lies in the 2.4 GHz band, hence there can be interference. One way to combat harmonic interference is with filters that try to diminish the emission of harmonics.
Civilian GPS band is at about 1575 MHz so beware of equipment interfering on that band.
HDMI cable
-One way to diminish interference. HDMI cable has a braided metallic shield around the cables inside. This shield is connected to ground and hence EMI generated within the HDMI cable doesn’t get out and interference from outside the cable doesn’t get in.
Separation
-Easiest and best way to reduce interference between cameras(can emit RFI noise), GPS units, VTx, and receivers. Spread components out on your aircraft. Output power from a transmitter decreases exponentially with distance from the transmitter so the farther away from your video Tx other components are, the better.
LC filters
-Help reduce electrical noise, especially going into the circuit that feeds your camera and VTx. An LC filter helps reduce rapid changes in voltage and/or current flow.
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