Introduction

RFC is being rushed due to the incoming of competition. Might never be merged officially but will be used for competition airframe. Done by @Daniel Puratich and reviewed by @Nathan Green . Started on evening of 5/11/2024 to get orders in just before competition but we’ll see time wise if I can swing it.

Table of Contents

Information

RF Filters

Why

• Everything needs a filter for competition as an attempt to filter out strong out of band signals to prevent ADC overload causing RF desense to our intended signals.

• we experienced this at last competition likely due to airport radar operating above a kW

• possible causes listed below but I will be SDRing on Friday at competition to validate this assumption.

• https://en.wikipedia.org/wiki/Airport_surveillance_radar

  • 2.6 to 2.9 ghz over a kW

• https://tc.canada.ca/sites/default/files/2021-09/AIM-2021-2_COM-E.pdf

  • Air Voice (VHF AM) 118 to 137 MHz ← most likely an intermittent problem area, airport has a pretty strong TX generally but I can’t really find specs online, guessing above 200 W, when I lived near an airport this is what I heard lol

  • TSR, 1.25 to 1.35GHz ← most likely our problem area if the airport has one of these lol

  • PAR, 9GHz to 9.18GHz ← possible?

  • ASDE, 16GHz ← likely are antennas really aren’t resonant way up here

  • SSR (ADS-B), 1.03GHz and 1.09GHz ← low amplitude no worries

Filtering Required RF Devices

• ELRS drone side bidir

  • MPN: EP1 TCXO Dual

  • HappyModel ExpressLRS TCXO EP1 Dual RX

    • Type: ISM2.4GHz

    • ESP32 PICO D4, dual SX1280(SX1281)

    • Antenna connector: IPEX MHF 1/U.FL

    • RF Frequency: 2.4GHz (2400~2480MHz)

    • Telemetry power: >19dbm

      • 80mW

    • Receiver protocol: CRSF

      • over UART

    • Input voltage: +5V DC @ “+” pad

    • PCB size: 19mm x 14mm

    • Weight: 1gram exclude antenna

    • Built-in a TCXO (temperature-compensated crystal oscillator)

  • 2.4G ELRS EP1 EP2 EP1DUAL TCXO Receiver

• Gemini bidir

  • SX1261

    • https://www.mouser.com/datasheet/2/761/DS_SX1261-2_V1.1-1307803.pdf

    • lora module used? see schem

• Tracking ant bidir

  • EP1 (not dual) ELRS

• controller bidir

  • TX16 build in transciever

• Video Tower VRX

  • rmrc 900/1.3 video receiver

• Video Goggles RX

  • 5.8ghz

  • transmitter is rush fpv something

No Filtering Required RF Devices

• TX Only

  • Drone VTX:

    • No filtering required, transmit only

    • https://flywoo.net/products/flywoo-1.2ghz-1.3ghz-video-transmitter-vtx-1g3-9ch

    • 9CH (1080,1120,1160,1200,1240,1280,1320,1360,1258MHz)

      • ITU Regions

        • https://en.wikipedia.org/wiki/23-centimeter_band

        • https://transition.fcc.gov/oet/spectrum/table/fcctable.pdf

        • Canada is ITU Region 2

      • Bandwidth

        • 20MHz for analog video, centered on these CH frequencies?

      • Guide: 1.2GHz -1.3GHz FPV Video System - Oscar Liang

        • “Based on reports from the community, a 400mW VTX is capable of doing 15 miles or more.”

        • “High power VTX could [cause ADC overload on] your radio receiver, causing [system] range issues long before your video gives up. “

        • https://s.click.aliexpress.com/e/_DmMRUKl TBS notch for 900MHz band (pass 1.3)

        • https://s.click.aliexpress.com/e/_DC313lr low pass (pass 1.3)

      • https://ised-isde.canada.ca/site/spectrum-management-telecommunications/en/learn-more/key-documents/consultations/canadian-table-frequency-allocations-sf10759#s2.2

        • same as FCC table for region 2

        • 960 - 1 164 MHz

        • 1 215 - 1 240 MHz

        • 1 240 - 1 300 MHz

          • Amateur 5.282 5.331 5.332 5.335 5.335A

        • 1 300 - 1 350 MHz

        • 1 350 - 1 390 MHz

        • https://ised-isde.canada.ca/site/spectrum-management-telecommunications/sites/default/files/attachments/2022/2018_Canadian_Radio_Spectrum_Chart.PDF

    • RF power output: 0.1mw(0, pit mode), 25mw(1), 200mw(2), 800mw(3)

      • 800mW peak

    • Button control and LED indicators

    • Built-in AGC microphone

    • 7-26V DC IN (2-6S LiPo)

    • Power Consumption: 40mA(0.1mw), 110mA(25mw), 150mA(200mw), 250mA(800mw) @12V IN

      • 250mA @ 12V = 3W => 26% Eff, 2.2 W heat in the sink

    • Video format: NTSC or PAL

    • Audio Subcarrier: 6.5 MHz & 6.0MHz, onboard microphone on 6.5MHz

    • RF Connector: MMCX

    • Weight: 12.2g

    • Size: board 35*35mm

  • Video Tower VTX

    • Tx only, no worries

• Not Practical to Filter

  • Phone Telemetry bidir

    • internal to phone, should be properly designed antennas in there

    • not possible to filter easily

  • GPS receiver

    • gps denied should be anticipated tho hopefully antenna gain helps us

    • not possible to filter easily

    • vtx and usb are both scary

Adapters

https://cdn.everythingrf.com/live/sma_rpsma-male-female_636715630002666648.jpg

Open

Filter Options

• https://www.nooelec.com/store/ham-filter-bank.html

  • meant for amateur radio, cheap

• https://www.l-com.com/bandpass-filter-rf-splitter-24-ghz-ultra-high-q-4-pole-outdoor-bandpass-filter-full-band

  • more expensive

  • cheaper if not waterproofed

    • https://www.l-com.com/bandpass-filter-rf-splitter-24-ghz-ultra-high-q-4-pole-indoor-bandpass-filter-full-band

    • only 200 dubloons

• https://www.l-com.com/bandpass-filter-rf-splitter-900-mhz-ultra-high-q-4-pole-outdoor-bandpass-filter-full-band

  • expensive

  • cheaper if not waterproofed

    • https://www.l-com.com/bandpass-filter-rf-splitter-900-mhz-ultra-high-q-4-pole-indoor-bandpass-filter-full-band

• https://www.aliexpress.com/item/1005004674064653.html

  • out of shipping time for ali express lol

• https://tektelic.com/expertise/cavity-filter/

  • for 900mhz lora

• https://www.videoaerialsystems.com/collections/2-4ghz-antennas/products/notch-filter-1-3ghz-stop-2-4ghz-pass

  • 1.3ghz pass (low pass)

• Challenge

  • i think i waitted too long to ali express cheap shit for airside. ground side idk if we wanna drop a hundred on a nice cavity filter. shipping just too late to do anything besides amazon and it's hecked. like running filters is not common at all for these systems, most filters are built for way more power than we need or are rlly shitty aliexpress ones or are cost optimized and somewhat reasonable provided u bring your own pcb which we outta time on. building our own cavity filter is a legitament option but we dont have a vna i trust above 500mhz.

• Build our own?

  • https://www.eevblog.com/forum/rf-microwave/building-and-tuning-a-narrow-band-2ghz-cavity-filter/

    • Interdigital Bandpass Filter Designer

    • http://lea.hamradio.si/~s53mv/cavity/cavity.html

SDR Setup

• Two Yagi’s

  • Dipoles total length is a halfwave

  • Two support 900MHz band, one is meant for 2.4GHz

• Normal antennas

  • will just use simple antennas for analysis at FT and prep for competition

Drone Antenna Locations

• Hardware locations physically on drone

  • general

    • Viewing the flight paths from conops

    • asked people

  • competition:

Open

• above diagram shows intended flight for task 1 and task 2

  • comp info:

    • task 2 is far riskier so antennas should all go on right side of drone

  • components

  • elrs

  • vtx

    • right side to optimize for task 2 approach

    • mounted with gromets because it matches the 30x30mm mounting pattern ontop of drone!

    • mech people will likely improve accordingly, i did my best lol

    • should have null pointed toward GPS antennas to avoid interference, nearby bands

  • gps

    • spread out on top, already mounted, no change

• putting the antennas on a VNA in person will be nice to verify assumptions

Ground Antenna Locations

Separate tower for video receiver and elrs

Battery Information

• We are running four batteries (and not six)

  • this was proven preferable from flight tests

• If the four arms are corners, then one battery per side

  • this has negligible impact on flight characteristics

Wiring Information

• there's a few blanks to fill in currently in the doc but I (@Daniel Puratich ) do not have time before competition, hopefully onsite EEs are available for fun

• if someone wants to fill in, plz do!

• probably not, sadge