Introduction

• We are worried about strong out of band signals affecting us significantly at competition as this was an issue at last year’s competition. See 2024-04-22 RF Electronics for context.

• We are addressing this for a higher priority 2.4ghz link with 2024-05-15 Digikey Order off the shelf bandpass filters, however, these are expensive and must be lightweight to mount on our drone and automated tower.

• For the 1.3GHz video system there is no size and weight requirements so a larger more effective filter seems in order. This document covers the design of such filter.

  • This is also a fun learning opportunity for meeee!

Table of Contents

Research

Filter Design

• scope cavity filter https://lea.hamradio.si/~s53mv/cavity/cavity.html

  • nice table given for designs with measurements

• https://core.ac.uk/download/pdf/286916696.pdf

• Need to use only ESMS rectangular tubing https://ems-stores.uwaterloo.ca/ems-site/aluminum-sq-rect-tubes

• Actually good textbook: https://link.springer.com/book/10.1007/978-3-642-32861-9

  • chap 7 for cvity filters

  • lots of formulae are given here but not exactly what i wanna do

• https://ieeexplore.ieee.org/document/4140435

  • very specific not very useful

• https://ia803103.us.archive.org/15/items/MicrowaveFiltersImpedanceMatchingNetworksAndCouplingStructures/Microwave Filters%2C Impedance-Matching Networks%2C and Coupling Structures.pdf

  • this one is pretty dense but section 8.13 discusses these

• calculators

  • https://www.changpuak.ch/electronics/Coaxial_Tank_VHF_Filter_Designer.php

  • https://www.changpuak.ch/electronics/interdigital_bandpass_filter_designer.php

  • https://www.changpuak.ch/electronics/combline_bandpass_filter_designer.php

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

  • digital filter, not what im going for lol

• Transition

  • https://www.microwaves101.com/encyclopedias/waveguide-to-coax-transitions

  • https://www.microwaves101.com/encyclopedias/waveguide-mathematics#guide

Our System

• Analog video bandwidth

  • https://en.wikipedia.org/wiki/NTSC#:~:text=An%20NTSC%20television%20channel%20as,lower%20bound%20of%20the%20channel. 6MHz bandwidth on these

• We are currently using the following devices:

  • Devices from 2024-04-22 RF Electronics | Filtering Required RF Devices

  • VTX: Flywoo 1.2GHz/1.3GHz Video transmitter VTX-1G3 9CH

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

    • each channel is 40 MHz wide so that’s the minimum bandwidth I need for this filter

  • VRX: https://www.readymaderc.com/products/details/rmrc-900mhz-1-3ghz-receiver-1258-with-rmrc-comtech-tuner

    • CH0:910MHZ

    • CH1:980MHZ

    • CH2:1010MHZ

    • CH3:1040MHZ

    • CH4:1080MHZ

    • CH5:1120MHZ

    • CH6:1160MHZ

    • CH7:1200MHZ

    • CH8:1240MHZ

    • CH9:1280MHZ

    • CHH:1320MHZ

    • CHC:1360MHZ

    • CHd:1258MHZ

    • CHE:1100MHZ

    • CHF:1140MHZ

    • We’re using Channel 9 on rmrc so 1280 MHz center roughly

• Band Information

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

  • https://en.wikipedia.org/wiki/Amateur_television#:~:text=2.4175%20GHz%20is%20used%20for,4%20MHz%20deviation%20are%20used.

    • 1.24 – 1.246

    • 1.252 – 1.258

    • 1.264 – 1.27

    • 1.276 – 1.282

    • 1.288 – 1.294

  • https://www.rac.ca/1240-mhz-23-cm-page/

    • there are three legal ATV channels where we are

      • 1240 – 1246

      • 1252 – 1258

      • 1276 – 1282

• Legal (Possible) Spots

  • RMRC Ch 8 “1240MHZ” should be 1240 – 1246 MHz so legal

  • RMRC Ch 9 “1280MHZ” should be 1276 – 1282 MHz so legal

  • RMRC doesn’t have a channel for 1252 – 1258 so we can’t use this one

• Passband should cover both legal spots

  • what’s in between hopefully isn’t an issue.

  • Just trying to filter out strong out of band signals with this filter.

First Pass

Parameters

• Minimum Pass Band: 1236MHz - 1286MHz

• Minimum Bandwidth = (1286-1236)/2 = 25 MHz

• Target Center Freq = 25 +1236 = 1261 = 1260 MHz

• Center Freq Wavelength = 299 792 458 / 1 260 000 000 = 0.2379 m = 23 cm

• Center Freq Quarter Wavelength = 0.05948 = 6cm = 2.4 inch

• Target Bandwidth = 40 MHz

• Target Pass Band: 1220Mhz - 1280 MHz

Ideal Dimensions

• Using the table from https://lea.hamradio.si/~s53mv/cavity/cavity.html for the initial dimensions.

  • “Fingers” are the three rods installed in the tube

  • “Probe” is the piece connected to the SMA

• Tube Size

  • Ideally one dimension is 1/4 wavelength, will do my best

  • Targeting 2.4inch for one dimension based on above calculations

• Finger Spacing

  • From table this is pretty clear I need to go with 40mm spacing

  • The distance between the “Fingers” is used to set the center frequency of the filter

• Tube Length

  • From table 180mm seems optimal with this finger spacing so 7 inch

• Finger Diameter

  • 10mm from table so 0.39 inch = 0.4 inch

  • 8mm also seems fine in table so = 0.31 inch

• Finger Length

  • this seems to matter a lot based on center frequency

  • I will be able to sand these down worst case though this is also tunable

  • 48mm on all of them seems reasonable so 1.9inch

  • 2inch is 50.8mm so it’s kind of a lot but could sand it down

Materials

• I am rushing this because machine shop closes for three day weekend!!

• Getting connectors for this via 2024-05-15 Digikey Order

  • ensured they have the dielectric on the output to keep the 50 ohms

  • went with ones with just two bolts hoping easier to secure

• Box Tubing

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

    • uses 60x40mm tubing so 2.4 x 1.5 inch exterior for similar center frequency designs

  • https://ems-stores.uwaterloo.ca/ems-site/aluminum-sq-rect-tubes

    • they really dont have anything that close

    • Alum Rect Tube 2 X 1 X .125 Wall so 2 x 1 inch

      • seems like the closest, inner diameter will be 1.8 which is actually quite far off

      • hopefully I can make this solve working off of the table provided

    • Alum Rect Tube 3 x 1 x .125 Wall so 3 x 1 inch

      • going from the inner diameter this will be closest on the 3 inch axis but the 1inch on the other one is gonna be tough to work with, will see if it solves, kinda gotta send it

    • Wall thickness

      • the .125 wall thickness is a bit thicker than the 0.01 inch used so

  • https://ems-stores.uwaterloo.ca/ems-site/steel-tubing

    • they have this in better sizes than aluminum

    • steel has less conductivity (~ 5x less) so it’s really not viable

  • https://www.homedepot.com/b/Hardware-Metal-Stock-Metal-Tubes/Aluminum/Square/N-5yc1vZ2fkos9vZ1z1czmvZ1z1czw8

    • not even close

  • Length

    • 7 inch, I can pick this so yea will just tell them

• Finger pieces

  • https://ems-stores.uwaterloo.ca/ems-site/aluminum-solid-rounds

  • diameter

    • must go with 0.25 or 0.5 inch

    • i have no idea what’s gonna be better but 1/4 inch seems like the way to go here because it’s closer

  • length

    • 1.9inch if they can do it, otherwise 2inch should be tunable with the screws and sanding down

• End Cover

  • 0.6mm sheet aluminum was used (could be thinner tho if they have?)

  • as thin as possible at EMS

  • Dimension options to cover 1x3inch tube with an extra 1/2 inch (reference used 0.4inch on each end so 0.8inch total) on bent part gives

    • 1.5x3

      • this one seems preferable

    • 1x3.5

  • https://ems-stores.uwaterloo.ca/ems-site/aluminum-sheets

    • Alum Sheet - 5052 H32 20 GA. - .032

• Fasteners

  • 4x for SMA conn bolts M3X3

  • 3x tuning bolts M3X15

  • 3x tuning counternut and lockwasher

  • 3x finger bolts

  • 8x end cover bolts M3 (M3X6 ?)

  • 8x end cover nylock nuts M3

• copper/brass rod for probe

  • EMS doesn't have anything good for this, will need to get inventive here, not sure where to get brass rods or thin copper tube. guide uses UT-085 semirigid shield but I don’t have any of that

  • We have steel rods that are for little red yard flags

    • I can solder to these easily

    • Can easily cut myself with wire cutters

    • width should be close enough hopefully

EMS Purchase

Website and sign says they close 4:30pm friday, but 3:45pm they were closed. This means not gonna happen for competition.

• Alum Rect Tube 3 x 1 x .125 Wall

  • 7 inches

  • 1x

• Alum Solid Rounds 1/4 - 6061

  • 3x

  • 1.9 inch (2inch ok)

• Alum Sheet - 5052 H32 20 GA. - .032

  • 1.5x3 inch

  • 2x

• Metric Steel Socket Head Cap Screw 3X10MM X 0.5

  • 20x

• Metric Steel Socket Head Cap Screw 3X20MM X 0.5

  • 10x

• Metric Steel Socket Head Cap Screw 3X16MM X 0.5

  • 10x

• MET NUTS M3 X 0.5

  • 20x

• MET LOCK/SPRING/WASHERS M3

  • 20x

Actual Dimensions

• measure and sharpie it out

Simulation

• As the EMS is closed for the weekend I have more time to simulate this circuit to hopefully get closer to acing it on the first try even if I can’t get the perfect tube size.

  • Simulate multiple tube sizes and spacings of components

• Software options

  • WIPL-D

    • https://wipl-d.com/wp-content/uploads/2019/02/MW_021_WIPL-D_Cavity_Filter_Design_and_Optimization_2018.pdf

    • https://wipl-d.com/support/download/demo/

    • demo available

  • wavecon

    • http://www.waveconsoft.com/parfil/

    • paid software no demos

  • MFilter

    • https://docs.keysight.com/display/genesys2010/MFilter

• I need to CAD up my thing before I can sim it

Assembly

• taps from EMS?

• should be able to drill with stuff in the bay and measure it out myself mostly

• soldering rod to SMA connector

Tuning

• Borrowing VNA from university

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

  • Screwing in the tuning screws should decrease the resonant frequency of each finger by approximately 5%.

  • “The useful tuning range of the complete filter is approximately +/- 3% around its nominal center frequency.”

• using epoxy or hotglue to secure the tuning bolts once done as I don’t have the recommended paint

• sharpie on parameters

Conclusion

No VNA yet, no materials, low on time to simulate. Will save this for later. It is good fun, cost effective, and useful so seems worth pursuing.