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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

Our System

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

  • Finger pieces

  • 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

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

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