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High pass filter schematics for TVI and examples of finished filters

A K9AY loop antenna project done with Far Circuits pc boards for the antenna switch and bandpass filter and preamp by K7SFN

Sometimes we need a little more output filtering than traditional designs offer. Look at a new filter that can give you that extra boost. QST article by W3NQN

Third-order Cauer filters can boost performance of multi-transmitter, multi-operator contest stations to the next level. The filters are practical and you don’t need expensive test equipment to align them.

Tips on winding wire coils used as inductive loads, traps, band-pass filters, are implemented in a number of amateur radio projects especially antenna projects

A simple project for an effective home made band pass filter, designed for the portable field day usage

Australia's largest range of Electronic Kits & Components In Stock for the Radio Experimenter, ATV, microwaves, HF Band pass filters

The direct conversion receiver described consists of only a bandpass filter, mixer IC, VFO and audio filter. With only 22 parts, this simple circuit should take a few hours at the weekend to construct.

Article explaining electronic network of low pass filters

How to design narrowband bandpass filters

Clean Up Your Signals with Band-Pass Filters

An home made low pass filter for 70 MHz by IK0VAQ

A combined 144MHz Low-Pass and 432 MHz Notch Filter by VE2ZAZ

Details the construction of an **HF converter** designed by M1GEO, George Smart, specifically to extend the frequency range of the FunCube Dongle Pro (FCD) for amateur radio reception. The FCD natively covers 64 to 1,700 MHz, but this project enables reception from 0 Hz to 64 MHz by up-converting signals to the FCD's operational range. It employs a **double-balanced mixer** with a 100 MHz local oscillator (LO) to translate incoming HF signals; for instance, a 1 MHz signal appears at 101 MHz within the FCD's passband. The design incorporates a 7th-order Chebyshev low-pass filter with a 62 MHz cutoff frequency at the input to mitigate image frequencies, ensuring cleaner spectral presentation. George provides the schematic, PCB masks, and Gerber files for replication, noting that Far Circuits also offers PCBs. The resource includes test results for the low-pass filter and measurements of LO leakage, identifying -36.8 dBm at 100 MHz as a potential sensitivity concern. M1GEO discusses potential improvements, such as adjusting the mixer's LO drive, adding a balance pot, or incorporating a post-mixer high-pass filter to reduce LO breakthrough. Audio recordings from 40m and 17m demonstrate the converter's performance with WRplus SDR software.

Optimizing weak signal reception on the HF bands, particularly in the presence of strong local QRM, often necessitates specialized receiving antenna systems. This resource details the _HI-Z Antennas_ product line, focusing on phased vertical arrays designed for superior noise rejection and directivity. It covers components such as the 4-Square and 8-Element array controllers, which allow for rapid switching of receive patterns, and dedicated low-noise preamplifiers to improve system sensitivity. The site also presents various bandpass filters, crucial for mitigating out-of-band interference and enhancing the dynamic range of the receiver. The HI-Z systems are engineered to provide significant front-to-back and side rejection, often yielding **20-30 dB** of attenuation to unwanted signals, which is critical for DXing and contesting. Users can achieve a notable reduction in local noise, allowing for the discernment of signals that would otherwise be buried. The array controllers facilitate quick pattern changes, enabling operators to null out interference or peak weak signals from distant stations, effectively extending the reach of their receive capabilities by improving the signal-to-noise ratio.

Will additional coax stubs improve Band Pass Filter performance?

Online high pass filter design page with introduction to high-pass filters by K7MEM

The _Sci.Electronics FAQ: Repair: RFI/EMI Info_ document, authored by Daniel 9V1ZV, provides a detailed analysis of computer-generated RFI/EMI, focusing on its impact on radio reception. It identifies common RFI sources such as CPU clock rates (e.g., 4.77 MHz to 80 MHz), video card oscillators (e.g., 14.316 MHz), and even keyboard microprocessors, all of which generate square-wave harmonics across HF and L-VHF regions. The resource outlines a systematic procedure for pinpointing RFI origins, including disconnecting peripherals and using a portable AM/SW receiver with a ferrite rod antenna to localize strong interference sources. The document categorizes RFI mitigation into shielding, filtering, and design problems, offering practical solutions for each. It recommends applying conductive sprays like _EMI-LAC_ or _EMV-LACK_ to plastic casings of radios, monitors, and CPUs to create effective Faraday cages, emphasizing proper grounding and avoiding short circuits. For filtering, the guide suggests using line filters, ferrite beads, and toroids on power and data lines, and small value capacitors (e.g., 0.01 uF for serial/parallel, 100 pF for video) to shunt RFI to ground. It also discusses the use of bandpass, high-pass, low-pass, and notch filters on the receiver front-end or antenna feed to combat specific in-band noise.

A homemade band pass filter for the 30 meter band based on the original VE7BPO design

Javascript filter design page with introduction to low pass filters by k7mem

Passive Band-Pass Filter, low-pass filter calculator

This unit uses four capacitors, only two of which are variables, and a single tapped coil. It is built in a die-cast box, and has been tested up to 80W input power on 70MHz by G4CJZ

Understand how high pass filters are done

VHF Broadcast Passband Filter (88 108 MHz) by DF9CY

HF Radio Preselectors,BCB filters,Low Pass filters,Band Pass filter

Homebrew Amateur Radio 440Mhz Interdigital Bandpass Filter

The program can be used to calculate inductors (coils) and their number of turns on ferrite cores, ferrite shells and air coils. These can be used for baluns, Ununs, bandpass filters, low pass filters, resonant circuits, and more. The technical specifications of the cores are already integrated in the program. Application is free and runs on Windows 32 bit versions only. To make it run on Windows 10 64 bit need to be unzipped in a single folder.

Hand made RF sweeper made to adjust the pass band of crystal filter

Mobile RFI, often manifesting as persistent noise in the receiver even with the antenna disconnected, frequently originates from the vehicle's power supply system. This guide details systematic troubleshooting steps, beginning with isolating the radio from the car's 12-volt supply to confirm the power system as the noise source. It emphasizes the critical importance of drawing power directly from the battery using **heavy gauge wire**, bypassing the fuse block to leverage the battery's natural capacitance for RFI suppression and ensuring a solid RF ground. Proper routing of power lines through the firewall is also covered, advocating for dedicated grommeted holes to prevent inductive coupling from other wiring harnesses. The article stresses the necessity of fusing both positive and negative leads from the battery, a crucial safety measure to prevent damage to the rig and mitigate high-current risks should the battery's engine block ground become compromised during service. Addressing **alternator whine**, a common high-pitched noise that varies with engine speed, the resource suggests checking battery connections and the alternator-to-battery harness for looseness or corrosion. It also mentions the utility of adding an external RF noise suppression capacitor in parallel with the alternator's internal capacitor for enhanced filtering, and the effectiveness of commercially available in-line power supply filters.

R3KBO is an amateur radio club producing and developing amateur radio kits and assembled products including HF VHF Power Amplifiers or RF Power Kits and amplifier parts, low pass filfers, band pass filters, sourge protections, splitters and combiners, antenna switches

These are QRP rated 3 Pole Butterworth filters based on a design by K4VX, feature simplicity, almost no insertion loss, and reasonable rejection on the other ham bands.

This low pass filter terminates all harmonics from the 2nd to above the 10th harmonic in a better than 15 db return loss load

Homemade LC Bandpass Filters for 20M, 30M, 40M In multi-station environments like special events, field day, portable operating, is very important to protect receivers from excessively strong signals. Bandpass filters help to protect your transceivers.

Combined 432 MHz High-Pass - 144 MHz Notch Filter By Bertrand Zauhar, VE2ZAZ

How to create a low pass filter able to reduce or even remove the noise generated from your car or truck alternator

Adding a 20m band pass filter for the Icom IC-706MkII(G) by ON7EQ

VE3NEA presents a detailed account of designing and constructing a **helical filter** specifically for 144 MHz satellite reception, addressing severe SDR receiver overloading from nearby Land Mobile Radio Network signals between 141 MHz and 143 MHz. The author's experience with RTL-SDR, RSP1a, and Airspy Mini receivers demonstrated the necessity of a narrowband solution to enable reception in the 145.8-146 MHz range with only 3 MHz separation from strong interference. The resource delves into the theoretical underpinnings of helical resonators, drawing upon Anatol I. Zverev's "Handbook of filter synthesis." It explains the equivalent circuit of a two-resonator passband filter, highlighting the critical role of stray capacitance (C3) in determining the coupling coefficient (K) and emphasizing the importance of **critical coupling** (K=1) for optimal filter performance. Misconceptions regarding tap positions and impedance transformation are clarified, with tap placement shown to control resonator loading and the trade-off between bandwidth and losses. Construction details include sourcing materials like 1/4" copper tubing and PCB, precise assembly techniques, and the use of tuning screws for frequency and coupling adjustments. Measurements performed with a NanoVNA revealed discrepancies from initial Coil64 calculations, necessitating adjustments to helix turns and tap positions to achieve the desired 3.5 dB passband loss and 40-50 dB suppression of interference.

The Kenwood TS-870S HF transceiver features two state-of-the-art 24-bit 20 MIPS DSP chips, providing over 100dB out-of-passband attenuation and CW bandwidth adjustable to 50 Hz. It operates across 160-10 meters with 100 watts output, incorporating digital filtering, a beat canceller, and 100 memory channels. The radio also includes a transmit equalizer, RX antenna input, and a K1 Logic Keyer, enhancing signal processing and operational flexibility for amateur radio operators. Advanced capabilities include IF stage DSP, dual noise reduction, and an auto notch filter, all contributing to superior signal reception and clarity. The TS-870S offers a variable AGC, voice equalizer, and an RS-232C port for computer control, with Windows™ software supplied. Its built-in automatic antenna tuner functions on all bands for both transmit and receive modes, streamlining station setup and operation. Available accessories such as the DRU-3A digital recording unit, SO-2 high stability crystal oscillator, and VS-2 voice synthesizer option further extend the transceiver's utility. The unit requires 13.8 VDC at 20.5 Amps and is supplied with an MC-43S hand microphone, making it a comprehensive station component.

A cavity filter, often a critical component in _duplexer_ designs, functions as a sharply tuned resonant circuit, allowing only specific frequencies to pass while attenuating others. These filters are essential for maintaining signal integrity in environments where multiple transmitters and receivers operate simultaneously on closely spaced frequencies, such as in repeater stations. The article details how these filters, sometimes referred to as _notch filters_, achieve high Q factors, which are crucial for their performance. Understanding the principles of cavity filters is fundamental for any amateur radio operator involved in repeater operation or designing custom RF front-ends. The discussion covers the basic circuitry and operational characteristics that enable these devices to provide significant isolation, often achieving **-80 dB** or more between transmit and receive paths. This level of isolation is vital for preventing receiver desensitization and intermodulation distortion. Properly tuned cavity filters ensure that a repeater can transmit and receive simultaneously on different frequencies without self-interference, a common challenge in VHF/UHF operations.

A CW sidetone bypass for the NESCAF audio filter, a small and low cost circuit that works great.

Online Constant - k Lowpass Filter calculator

Butterworth Highpass Filter Designer

Building the W3NQN bandpass filters

Constructing a high-performance RF spectrum analyzer up to 1000 MHz requires careful attention to component selection, shielding, and circuit isolation. This resource details a project that improves upon the _Spectrum Analyzer for the Radio Amateur_ design by Wes Hayward (W7ZOI) and Terry White (K7TAU), incorporating ideas from Scotty Sprowls' project, particularly his 1013.3 MHz IF bandpass cavity filter. The analyzer utilizes a Mini-Circuits SRA-11 mixer with a sweeping local oscillator from 1013 to 2013 MHz, feeding into a 4-pole copper pipe cavity filter. The design employs a second SRA-11 mixer with a fixed 1024 MHz LO to produce a 10.7 MHz final IF. This signal then passes through narrowband resolution filters and is processed by Analog Devices AD603 and AD8307 ICs for IF amplification and logarithmic detection, driving an oscilloscope in X/Y mode. The project emphasizes modular construction, using salvaged components and double-sided FR4 material for PCBs, with critical notes on minimizing spurious images through effective shielding and proper voltage regulation for each module. Key components include a Z-Communications V585ME48 VCO for the first LO and a Z-Comm V583ME01 VCO controlled by a Motorola MC145151 PLL for the second LO. An optional Hittite HMC307 step attenuator and K&L 5L121-1000/T5000-O/O low-pass filter manage RF input. Tuning procedures for the 10.7 MHz IF resolution filter are also detailed, showing before-and-after spectrum views.

A modification of the UV3R Baofeng handheld transceiver to improve the built in low pass filter

How to build a low-pass filter to remove alternator whine from your car

Operating in the **microwave** spectrum, Response Microwave, Inc. specializes in the design and manufacturing of RF and microwave signal processing components and subsystems. The company's product line encompasses a wide array of offerings, including Connectivity Series components, rotary joints, phase shifters, cable assemblies, surge protectors, terminations, Hybridline/Couperline products, circulators/isolators, directional couplers, quadrature hybrids, attenuators, custom assemblies, filters/diplexers, DC blocks & bias tees, power dividers/combiners, laser diodes & drivers, high-frequency connectors, and precision test accessories. This extensive catalog supports various applications requiring precise signal manipulation and transmission at elevated frequencies. The resource provides access to a comprehensive product catalog and a dedicated connector catalog, detailing specifications for components like **high-frequency connectors** and test cables. While specific performance data or comparative analyses are not directly presented on the main page, the breadth of products indicates a focus on providing foundational building blocks for microwave systems. The company emphasizes customer service and aims to be a reliable source for RF/Microwave/Optics product requirements, serving a growing customer base with its specialized component offerings.

Examples of absorbtive low-pass filters in which the unwanted high-frequency energy is separated from the wanted output and then dissipated in matched resistors

30m lowpass filter for the Softrock Ensemble RXTX tranceiver.

Thsi article describes a microcontroller driven semi-automatic antenna tuner capable of handling power levels up to 150 watts. The device is a low pass filter tuner manually tuned by setting the optimized L/C combination by hand and then storing the values into the EEPROM of the mictrocontroller to recall them later (seperately for each band from 80 to 10 meters including WARC bands)