KR4BAD Amateur Operator

Author: David L Whitehurst

  • From Prompt to PCB: How AI Helps Me Build World-Class RF Filters

    From Prompt to PCB: How AI Helps Me Build World-Class RF Filters

    AI is going to help me be a true success! While a large part of the population is afraid that AI is going to take their job, I remember a statement from Tony Robbins, the Self-Help, Success-Guy, “you are not going to lose your job to AI, you are going to lose your job to someone that knows how to use AI.”

    Here’s my prompt (what I ask AI) today. “I’ve chosen components and calculated resonances, etc. for a 20 meter band pass filter using SMD components, air-core inductors (Coilcraft), and 0805 high-Q capacitors with female SMA connectors on the ends. Can you talk to me about things to consider as I design my PCB board to be really compact and use RFI shielding clips for an aluminum enclosure? Anything that I should focus on specifically?”

    And, here’s the response. Remember, it’s mine to use, author, etc.

    You’re in a really fun part of the project now — the physics starts to show up in the copper. Let’s talk specifically about what matters for a compact 20 m BPF PCB inside a shielded aluminum can.

    1. Board stackup and grounding

    This is the most important foundation.

    • Solid ground plane:
      • Use a continuous ground plane on the layer directly under the filter components.
      • Don’t route signals in that ground layer under the filter if you can avoid it.
    • Short return paths:
      • Every cap/inductor junction should have a very short path to ground (via right next to the pad).
      • For shunt capacitors, put the via as close as manufacturable (0.2–0.5 mm away).
    • Many ground vias:
      • Stitch ground around the filter area (via fence along the edges and around the SMA footprints).
      • Aim for via spacing on the order of a few mm in the RF path region.

    The idea: the filter lives in a “RF cavity” defined by the ground plane and the metal lid; your PCB layout is the boundary condition.

    2. Signal routing and physical layout

    At 14 MHz, you’re not in microwave territory, but layout still matters a lot for coupling and stability.

    • Keep the filter linear and symmetrical:
      • Lay out the network in a straight line: SMA → series L → shunt C → series L → shunt C … → SMA.
      • Avoid dog‑legs and wildly asymmetrical element placement.
    • Keep traces short but sane:
      • Use short, reasonably wide traces between components (you’re roughly in 50 Ω land, but not truly transmission‑line dominated at 14 MHz).
      • Don’t obsess over exact 50 Ω microstrip, but avoid long skinny runs.
    • Minimize parallelism:
      • Don’t run input and output traces closely parallel — route them in a way that minimizes direct capacitive coupling (no “looping back” alongside each other).
    • Inductor orientation:
      • Rotate adjacent air‑core inductors 90° to each other wherever possible.
      • Maximize physical distance between inductors that aren’t meant to couple; even a few extra mm helps.

    You’re trying to preserve the intended L/C coupling and avoid creating a mystery extra coupling path that changes your filter shape.

    3. SMA connector integration

    Female SMAs on a compact board inside a shield can get tricky mechanically.

    • Grounding the connectors:
      • Ensure the SMA’s ground tabs or edge‑mount ground are well tied into the ground plane with multiple vias right at the connector.
      • Don’t rely on just the mechanical tab; stitch it with vias all around.
    • Mechanical registration with enclosure:
      • Think in 3D: where the SMAs sit relative to the enclosure wall or hole.
      • Make sure there’s enough clearance so the cable can mate without pushing the PCB or stressing solder joints.
    • Shield continuity at the connector:
      • Ideally, the SMA body and the enclosure are at the same RF ground potential.
      • If the SMA isn’t panel‑mounted but just pokes through a hole, you may want extra ground fingers or copper pads near that opening.

    4. RFI shielding clips and aluminum enclosure

    With PCB‑mount shield clips + aluminum lid/box, some specifics matter.

    • Placing the clips:
      • Put clips on grounded copper regions, not over signal traces.
      • Use ground via stitching under/near the clip pads so each clip connection has a low‑impedance path to the main ground plane.
    • Shield “wall” on the PCB:
      • Run a ground copper ring around the filter area inside where the shield will sit.
      • Stitch that ring with vias to the ground plane (this becomes your RF “gasket” under the lid).
    • Gap control:
      • Try to avoid large gaps between the lid and the PCB copper ring. Even at HF it’s not critical, but better contact = less chance of weird coupling.
    • Aluminum is not solderable:
      • Your electrical connection is via the clips to a copper pad, not the lid itself.
      • Ensure that the lid or box is mechanically pressed against those clips or edges so there’s real metal‑to‑metal contact.

    If your board sits inside a larger aluminum box, you can use both the PCB shield can and the external box as nested shielding — very effective.

    5. Compactness vs coupling

    When you shrink everything, coupling wants to creep in; this is where most “mystery behavior” comes from.

    • Watch in/out isolation:
      • Keep input and output physically separated by the filter network and some ground copper.
      • Don’t let the SMA inputs “see” each other over the top of the filter via stray capacitance.
    • Use ground “fences” between sections:
      • You can place thin grounded copper strips between sections of the filter (with vias), acting as mini bulkheads.
    • Guard traces:
      • Put grounded copper between sensitive nodes if you’re worried about coupling — especially between input and output ends.

    Compact is good, but don’t make it so compact that inductors and traces are stacked over each other.

    6. Practical tuning/debugging considerations

    Even with careful calculation, the real PCB will need a little love.

    • Leave some tuning flexibility:
      • Use parallel pads so you can add or change capacitors (e.g., room for small parallel trimmers or extra 0805s).
      • Make it easy to snip out an inductor turn or stretch/compress them if you ever go custom‑wound later.
    • Test points:
      • At minimum, expose input and output SMA and make sure you can easily connect a VNA or sig gen + power meter without contortions.
    • Ground test pad:
      • Provide a good ground pad near the filter for probing, not just the SMA body.

    This lets you actually see how your layout choices affected the filter response — and refine future versions.

    7. Specific “watch this” items for your setup

    Given what you described (SMD, Coilcraft air‑cores, 0805 high‑Q, shield clips, aluminum box):

    • Air‑core inductors:
      • Keep them slightly away from the enclosure walls and lid; very close metal will change inductance and Q.
    • High‑Q capacitors:
      • Great choice. Just keep their leads short and put ground vias right beside shunt cap grounds.
    • Aluminum box:
      • Plan screw locations or standoffs so the PCB ground has several solid mechanical + electrical bonds to the box.
      • Avoid routing high‑impedance nodes right under screw heads or standoffs.

    If you’d like, describe (or sketch verbally) your basic layout: SMA on each end, number of poles, and board size target. I can then walk through a more concrete “place this here, avoid this there” pass tailored to your exact filter.

    It’s me again! That’s quite some education there. Go learn AI.

    My About page provides the background of my project, the Freedom7 HF Transceiver.

    If this story resonates, comments are welcome. You can also reach me at david [at] kr4bad-dot-communications. no com

    And if you believe understanding matters more than black boxes, you can subscribe to my WordPress https://kr4bad.com/?subscribe=1.

    73 KR4BAD David

  • 3-Pole Butterworth Series-Coupled Bandpass Filters

    3-Pole Butterworth Series-Coupled Bandpass Filters

    I started my band-pass designs using the Chebyshev model. I didn’t do my homework. I just found a schematic, chose some values based on someone else’s design using toroids, and totally ignored the “Q” values of components. I built 7 filters and also tested them on a NanoVNA. I didn’t think anything of the 3 or greater dB insertion loss. I was verifying the passband graph shape. Everything looked good. It was not. I’ve been doing lots of homework and here’s some of it.

    When you design a 3‑pole Butterworth series‑coupled band‑pass filter, one of the first surprises is that the parallel LC tanks don’t resonate at the same frequency as the filter’s actual passband center. For example, you might calculate:

    • Tank resonance: 14.029 MHz
    • Filter center frequency: 14.2 MHz

    At first glance, this looks contradictory. Why would the tanks be tuned below the desired center frequency?

    The answer is simple once you understand how coupled resonators behave.

    1. The tanks have a “free” resonance — but the filter has a “forced” resonance

    Each LC tank has its own natural resonance frequency:

    fp=12πLC

    At this frequency, a parallel LC tank has maximum impedance. If the tank were sitting alone on a bench, it would peak at 14.029 MHz.

    But in a real filter, the tanks are not alone. They are:

    • coupled to each other,
    • loaded by the source and load impedances,
    • and constrained by the Butterworth response requirements.

    Once you connect everything together, the system develops multiple resonant modes, and the collective behavior shifts the passband center upward.

    This is why the filter’s true center ends up at 14.2 MHz, not 14.029 MHz.

    2. Coupling “pulls” the resonant modes apart

    A 3‑pole filter has three resonant modes. If the tanks were uncoupled, all three would sit at 14.029 MHz.

    But coupling splits them into three slightly different frequencies:

    • one a bit below the tank resonance,
    • one near it,
    • one above it.

    The Butterworth design equations choose the coupling so that the middle mode becomes the passband center, and that mode ends up at your design frequency (≈14.2 MHz).

    So the tanks are intentionally tuned a little low so that, once everything interacts, the system resonance lands exactly where you want it.

    3. Think of it like tuning three musical instruments

    Imagine three violins, each tuned to 14.029 MHz. When they play together and acoustically couple, the ensemble’s strongest note shifts slightly upward.

    The individual strings haven’t changed tuning — but the system resonance has.

    Your filter works the same way.

    4. What this means for your frequency‑response plot

    If you graph insertion loss (dB) vs. frequency:

    • At 14.029 MHz Each tank is at maximum impedance, but the filter is not at maximum transmission. You’re on the lower slope of the passband.
    • At 14.2 MHz The three resonant modes combine to produce the minimum insertion loss. This is the true center of the Butterworth passband.
    • Above and below The response rises smoothly, with the classic maximally‑flat Butterworth shape.

    So the filter’s “flat top” is centered at 14.2 MHz even though the tanks themselves peak at 14.029 MHz.

    5. The relationship in one sentence

    14.029 MHz is the natural resonance of each individual tank. 14.2 MHz is the resonance of the entire coupled system.

    The difference is not an error — it’s exactly how a properly designed 3‑pole Butterworth band‑pass filter is supposed to behave.

    My About page provides the background of my project, the Freedom7 HF Transceiver.

    If this story resonates, comments are welcome. You can also reach me at david [at] kr4bad-dot-communications. no com

    And if you believe understanding matters more than black boxes, you can subscribe to my WordPress https://kr4bad.com/?subscribe=1.

    73 KR4BAD David

  • Why I’m Building My Own HF SSB Transceiver

    Why I’m Building My Own HF SSB Transceiver

    Every radio operator has a moment when the hobby stops being just about making contacts and starts being about understanding what makes those contacts possible. For me, that moment arrived this past year and when I renewed my FCC license. I used to be KJ4GIZ long ago and that call had expired.

    I’ve been on the air for a while, but I realized something: I could operate an HF Single Side Band transceiver… yet I couldn’t explain one to others. Not really. Not down to the level where I could point to a signal path and say, “This is where the magic happens.”

    So I decided to build one.

    Not buy one. Not assemble a kit. Actually build one — stage by stage, circuit by circuit — until I could look at the finished radio and say two things with complete honesty:

    1. I finally understand how an SSB transceiver works.
    2. I made this myself.

    That second part matters more than I expected. There’s something deeply satisfying about the idea that when someone hears me on the air, they’re hearing a signal that came from a radio I built with my own hands. It’s like catching a fish with a lure you carved yourself — the contact means more because you made the tool.

    And yes, I’ve had help. Not from a mentor sitting next to me at a workbench, but from a different kind of companion: Microsoft Copilot. I’ve been using it to check circuit theory, explore design options, understand mixer behavior, and even sanity‑check my math. It’s been like having a patient, knowledgeable Elmer available 24/7 who never gets tired of my questions.

    But the deeper question — the one I want to share with the JARS (Johnston County Radio Society) members — is this:

    Why does this matter to my radio club?

    Because this project isn’t just about me. It’s about us.

    • It reminds us that amateur radio is still a hands‑on, experimental hobby. We don’t have to be just consumers of equipment. We can still be builders, tinkerers, experimenters — the very spirit amateur radio was founded on.
    • It shows that learning the fundamentals is still exciting. Understanding how SSB works isn’t just academic. It makes you a better operator, a better troubleshooter, and a more confident communicator.
    • It demonstrates that modern tools can empower old‑school skills. AI doesn’t replace the soldering iron. It just helps you understand what you’re soldering and why it matters.
    • It has shown that electronic projects today can be controlled with microcomputers. Modern electronics has seriously advanced beyond my school education. This project gives me a reason to own the ARRL Radio Handbook
    • It might inspire someone else in the club to build something too. Not necessarily a full transceiver — maybe a filter, a tuner, a preamp, or even a simple oscillator. But one person building something often sparks another.

    In February, when I stand up at the club meeting, I won’t just be showing off a radio. I’ll be sharing a journey — one that blends curiosity, craftsmanship, and a little help from a digital friend.

    And when I finally make that first contact on the air, I’ll be able to say:

    “You’re coming through loud and clear… on a radio I built myself.”

    My About page provides the background of my project, the Freedom7 HF Transceiver.

    If this story resonates, comments are welcome. You can also reach me at david [at] kr4bad-dot-communications. no com

    And if you believe understanding matters more than black boxes, you can subscribe to my WordPress https://kr4bad.com/?subscribe=1.

    73 KR4BAD David

  • Testing

    Testing

    This builder supports fast failure. Read on … you’ll understand. Or, build on and you’ll understand too.

    The first modular component I designed and created was the PCB for seven unique band-pass filters. They are passive and I could use a NanoVNA to actually show the performance curve with the RF spectrum from the NanoVNA device. I have created the front-switching component for these band-pass filters and also a RX/TX switching component. There are many things to test. This post will consider the tools I currently have and the methods I will use with these tools.

    I have an inexpensive, but serviceable oscilloscope and a pretty decent smart multimeter. The multimeter can check capacitors, diode forward-voltage, and even temperature. I have breadboards and various power supplies with sufficient amperage needed. I try to prototype and test each unique network task, chip, configuration, digital logic, etc before PCB manufacture. I have LTSpice for circuit modeling. I used LTSpice heavily during my bandpass filter design. I did not model the RX/TX switching schematic.

    My first test of the RX/TX switching component was to power it on with 5V and adjust a potentiometer for my 28V biasing (block) against schottky diodes. I powered the module and a small inductor started smoking. Hard stop! I had to completely rethink my design and testing. I should have considered everything prior to making the PCB and choosing components. This was all prior to any real testing.

    My lesson learned here is to consider amperage in all cases. And while I’m very excited with these SMD components, again I need to pay attention to voltage, amperage, wattage, etc. or component rating basically. My RX/TX switching component is pretty solid design on the receive side because the RF power is nothing really. The transmitting side completely forgot the RF power it would see. I really need an interface between the transmitting side and the central antenna connector.

    I also learned that I need a signal generator. I need a unique fixed RF frequency to send through components. I have a NanoVNA and I’ve been told that it can emit fixed frequencies. I will learn about the NanoVNA or I will make a testing signal generator using the Si5351a that I will use for my VFO.

    I’m trying to make ultimate use of the tools that I have. This radio project has been expensive. I need to use my head for something besides a hat-rack like my Dad always said. I think I have what I need to get me through most of what I need to test but that means, more focus on design. I also need to write down my tests regardless of simplicity.

    In conclusion, I’ve determined that more effort is needed before schematic, component, and PCB acceptance. Every circuit that isn’t completely clear in my mind should be modeled if possible in LTSpice. And, the component BOMs should be scrutinized for ratings, specs, etc. beyond just PCB footprint. More of everything needs to be written down.

    “Support fast failures” … Tom Peters

    My About page provides the background of my project, the Freedom7 HF Transceiver.

    If this story resonates, comments are welcome. You can also reach me at david [at] kr4bad-dot-communications. no com

    And if you believe understanding matters more than black boxes, you can subscribe to my WordPress https://kr4bad.com/?subscribe=1.

    73 KR4BAD David

  • Modular

    Modular

    The Freedom7 HF Transceiver has yet to receive or transmit however, I’m making progress and I’m designing components in a modular fashion. The image here below in this post shows my solution to Receive and Transmit switching. The screw terminal block on the side of the PCB has 4 connections for 5v, ground, receive digital, and transmit digital. The two RX/TX connections will be HIGH/LOW or LOW/HIGH respectively at any time.

    RX/TX Switching

    This is but one module called RX/TX Switching and it’s made to perform one task. This module will act as a stand-alone design that could be improved and replaced with a better module at a later time. I will share that I’ve done one PCB earlier e.g. with no screw terminals but later realized that solder pads were not supportive of a true modular concept. This is a work in progress. Future PCBs will be easy to implement and change out. Also note that a production-ready version of Freedom7 will probably be much different in high-level design. These modules will support my Freedom7 working prototype.

    I’ll share an early schematic but talk to the upper part of this. There are two MOSFETs that act as a 28v switch. Where did we get 28 volts you may ask? Well, I used a boost regulator MT3608 that converts 5v to 28v, with the help of a variable potentiometer. Either side will be biased at any time, i.e. the 1N5711 Schottky diode will be reverse-biased with 28v at the cathode side of the diode, thereby greatly blocking the flow of RF. This is not perfect but the small leakage that can occur is also choked with a 10 microhenry inductor and 3.3k resistor. I have calculations on these values. P.S. I also should call the digital control inputs just RX_BIAS and TX_BIAS.

    My About page provides the background of my project, the Freedom7 HF Transceiver.

    If this story resonates, comments are welcome. You can also reach me at david [at] kr4bad-dot-communications. no com

    And if you believe understanding matters more than black boxes, you can subscribe to my WordPress https://kr4bad.com/?subscribe=1.

    73 KR4BAD David

  • Control

    Control

    This project has been fun so far, but it’s hard to see the final, working product. I began this journey with the Arduino Nano R3 to be my choice of digital microcontroller. With the Nano, one can create it’s firmware using the Arduino IDE on any computer. The Arduino Nano is very popular, I got 3 of them on Amazon for like $13 with USB cables. I was hooked. I had a little computer that I could program with digital pins I could use to control things for my radio. I had my microcontroller.

    I initially had visions of 7 bands, 10m, 15m, 17m, 20m, 40m, 80m, and 160m. I also needed a way to push a button or cycle through a single button to select my band. A computer program, called a sketch for the Arduino, could monitor digital pins that are grounded with a momentary switch. A loop method or cyclic listening function would monitor 7 digital pins and then do something else based on the grounded pin. With the selection of these 7 pins, they are now reserved for this radio operation function or band selection.

    The Arduino has analog and digital pins. Some of the analog pins can be made digital but the digital HIGH is labeled floating. Normally the digital pin would be HIGH or 5V but for the analog pin it can drift. The solution is a pull-up resistor and 5V applied. The values are based on pin usage so I’ll not discuss that here. I will however, say that the digital pins are priceless because they can be used as sensors (incoming, grounded, sensing, etc.) or they can be digital representation. The analog pins can be used as sensing too but in an analog way, i.e. they can sense variations from 0 volts to 5 volts.

    I have tested a makeshift VFO with a single frequency. I have wired and tested an encoder with this VFO where 4 pins are used on the Arduino that do not interfere with the band select pins. Two pins are currently used to connect with another microcontroller, the Arduino Mega 2560. These pins are serial transmit and serial receive. Two pins are currently used for the I2C protocol. I have tested those with a programmable clock generator chip Si5351a that uses the two pins to send an address and then communicate with that device. The two pins on the Arduino can be an I2C bus if the pins are held high with the right pull-up resistors. I’ve tested continuous frequency memory with each band. I have three dedicated pins for addressing of RF switches.

    I also need more pins which I do not have for new features that have not been designed and tested. I ordered a breakout board with the MCP23017 GPIO extender that will give me 16 more digital pins for use with the Arduino. I have not tested it yet. Enter light bulb moment.

    Look at this post’s featured image and notice the Arduino Mega, shield and TFT display. Notice the convenience board with screw terminals around three sides. Notice the only two wires. They are serial transmit and receive. You can’t see the ground from the Nano in the photo. Well, you can see that wire crossing over the shield. Only three wires into the complex being. And, notice the little lower life form to the left. You don’t see the momentary select wires into the terminals but all the pins are reserved for something.

    Could the Arduino Mega be used for everything and can I use just one complex life form? Yes.

    I’ll take an aside on this post to tell you that I’m focused on modular components for my radio. I’ve also made some solid PCBs, i.e. I’m satisfied that they meet my requirements for the proof of concept (POC). But I’m glad that I’ve only done some draft schematics for the controller and display focus.

    UPDATE: I’m not going to re-write this posting but I’m now using two Arduino Mega 2560 microcontrollers, one solely for display and the other strictly for control.

    My About page provides the background of my project, the Freedom7 HF Transceiver.

    If this story resonates, comments are welcome. You can also reach me at david [at] kr4bad-dot-communications. no com

    And if you believe understanding matters more than black boxes, you can subscribe to my WordPress https://kr4bad.com/?subscribe=1.

    73 KR4BAD David

  • Document

    Document

    The Freedom7 HF Transceiver project really consists of multiple projects. One is pure design and experimentation. Another is product assembly, manufacture, and evolution. Lastly, an overlooked but oh so important project of documenting, accounting, journaling, recordkeeping, and just describing the parent project in detail for well-informed progress, decision-making assistance, and reduced anxiety moving forward.

    I generally push into new design and experimentation with ease. Yet, I begin to stress when it comes to assembling a vision of the completed product. And, I almost paralyze at the thought of documentation for something I’ve figured out completely. I am currently procrastinating the documentation for the first components I’ve completed and tested. These are passive LC band-pass filters.

    With this post, I’ll outline how I plan to break this procrastination and do each component’s documentation. I have been using Donald Knuth’s TeX typesetting system to produce my most important and formal documents. I’ll share my outline for breaking this procrastination and also produce a PDF sharing my band-pass component’s TeX documentation.

    Here are the numbered but unordered considerations for documenting each Freedom7 HF Transceiver component:

    1. Series of iterations of component on paper from simple box to full detail
    2. What is it and what does it do?
    3. What are its interfaces?
    4. Does it need power? Voltage? Amperage need?
    5. Active or Passive and why? Pros and cons?
    6. Does it need outside (external) control, configuration, tuning (once or operator), etc?
    7. Does it require testing, tuning during manufacture, adjustment, trials, reset (internal or operator), etc.
    8. Shielding?
    9. Ground plane concerns?
    10. Possible floating pins?
    11. Consists of the following parts? Footprints for PCB? Costs?
    12. Calculations, modeling (LTSpice), paper trials, etc. or any deep technical analysis?

    Here’s an example TeX document that’s unfinished. I’ll add the considerations shown above and replace this when the formal document is complete. This is provided here to show the type (TeX) of document I’ll use to reference each component as I go.

    Loader Loading…
    EAD Logo Taking too long?

    Reload Reload document
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    My About page provides the background of my project, the Freedom7 HF Transceiver.

    If this story resonates, comments are welcome. You can also reach me at david [at] kr4bad-dot-communications. no com

    And if you believe understanding matters more than black boxes, you can subscribe to my WordPress https://kr4bad.com/?subscribe=1.

    73 KR4BAD David

  • How To?

    How To?

    band-switching

    My technician class license renewal was pretty easy and I struggled a bit before my general. The vision of a DIY Ham radio came through my brain, quite obnoxiously, interrupting my study for my extra about 2 weeks after I paid ARRL too much for a study guide. Now just after the ADHD kicked in, the real question was, “How am I going to make my own HF radio?” or “How-to?”.

    I’m here to tell you the answer to the how-to question is easy. AI is not your enemy. In fact, I wish that I had AI when I was in college. I would be addressed as Dr. Whitehurst today. But beware because with everything AI gives you, it comes with no shame, no caveat, no hesitation, etc. You can’t accept all AI responses as truth or gospel. You can however, use it to accelerate the creation of a DIY HAM radio.

    I’m going to use the post image to describe how I used AI to arrive at my design and how I chosen components to accomplish a task within the larger component of RF routing into selected band-pass filters. I first want to make this analogous to fishing. And, if I didn’t catch the big fish it was because “you’re not holding your mouth right” according to my dad. With AI, you’re not going to get the best answer when “you didn’t give it the right prompt”. Let’s begin a session with AI and I’ll you’ll begin to see how this fishing trip goes.

    I’ll provide a background to describe the need for the circuit above and we’ll begin just after the requirements. The Freedom7 HF Transceiver decidedly selected 7 bands, 10m, 15m, 17m, 20m, 40m, 80m, 160m for use. I now knew that I would have 7 bandpass filters. I could see my seven filters just after the antenna came into the radio enclosure. Wait a minute, we need a splitter of sorts and really a switching mechanism because our RF doesn’t need to be wasted on all filters. We have this weak RF signal and we need to route it through a band-pass filter and selectively narrowing the spectrum to the band chosen by the radio operator.

    We now need to think of components, interfaces, and testing. I could see this thing or black box with one RF input and seven outputs. These interfaces would be SMA. Easy right? So what exactly does this black box need beyond the single input and then outputs into each band-pass filter? How do we select or switch into the output SMA? I could think of two choices: 1) mechanical switch or 2) digital selection. I liked the digital selection because that’s what I learned most about in school. Enter AI.

    I’ll skip over the part here where the radio operator will push a momentary switch for the selection of one of the seven bands. Assume that we are using microcontrollers inside and when the operator pushes the button for 20 meters that the microcontroller can send the information needed to drive a chip to route the RF signal to a unique RF output. Let’s now ask AI (Microsoft CoPilot) if there are available chips to route RF signals. We’ll ask the question and I’ll share the AI response here. Ask AI very clearly what you want. And, if backstory is needed give it.

    Here goes … Prompt: I’m making a homebrew HF transceiver. The RF ranges between 1.8 and 30 Mhz. Are there IC chips that can route input RF to many outputs based on digital selection?

    CoPilot Response: Yes, there are RF switch ICs that can route signals in the 1.8–30 MHz range to multiple outputs using digital control. These are typically SPxT (Single-Pole x-Throw) switches designed for wideband applications.

    🔌 Recommended RF Switch ICs for HF Transceivers

    Here are some ICs suitable for routing RF signals in your HF range:

    ICTypeFrequency RangeControlNotes
    ADG904BRUZSP4TDC to 2.5 GHzCMOSAbsorptive switch with low insertion loss analog.com
    ADRF5051SP4T9 kHz to 20 GHzCMOSNonreflective, high isolation analog.com
    ADRF5060SP3T + Bypass100 MHz to 20 GHzCMOSRF path selector, may need bandpass filtering analog.com
    PE42422 (pSemi)SPDT10 MHz to 8 GHzCMOSHigh linearity, low loss pSemi
    Mini-Circuits USB-RF SwitchesSPDT–SP10TDC to 50 GHzUSB/digitalModular, ideal for test setups Mini Circuits

    Sources: Mouser Electronics analog.com pSemi Mini Circuits

    🧠 How to Use These in Your Design

    • Digital control: Most switches use CMOS logic (e.g., 3.3V or 5V) for selection.
    • Routing options: Choose SP4T or SP6T depending on how many outputs you need.
    • Insertion loss: Look for <1 dB to preserve signal strength.
    • Isolation: Higher isolation (>30 dB) prevents crosstalk between outputs.
    • Absorptive vs Reflective: Absorptive switches terminate unused ports to prevent reflections—ideal for RF.

    🛠️ Application Ideas

    • Band switching: Route different frequency bands to separate filters or amplifiers.
    • Antenna selection: Switch between multiple antennas based on mode or band.
    • Signal routing: Direct RX/TX paths to different modules or test points.

    Would you like help selecting a switch for a specific number of outputs or integrating one into your schematic?

    End CoPilot

    I did not edit the CoPilot response but you notice that the ADG904BRUZ chip was first in the list. I knew nothing about this chip until I asked AI. I held my mouth right that time, and caught the big fish. I knew that my next step was to find my Golden Field Guide to Fish and look him up. Or, better yet search for a datasheet on the ADG904BRUZ chip.

    My microcontrollers are being used for various tasks but they both (yes 2) are powered by 5V. This chip requires a step-down regulator.

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    I’ll not belabor this post with all of my design details here but I do want to emphasize how important AI can be in figuring all this out. I’ll leave you with some homework. Ask Copilot how two ADG904BRUZ chips could be used with a microcontroller to drive the choice of 7 unique RF outputs. You will be amazed. Just ask it your question.

    I’ll end my post today with the following. AI is not always right. I have caught outright untrue responses/results. I have also called AI on these facts and AI provided a second look and agreed with cause for debate. I’ll write more about this in the future. AI is here so use it for what it is. Learn, learn, … and learn some more.

    My About page provides the background of my project, the Freedom7 HF Transceiver.

    If this story resonates, comments are welcome. You can also reach me at david [at] kr4bad-dot-communications. no com

    And if you believe understanding matters more than black boxes, you can subscribe to my WordPress https://kr4bad.com/?subscribe=1.

    73 KR4BAD David

  • Reference

    Reference

    Hilberling PT-8000A

    No, it’s not the Freedom7, but it’s what I would consider the reference specification for measuring the quality of an HF transceiver. The radio you see above is the Hilberling PT-8000A. I’m sharing this because I needed something to guide my ideas for the Freedom7 HF Transceiver that’s my current DIY/Homebrew project. I use this radio like I’ve used things that my father has created to help me judge the quality of things that I have made with my hands.

    This post is to describe how I will move from inception of the Freedom7 to a physical radio, sitting on my desk, and fully operational. And, where my HF radio is a DIY effort, I continually look at the best-in-breed radios for inspiration during this effort.

    I also want to share my architectural strategy here. This strategy is mentioned briefly on the bottom of this blog’s main landing page. These three concepts are: Component, Interfaces, and Testability. This radio is a huge undertaking but I am an Enterprise IT Architect by trade and my mind immediately goes to a division of the whole into bite-sized chunks. These chunks are really components. Look at the Hilberling radio above and what do you immediately see as a component? I see a Power Supply. Okay, that’s component 1. Other components may be more difficult to see. Ask yourself how you saw the Power Supply as a component? It’s because the Power Supply has visible boundaries. It’s in its own enclosure. You can just see it visibly as being it’s own component. This is but one needed component for the Freedom7 HF Transceiver. We will need power to light all of the components.

    I’ll just mention a few other components and then I’ll describe the importance of Interfaces. We’ll need band pass filters, a VFO, mixer, modulator, demodulator, amplifiers, RX/TX switching, filter selection, display, control systems, and more. Instead of focusing on specifics, I mainly want to emphasize that the radio is comprised of a collection of components. And, these components have boundaries and contexts. But most importantly, these components communicate, are powered, and are controlled or configured through interfaces.

    Components are bounded by context, borders, boundaries, edges of PCBs, lol, something that keeps the context within. My point is that every component can be defined as a functional item that does something to contribute to the overall operation of the HF transceiver. These components all have some form of interface. These interfaces are analogous to wires that may be coming from a black box. If the black box is the component we know that something happens within the black box and the wires would be the only way in or out of the box. These wires or interfaces have a function and a protocol for how they are used.

    Let’s discuss the component interface and how it’s modeled for any HF radio component. Remember that these components can be modeled on paper, along with their interface definitions, and then we can answer questions like 1) is it testable? 2) how would we test it? We’ll discuss that next but think about the component and how it must have interfaces of some kind.

    I’ll describe the band pass filter as a component and only describe this (sub) component at a high level with a single sentence. We’ll analyze the sentence and then define its interfaces. The Freedom7 band pass filter is a passive LC design that accepts RF at the width of the ARRL HF spectrum or 1.8-30 MHz and allows a narrower bandwidth with roll-off of approx. -40 dB on both sides of the narrowed bandwidth and minimal insertion loss (approx. -6 dB) across the translated bandwidth. That’s quite a mouthful but it tells me a lot about this new black box I need for my new Freedom7 HF Transceiver.

    The Freedom 7 HF Transceiver will support 7 bands across the ARRL accepted HF spectum, 1.8-30 MHz. These are 10m, 15m, 17m, 20m, 40m, 80m, and 160m. Given 7 bands, we’ll need 7 bandpass filter components for receive and 7 bandpass filter components for transmit. I’ll now describe the interfaces for each bandpass filter. Since the filters are passive, they do not require power. Each filter consists of 3 series LC traps with additional drop capacitors to shape and narrow the bandwidth. The filters are simple with RF in and RF out. Two interfaces per filter.

    I used the Freedom7 bandpass filter as a component example only and to show that the component has a single function and two simple interfaces, radio frequency in and out. With the component and interfaces defined, we can now ask if the component is testable and give an analysis for testing.

    First, yes, the bandpass filter is testable. Each bandpass filter needs an HF-wide RF signal and we need a way to see the shape of the dB cut over the width of the input signal. Designing each filter was done with the help of LTSpice and LTSpice can render the shape of the filter response. I’ll go into the design of components in other posts but just mention that LTSpice was used early-on to provide expectation for this analysis (testing) I’m discussing here. The Nano VNA is a popular spectrum analyzer that I happen to own and it was used to show each filter response.

    I hope that I’ve clearly described the modular component architecture strategy that I’m using to create this DIY HAM radio I call the Freedom7. I’ll be posting detailed articles soon and I will follow the component, interface, and testability concepts with each component. This post was about reference and my goal was to share with you my quest for only the best quality using quality examples like the Hilberling and also reference to sound architecture practices. I just want to share with my HAM friends, my Freedom7 HF Transceiver and how I’m doing it. It’s exciting and I want to share. I hope that I can spark others to work with me in the future. I can’t afford the Hilberling, but I certainly can aspire to do it myself.

    P.S. Here’s a link to the Hilberling https://www.hilberling.de/en/hamradio/. My picture above is antiqued or filtered as part of this blog theme. I could have rendered it in full color but it’s not a post about the Hilberling; that’s just something to dream about Christmas eve.

    My About page provides the background of my project, the Freedom7 HF Transceiver.

    If this story resonates, comments are welcome. You can also reach me at david [at] kr4bad-dot-communications. no com

    And if you believe understanding matters more than black boxes, you can subscribe to my WordPress https://kr4bad.com/?subscribe=1.

    73 KR4BAD David

  • Freedom7

    Freedom7

    I renewed an outdated HAM license in February 2025. I’m now KR4BAD and licensed as a General. With this old hobby startup, my itch to make my own radio was strong. I had built a TenTec HF receiver and a CW transceiver years ago, but these were very simple and they were kits. I need to build my own HF radio.

    I’ve played guitar since I was four years old and I’ve already made my own electric guitar tube amplifier about 10 years ago. Now that I’m doing DX and POTA hunting on the front porch, I’ve been itching to create my own radio and use it to talk to someone far away. Enter the idea for the Freedom7 HF Transceiver.

    The Freedom7 HF Transceiver will provide SSB operation for 7 HF bands, 10m, 15m, 17m, 20m, 40m, 80m, and 160m. It will be designed and built by David L Whitehurst or KR4BAD, Johnston County Amateur Radio Society (JARS) member of Fuquay Varina, North Carolina. It may be built by others as I share my journey and others help me test and improve components.

    I am currently using a Arduino Nano R3 and an Arduino Mega 2560. These microcontrollers are doing things like band selection, band switching, RX/TX switching, VFO specification, and TFT operation display. I am designing things using KiCAD and sending Gerber/Drill files to a manufacturer for my PCBs.

    I have a lot to do but I still wanted to share my activity and let you know that I have a plan and it’s happening. The Freedom7 HF Transceiver is being designed, built, and tested one piece at a time, just like Johnny Cash sang about. Please consider leaving your email on the main page. I’m gathering emails so I gauge interest in what I’m doing. And, don’t tell everyone, but I might offer kits for the components after I get a better cadence on progress.

    My About page provides the background of my project, the Freedom7 HF Transceiver.

    If this story resonates, comments are welcome. You can also reach me at david [at] kr4bad-dot-communications. no com

    And if you believe understanding matters more than black boxes, you can subscribe to my WordPress https://kr4bad.com/?subscribe=1.

    73 KR4BAD David