# ai7sg.com


## Posts

- [2 meter DF board](https://ai7sg.com/index.php/2026/03/05/2-meter-df-board/): There was a delay getting my 20 meter polar modulation prototype back, so I decided to design a 2-channel phase coherent direction finding system for 2 meters.I decided to see just how inexpensively I could do it. The answer kind of surprised me. $4.51 in parts, plus my trusty Pico2!I wasn’t about to build it like that. It was a pretty bad design. But there was room to triple my BOM and make it decent. So that’s what I did. SMA inputs. ESD / RF limiter. Band pass filter to reject FM & TV. RF preamps. Dual conversion superhet design. […]
- [LTSpice Polar Modulation Schematic](https://ai7sg.com/index.php/2025/12/27/ltspice-polar-modulation-schematic/): I recently posted a Youtube video about the radio I’m building, and I promised to share the LTSpice file. You’ll have to install LTSpice and the EPC GAN library to run it. Here is the video: https://www.youtube.com/watch?v=zcixl9Ds5KQ Here is the LTSpice Schematic: Please note the included window function. When you take the FFT of the windowed RF waveform you’ll get a lot more dynamic range. Let me know if you’d like a deep dive on the circuit. I could put a video together.
- [My New SDR-like 20m Polar Modulated Radio is Almost Here!](https://ai7sg.com/index.php/2025/07/04/my-new-sdr-like-20m-polar-modulated-radio-is-almost-here/): I obviously haven’t been posting much lately, but I have been working on a new radio design for about the past 12 months. I switched from class K amplifier to class D for more efficiency at lower voltages, and to take advantage of a nifty TI part, the LMG1210. https://www.ti.com/product/LMG1210 This should enable me to output about 320 watts peak off a single 6S LiPo battery. I coughed up the tariffs and ordered it from JLCPCB back in June. It should be here next week! Interestingly enough, this polar modulation technique I’ve been using for years is back in the […]
- [New Antenna: Simple 20m horizontal half-wave dipole](https://ai7sg.com/index.php/2024/05/26/new-antenna-simple-20m-horizontal-half-wave-dipole/): I signed up at the local club to be an activator for lctota.org, so my 2 element beam pointed at New Zealand wasn’t really ideal. I really needed an antenna that would cover as much of the U.S. as possible, so I went with a simple horizontal dipole facing East / West. I have a great 3rd floor balcony over a downhill slope. I’m using my telescoping mast to get the bulk of the antenna in the air, and started with aluminum tape over 3′ chimney brush rods for the far 1/4 wave (coax and a common mode choke for […]
- [New Spring Project - a travelling companion for my IC-7300](https://ai7sg.com/index.php/2024/04/15/new-spring-project-a-travelling-companion-for-my-ic-7300/): I think work is going to be too busy for my grand project. But a fun, simple project for this spring: add a power supply output (13.8V 20A) to my 20 meter amp for my IC-7300, and package it up in an attractive, slim box. Maybe double the output power while I’m at it, just for fun. Perfect for Parks on the air, or even SOTA if I’m feeling ambitious and get back in shape…
- [New Project: My Summer Radio](https://ai7sg.com/index.php/2024/03/30/new-project-my-summer-radio/): I think I’ve settled on my next project. I’m getting sick of random polarization fading ruining a good conversation. So I think my next project will be a 10 meter radio set up to support circular polarization, battery powered with a fiber optic interface so there are no cables to mess with my antenna pattern. I’m thinking I’ll add the ability to add additional coherent channels because phased arrays sound fun as well. I’ll keep you posted on the progress!
- [What is a Class F amplifier and why does it matter?](https://ai7sg.com/index.php/2024/02/19/what-is-a-class-f-amplifier-and-why-does-it-matter/): The short answer is efficiency. A true linear amplifier is limited in its efficiency. A realistic number is 60%, meaning if your output is 300 watts, you’ve got 200 watts of heat to get rid of. A class F amplifier is NOT a linear amplifier. The transistor is driven with closer to a square wave than a sine wave, so it acts as a switch, not an amplifier. This means there is no physical limit to its efficiency, but a realistic number might be 90%. So for that same 300 watt output, there is now only 33 watts of waste […]
- [My Amplifier Works!](https://ai7sg.com/index.php/2024/02/18/my-amplifier-works/): The tests yesterday went well enough that I was comfortable getting on the air. So far the GaN FET is holding strong! I think the tweaks to the circuits improved efficiency (around 90% PAE I think) and removed the risk of a 200 amp current surge as it turned off. The Class F portion of the amp is super sensitive to SWR, and it’s windy, so I’m watching my antenna carefully. The Class H envelope-tracking-power-supply-modulating portion appears to have enough bandwidth that my audio is good. That’s a bit of a relief. Also, the 60V 20A supply I’m using to […]
- [Class H & F Amplifier Progress](https://ai7sg.com/index.php/2024/02/04/class-h-f-amplifier-progress/): The surface mount parts are on! I still need to do all the through hole parts, and then test the darn thing… more updates later today. Update 1: Through hole parts are on, mechanicals look good. Need to add heat pipes from the FETs still, but the fan is nice and quiet (which was the goal). So far two errors, one fixed. First, I inverted a signal I shouldn’t have, so had to flip it. Fortunately I had the right part on hand. Fixed! Second, I need the power supply to be responsive, but it looks like I over-tweaked the […]
- [My meander Yagi turned into a meander Moxon](https://ai7sg.com/index.php/2024/02/04/my-meander-yagi-turned-into-a-meander-moxon/): As expected, the antenna did not perform as modeled on my balcony. I think my F/B ratio is not great, but I seem to have a strong beam towards CA / NM / AZ. Once I mount it above the railing, I should be able to turn it , but I’m guessing South will always be the dominant direction. I really need a drone now to map out my antenna pattern… The impedance was much higher than expected. I don’t know why, but even without a balun (just a coax choke) it’s in range of the IC-7300 internal tuner across […]
- [Meander Yagi V2](https://ai7sg.com/index.php/2024/01/28/meander-yagi-v2/): I took some measurements of my balcony, and modified my EZNEC file, adding some straight sections at the end. The goal is to make it easy to disassemble and tune, so I bought 4 cheap telescoping sections (max length 74 cm) from Amazon (4 pack for $12). EZNEC says the reflector should only be 4 inches longer than the driven element. The idea is to have 4 identical “meander” sections of 2 inch aluminum tape wrapped around a 5 foot by 2 foot PVC frame. The reflector gets 2 and the driven element gets 2. Then there will be a […]
- [Learning EZNEC - My Meander Yagi](https://ai7sg.com/index.php/2024/01/28/learning-eznec-my-meander-yagi/): Years ago, (probably the late ’90s) I played with EZNEC and haven’t really used it much… until today. I was trying to find information on reduced size yagis using meander techniques. There really wasn’t much out there, so I tried my own. Ignore the W8JK name – I guess I forgot to update the comments on my antenna. Here is the file if you want to look at it or make tweaks: My goal was to get something I could build in a 0.2 x 0.2 wavelength planar shape that had some directivity. It was pretty clear that to get […]
- [My Amplifier Project: Class H/F Single-Band High Efficiency Amp](https://ai7sg.com/index.php/2024/01/15/my-amplifier-project-class-h-f-single-band-high-efficiency-amp/): The idea is to design a clean switch-mode power supply that can envelope-track (class H) a SSB or digital narrow-band signal, and use it to drive a Class F ultra-low-loss amplifier with some extra harmonic filtering. Simulations look pretty good, except for how sensitive it is to reactive loads. Targeting 20 meters, 1 kW, >85% efficient for my first design. Looking at https://gansystems.com/gan-transistors/gs66516t/ for the RF FET, cooled with a heat pipe and a small CPU fan at half power. The power supply is a special blend of circuitry built around this: https://www.analog.com/en/products/ltc7891.html (The power supply FETs will be cooled […]
- [My Second Antenna Project: The Mag Loop](https://ai7sg.com/index.php/2024/01/15/my-second-antenna-project-the-mag-loop/): For most of my career, the goal has been isolating from magnetic fields coupling where they shouldn’t, so building something to intentionally radiate was a refreshing change of pace. Design I already had some 4 inch aluminum tape and 1 inch copper tape. My first thought was to cover some PVC pipes, but I didn’t have them and didn’t feel like going into town. I also wasn’t sure what I’d do for a tuning capacitor. I’ve always thought corrugated plastic (your typical yard sign) had some potential so I decided to see how it worked structurally for a magnetic transmitting […]
- [My Favorite Free Tools](https://ai7sg.com/index.php/2024/01/14/my-favorite-free-tools/): If you’ve ever tried to simulate a circuit using nothing but paper and pencil, or tried to match your antenna to 50 ohms by making an educated guess, you’ve probably found yourself looking for some of these tools. Everyone has their favorite tools. Here’s my list! Impedance Matching https://www.analog.com/en/design-center/interactive-design-tools/rf-impedance-matching-calculator.html Just a handy, free calculator when you don’t want to do the math. When I wanted to match to my mag loop to 50 ohms with a capacitor instead of small loop, this is what I used. https://www.will-kelsey.com/smith_chart/ A really easy-to-use Smith Chart matching tool. Circuit Simulation https://www.analog.com/en/design-center/design-tools-and-calculators/ltspice-simulator.html This software is […]
- [My First Antenna Project](https://ai7sg.com/index.php/2024/01/14/a-journey-into-the-world-of-radios-amplifiers-antennas-and-more-3/): I have a big attic. When we bought this house, and realized just *how much* attic space there was, I knew I would find something creative to do with the space. This was before I got into ham radio, but the giant attic (in spite of the rain gutters messing with my antenna patterns) has definitely been the most convenient place to explore antenna ideas. Currently my attic is home to a 40 meter full wave loop that resonates at 10, 20, and 40, but more about that later. Getting Started: A NanoVNA, wire, duct tape, and a dream My […]
- [Introduction](https://ai7sg.com/index.php/2024/01/14/a-journey-into-the-world-of-amateur-radio-exploring-radios-amplifiers-antennas-and-more-2/): Welcome, fellow Amateur Radio enthusiast! As an RF Engineer and new ham, I am excited to share my ideas, experiments, and mistakes I make along the way with you through this blog, as I have learned so much from the community in my first few months. Whether you are just starting out or have 50 years of experience, I hope to offer some interesting, creative ideas, with a goal of making the hobby more approachable for newcomers, and maybe eventually more affordable for all! If nothing else, I will try to keep a running list of the tools I find […]


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