RF design is a black art for several main reasons: textbooks, Smith charts, CAD tools and components. Academic texts on the topic are expensive, full of mathematical formalism and written by men with no practical experience who cannot lay out an RF board themselves. They often explain matching networks with Smith charts which are a useful but abstruse and esoteric tool best avoided by beginners. Many discuss component design which is not interesting to the radio hobbyist or novice RF engineer. Some even discuss microstrip component design which is useless when wavelengths are longer such as those in the ham bands.
Books written for the ARRL by engineers with practical experience often include plans that require rare or discontinued through-hole (THT) components that are increasingly difficult to find due to the popularity of surface mount technology (SMT). They discuss construction methods such as “ugly construction,” “Manhattan Breadboarding,” “Perfboard,” and “Dead Bug” which require THT components and result in circuits that are not repeatable, manufacturable, or robust-enough for field deployment. They allude to the increasing popularity of PCB design but don’t explain it.
CAD tools are another hurdle for hobbyists and novices. There are many CAD tools on the market for designing circuits and laying them out on a PCB. Most are a prohibitively-expensive way of designing a board that should cost less than $10 to manufacture. Most CAD tools are difficult to use, even if you pay a lot of money for them.
Finally, there is the issue of the components themselves that I alluded to above. THT is becoming rare. SMT is common. This makes it cheaper and state-of-the-art. For example, this Freescale SMD power MOSFET is only $3 for 3 Watts which is enough power to reach quite a distance with code or PSK. Through-hole technologies at the same or greater wattage are about 10x the cost. When tinkering, you’re going to damage components. Cheaper components are better for tinkering.
Taken together, these difficulties are preventing novice engineers and amateurs from learning RF design and homebrewing radios. There are definitely amateurs learning RF PCB design, though. This presentation is by Michael Ossman, the founder and lead engineer at Great Scott Gadgets which makes the HackRF One. He has no engineering degree but learned how to do RF design on his own. Bear in mind, as you watch, that few electrical engineers can lay out a board like he can. He has really developed a great process for doing so.
Here are a few cautions not covered in the video:
- Use SMT/SMD package sizes that you can confidently see with low-cost magnification and solder. This takes practice. Don’t pick microscopic package sizes that require “reverse flow” soldering or some expensive process.
- Complete transceiver ICs on Digikey, Maxim, Mouser, and NXP are usually for commercial bands. I looked at HackRF One board and the Maxim transceiver ICs do not cover the lower bands. Hams operating on HF will have to use microcontrollers, ADCs, DACs, DDSs and other components to develop their own transceivers. This is really the goal of this – and subsequent – articles.
- Choose CAD tools wisely. A Chinese guy on the GreatScott YouTube channel demonstrates the free Chinese CAD tool that sources Chinese parts. Cheap! But you get what you pay for and who knows when the next trade war will be? Michael Ossman bellyached about the current trade war on his blog. Maybe he shouldn’t have sourced his parts and boards from the other side of the planet. Also, EasyEDA is a web-based application running (likely) on Chinese servers. If you use it, you will give the Chinese your design and they’ll list it on Amazon or AliBaba for peanuts before you are able to sell one at full price if that’s your goal. Use EAGLE instead. This is a cheap circuit design and layout tool used by most hobbyists and universities which saves all designs on your computer with universal manufac. You don’t need the auto-router. Learn here. Trace sizes for specific RF frequencies can be calculated using an online calculator like this one.
- Ossman made a couple of math errors which are corrected in the comments.
Since I am trying to learn this myself to make an HF data packet transceiver, I’m start by learning EAGLE and try to layout a microcontroller and an Analog Devices DDS on a board to generate some PSK and FSK. Future progress will be posted as it happens.