“Shut up and build a radio”! OK – we get it. Perhaps the growing level of AI slop -- and sheer number of presenters just talking , or reposting, or reacting to others‘ content may have peeved you dear emailer. It’s true – we also feel sad that so little actual design & construction (or radio listening) gets done nowadays. We also agree that while talk & discourse generally feels good --- we also need some action. Actually going and doing – not just yakking about building gear (with or without hand-wringing) seems like a good plan to us!
Rest assured, the emailer was a polite viewer from Singapore. His comment was in the vein of "Shut Up 'n Play Yer Guitar" a triad of albums released by Frank Zappa in 1981.
Well, we have a BIG secret – this post contains stuff that other radio homebrewers do NOT want you to know about --- oops, sorry – that is what you are supposed to say when you have a YouTube channel and seek many clicks. Our bad. It’s so tempting to be a normie.
For decades, we’ve explored radios, guitar amps and other gear at the component level. Like you wrote – we don’t build many radios. So Simple Radio staff aren’t really radio builders. instead, we prefer to explore the various circuits that make up radios in an attempt to try to understand how or why that particular circuit got designed -- rather than building complete radios.
The 3 main reasons include:
We already have way too many radios and lack the space to store any more. We’re in the purge or declutter phase of life. Remember, when you die -- someone else has to get rid of all your
junkstuff.We prefer to focus on designing, or hopefully understanding all the little circuits that make up radios for the joy & sake of learning
We do this fun
Having said all that – yep, we will comply with your wish. We’ll try to make a complete radio for our viewers. A simple superhet based on the SC-7 from Wes, W7ZOI in his book EMRFD. Wes, W7ZOI. Let’s go!
Above — All superhet builds begin with a bag of crystals -- 9 MHz. Three for the crystal ladder filter and 1 for the BFO.
Above — Front end. This radio goes on 14 MHz with a 9 MHz IF and 5 VDC. We no longer use 9 volt batteries for anything and feel glad we've got low-cost, rechargable 5 volt power supplies in abundance. We still have to do experiments to figure out how much current to run in the unbalanced mixer. A tuned mixer tank helps to reduce some of the LO energy at the mixer output node.
The mixer is the W7ZOI replacement for the 2-gate MOSFET popular in the 1970s ( still used in some analog VHF receivers today, but mostly as a 2nd IF. e.g. the ICOM ICA200 Aviation transceiver).
Above — Design of the input bandpass filter. Most builders make filters with software since the math is often complex. We used a combination of hand calculations, software and bench experiments to derive the filter. Each tank needs tuning and that is it. Should be reproducable -- or design your own. No need to match the tank to the JFET gate node as the 22K resistor does this in earnest. The JFET mixer is temporarily configured as a source follower -- this provides a low impedance output so you can stick it in a 50 Ω sweep system.
Above — An SMA output port was temporarily soldered on this long breadboard. DC voltage got applied via aligator clips and the filter was swept and tuned. This long, unshielded board in a bit unwieldy, but we got it done!
Above — A photo of the tracking generator - spectrum analyzer screen. It was a super hot day and our nearby central air conditioner's high speed fan was spewing some lower HF interference. Normally, we shut off this HVAC noise source during bench experiments, but knowing it is not in the area of interest [14 MHz], and to avoid complaints from our family + several overheated visitors -- we chose to keep the AC on.
Above — The filter was peaked & looked OK. The insertion loss = 3.7 dB. We removed the 10 pF series cap and tried a trimmer in its place + additionally, built a series/parallel capacitor network using series + shunt (parallel) trimmer caps. With tweakable caps, insertion losses of <=3 dB were realized, however, this makes the design less reproducable since now 3 or 4 caps tune the filter instead of just the original 2 tank trimmer capacitors. Simple radios incur compromises. We went back to the 10p series cap as shown. This reasonably matches the filter input to 50 Ω and does not load the first tank excessively.
The original JFET can now be configured as a mixer per the top schematic. Then mixer experiments are on. OK -- we are trying to make a radio. Thanks to you!
Soundtrack of these experiments
We always experiment with music on. Creative music spawns creative bench work. For the past few years, to support independent or non-major label bands/musicians, we listen to Indie music. We also never stopped buying CDs and do not stream. We do not listen to AI music.
BAND: The Kills , ALBUM: Blood Pressures from 2011. We are die hard The Kills fans.
