In yesterday's post I was babbling about a matchbox sized SSB Transceiver. I must have been suffering from the heat we have been experiencing in SoCal. Small is not always a good thing. Today's post will look at what shrinking the size down of a homebrew rig really means.
I can remember being issued a Motorla cell phone (1990's) as a part of my job. It was fat and in no way easily slipped into your pocket. Today's cell phones easily fit into any pocket in your tight designer jeans. Portability is the key operative word and so it would be with a ham rig, thus the shrink down syndrome.
Since we are focusing on a homebrew radio, the builder has total control over the size and final configuration.
My foray into creating a shirt pocket SSB transceiver started back in 2007. https://www.jessystems.com/morexcvrs.html
That project didn't end up being shirt pocket size but did provide some important baseline data. Somewhat reincarnated it is in a box 7X7X2 or almost 100 cubic inches. A later successful shirt pocket sized SSB Transceiver was 16 cubic inches or about 1/6 the size.
The shrinking process in part depends on technology advancement. The Xiao RP2040 is smaller than an Arduino Nano (and a lot more powerful). So that new tool takes up less space and that contributes to a reduction in size.
But let us explore some of the negatives about a size reduction.
1. I/O -- we need panel space for controls, displays, power input, mic and audio jacks, antennas and external controls. The I/O drove the 2X2 inch front and back panels. That was the smallest I could take it.
2. Following 1, is the minimum number of rig controls that you need. Two musts are audio and Tuning. For mic gain a 1/4 inch in diameter hole in the back panel gave access to a board mounted trimmer pot which was the mic gain.
3. Following 2, is that compromises were made in the design to eliminate circuitry and circuit features. No AGC circuitry was included but that drove the need for an audio gain control. I also eliminated a Receiver RF Amp stage (a mistake). Later technology enabled me to add a simple BFR106 RF amp stage on a small PC board .5X1 inch and literally a tenth of an inch high. It was tack soldered to the mainboard. A big improvement driven by SMD technology.
4. Method of building was dramatically changed. Thanks to a tip from KB1GMX, I used a base plate with a vertical spline so I could have circuits on the base plate and then small circuit boards vertically mounted to the spline. Pass through holes facilitated wiring of the various boards. This is creativity driven and more than a flat piece of pine board.
5. Heat and Unwanted Feedback were major issues! Jam stuff together and that reduces circuit cooling. Jam stuff together and you have set that stage for unwanted coupling and possibilities for oscillation. There was a lot of cut and try to get the right board placement.
6. Often forgotten in both commercial and homebrew rigs is the need for service. Where you have jam packed everything in every space -- when it comes time for servicing problems like subthreshold conduction, you are stuck. In fact, that reminds me of what happened with V1 of the Shirt Pocket Transceiver. I was probing with a screwdriver and because of everything being so close -- a giant short, a giant mushroom cloud over the bench and a giant number of smoked parts! It was not possible to rebuild it back to original and the salvaged pieces were built into another (larger) rig.
7. FFS (Fat Finger Syndrome) is an issue with small control knobs. The original Shirt Pocket rig used a crystal switched Super VXO tuned with a 10K pot. Separating stations took a deft hand not unlike that required for success in the back seat of the 57VW Beetle.
8. The add on devices. The 16 cubic inches had no room for a speaker and thus earphone operation or an external speaker to bring along. There was no nice digital display, so the dial scale markings were only approximate.
9. The tough question was it worth the shrink down. Yes, it is a novelty and has made contacts but a bit bigger would have been better.
TYGNYB.
73's
Pete N6QW