Back in the day, it was common for some or all of a ham radio
station to be homemade. Hams would buy electronic components and
assemble them into equipment. I once built an amateur radio TV
station that could transmit still images. I mostly used parts
salvaged from old junked TV sets. I never managed to contact
anyone with it, but it was fun anyway. My real station consisted
of a commercial Hallicrafters receiver and a Heathkit transmitter built
from a kit, plus some homemade odds and ends.
With the invention of integrated circuits and robots to assemble
circuit boards, it became rare to hear homemade equipment on the
air. Today most hams use commercial equipment. The main
exceptions are QRP operators. QRP means low power. These
hams still make equipment, either their own designs or kits, and
operate CW (morse code) at a few watts output.
The development of Digital Signal Processing (DSP) combined with modern integrated circuits has revolutionized electronics. Our analog signals are now digitized; that is, converted to numbers. The numbers can then be manipulated mathematically, and can be stored as numbers or even transmitted as numbers. Audio has gone digital. We now listen to CD's instead of LP's, and audio production is entirely a digital process that resembles word processing; think cut and paste. TV is about to go digital, as analog broadcasting is being replaced with digital broadcasting. But radio has been a holdout. Satellite radio is digital (actually, satellite-anything is digital). In Europe there is some digital radio. There is a digital standard for international broadcasters called Digital Radio Mondial (DRM), and a few broadcasters use it. Ham radio operators also do some digital transmitting. But today, virtually all radio, whether domestic or international is still analog.
Radio construction is mostly analog too. Transmitters and
receivers are make mostly from analog parts. A modern ham
transceiver (combination transmitter and receiver) is mostly analog,
may have a digital IF; that is, some digital filters near the end of
the signal path.
But today it is possible to construct radio transmitters and
receivers using mostly digital parts, and with a minimum of analog
parts. The antenna signal is converted to numbers as soon as
practical, and all subsequent processing is done using DSP. The
result is a Software Defined Radio. It is "software defined"
because if you want to change what the radio does, you just change the
software. The small analog bit probably stays the same.
Today hams can build a software defined radio fairly easily.
The digital part consists of crunching numbers, and a ham can use a
personal computer to do that. So with a personal computer plus
some software available for free on the
Internet, plus a few analog parts, hams can again build their own
equipment. I find this to be very exciting.
Let's take an example. We need an antenna to receive radio
signals. That part of radio never changes; a good antenna makes
any radio a lot better. We connect the antenna to a black box
that spits out numbers. We send the numbers to a personal
computer that uses DSP software to tune, filter and demodulate (convert
it to digital audio). The PC then sends the digital audio to its
sound card, and then to speakers. We then have a radio that does
most of the work of receiving a signal in a PC.
So what is in the black box? A minimal cost version uses the
PC sound card recording input as part of the black box. The
flow is antenna to a mixer (convert to near-audio frequency) to PC
sound card input for conversion to digital numbers. A version of
this radio for the 40 meter amateur band is the "Softrock-40" and costs
about $30. And performance is quite good. You can make this
receiver in a day. The equivalent analog version would have
dozens (maybe hundreds) of parts, and may not work as well.
Instead of a mixer and sound card, the black box could be a preamp
and a modern analog to digital converter (ADC) that
converts the antenna signals directly to numbers. It is not clear
to me which is better. I have both systems, so maybe I will find