For those who know me, it is nothing new that among other things I am somewhat of a radio technologies enthusiast as well.
In the current state of affairs, it is an even more interesting field, considering how much it can be done without a significant financial investment. Radio technologies have evolved tremendously in a short time span. Some of that evolution can be directly attributed to the large demand caused by the mobile communications industry, but also by the computer and digital broadcast industry. All of these called for the development of highly integrated semiconductors capable of many functions, from the analog frontend and digital processing, to applications.
With an increased supply of generic digital radio components, it became possible for RF enthusiasts to think of designing low cost Software Defined Radios (SDR), and repurposing certain radio devices by taking advantage of the underlying SDR architecture.
But until a certain time, SDRs were a niche product, and among the first devices made available to the public at large, was the USRP (Universal Software Radio Peripheral). This was in fact a family of devices created by Ettus Research (a subsidiary of National Instruments), and these had (and still have) a modular design, where the user can select which TX and RX frontends are best suited for his application. These devices have very impressive specs, but its price range makes these somewhat inaccessible for some of the hobbyist community.
Soon after, digital video broadcast became a ubiquitous reality, leading to the appearance of cheap hardware designed for receiving and decoding such transmissions. The DVB-T USB dongles were among these devices. Serving primarily for capturing the DVB-T broadcasts in the personal computer, these became a precursor platform for much of the RF hacking and development activities that are present today:
Soon the RF hobbyist/hacking community realized how versatile these devices were, because of the broad spectral range (it can span from 15 MHz to 1766 MHz - not bad for a device which costs around 10 or 20 Euros).
By using a modified device driver, it became possible for applications to have full control of the tuner and ADC chip parameters. While the DSP heavy lifting has to be done on the host computer, these dongles provide the RF frontend and digital conversion required for the rest of the radio implementation to be fully done on the software side.
Some HAM oriented versions of these dongles have since popped up in the market. These share the same core hardware components, such as the popular Realtek RTL2832U chip and the R820T2 tuner, but add better frequency stability and some features such as a built-in bias tee (a circuit that enables power to be provided to external devices connected to the RF path, i.e. between the antenna and the RTL-SDR), and a switch allowing the tuner to be bypassed, and signals between DC and 30 MHz to be directly fed to the RTL-SDR chip. This in practice extends the band coverage to HF, albeit without any preselection and limited sensitivity.
This is however just an example, and today there are multiple projects and products providing equivalent or better SDR functions.
One example is the HacRF One. This is a device that gained some traction and popularity, and was developed as a fully open source project:
One of its main characteristics is the fact that it is also capable of transmission, albeit at relatively low TX levels (it can reach at most about 30 mW in some bands). Even though TX power varies greatly depending on the band, it can transmit in virtually all of the covered frequency bands (which spans from about 10 MHz to 6000 MHz - a reasonably impressive range).
It does have its shortcomings as well. The unimpressive TX output limits its use without additional hardware. Also (and more importantly) given its poor dynamic range (partly attributed to its 8-bit DAC/ADC chip) and filtering, it produces harmonics which become particularly relevant when an amplifier is used in front of the device. For RX, image rejection is also a thing, and sensitivity can be relatively poor - not because of insufficient RF amplification, but because of the lack of preselection which will easily saturate the amplifier stages with signals outside of the tuned frequency.
Nevertheless it is still a very practical device, given its feature set, and the fact that it is currently not very expensive. Depending on the site, it can be purchased for under 100 Euros.
Alternatively it can also be found as part of another product - the portapack: