UV-K5 Band Modding: Contemplating the "27 MHz" Band Lowering Projects

I've been sitting on my to-be-modded UV-K5 for a while now in hopes of having low-cost access to more bands. In my diversion from this project, others have put together some good work, though at the low end of the range. The ludwich66 GitHub wiki tracking UV-K5 projects covers them on the Hardware_Mods_27Mhz page. The two sets of PDFs were uploaded back in July 2024. Yeah, I'm behind a bit. Let's dive into what's been implemented.

The first is the N5SIM document advertises "the transmission of 10/11/12 meter" signals, but tries to be extremely emphatic about legal operation and all that. I'd say please tune to 29.6 MHz and expect a fair few will be tuned about 2.5 MHz lower. Oh well. As for the technical portion of the document, it's a straightforward guide with parts list and construction steps. The receiver modifications to operate top HF are given without any particular test procedure. The transmitter suggests disabling the final transistor and checking the driver output before moving along. Reconstructing the PA filter is started by clearing both the VHF and UHF filter structures, then rebuilding with three custom coils. The schematic given helps a lot with figuring what the image is actually showing. Overall, a good set of instructions for the decently solder-experienced, but doesn't include the technical discussion that presumably happened alongside the development.

The second is PU4WLG's document that's a pretty decent machine translation of the original Portuguese version. This one gives a solid introduction to the UV-K5 capabilities and limitations of the target modifications particularly with regard to interoperation with proper CB radios providing AM and sideband modes rather than the custom firmware hacks for FM to work "well enough" with those modes when attempted by the BK4819 chip. Not really subtle on the application.

Still, the guide starts from assuming you already have a wideband firmware modded radio and demonstrates why hardware modifications are required: dumping harmonics of 1W each into broadcast FM radio and airband when trying to jump on a CB channel is a poor plan. A reason to get caught, bad for the radio/antenna, and bad for serving communication from the UV-K5 in the first place. So follows a guide and, critically, additional VNA traces illustrating the before/after intent for the modifications on the new low-band receive. The VHF bank is replaced for 28 MHz and the UHF bank is expanded for listening 210-600 MHz. The performance of the UHF bank modification is also illustrated from VNA traces; that'll be bad for selectivity and that's how being broadband goes with these components. Unfortunately there's not any specific information on the instrumentation technique. More of that might be apparent in the YouTube (WanderLucioGomes) videos of the modifications and performance examination.

The fun bit is stepping through the transmit side analysis. This starts with tuning down the driver, with spectrum analyzer results, to operate successfully at 27 MHz rather than VHF+. That also comes with disabling the UHF transmit path operation since those components are being scavenged and the goal of the mod isn't to transmit above 400 MHz. The process and schematic/values are very similar to the other document. Such collaboration is a good sign that folks are remaining connected, even if I am not. Either that or there's only so many ways to feed an amp transistor and filter its output! The results are also illustrated with spectrum and VNA traces demonstrated the intended performance is achieved.

Of course, trying to tune in 900 and 1300 MHz is a bit more of a challenge, but one worth pursing. More to consider on my end for that!