At the beginning of this month we posted a review by Bjarne Mjelde who reviewed the MW DXing performance of the Airspy HF+. One thing he found was that by performing a simple capacitor bypass modification, the performance of the HF+ below 15 MHz could be significantly improved. Over on the SWLing blog we've seen a post that gives a bit more information for those wanting to perform the R3 bypass modification on their HF+'s. There Yousseff, head of the Airspy team wrote:
During the early phases of the design R3 was a place holder for a 0 ohms resistor that allows experimenters to customize the input impedance. For example:
- A 300 pF capacitor will naturally filter the LW/MW bands for better performance in the HAM bands
- A 10µH inductor would allow the use of electrically short antennas (E-Field probes) for MW and LW
- A short (or high value capacitor) would get you the nominal 50 ohms impedance over the entire band, but then it’s the responsibility of the user to make sure his antenna has the right gain at the right band
- A custom filter can also be inserted between the SMA and the tuner block if so desired.
R3 and the nearby resistors have been intentionally left outside of the RF shield, and their size was picked to be big enough to allow anyone to play with them. You will notice the size difference with the rest of the components.
In general, unless one knows what he’s doing, it’s not recommended to alter a working system. “If it’s working, don’t fix it”. But, we are hobbyists, and not doing so leaves an uncomfortable feeling of something unachieved. Most brands addressing the hobby market leave some tweaks and even label them in the PCB.
The main purpose of the HF+ is the best possible performance on HF at an affordable price. This is to incite HAMs to get started with this wonderful technology while using an SDR that isn’t worse than their existing analog rig.
The MW/LW/VLF crowd may have slightly different requirements, but that can be addressed by shorting a resistor.