For reference purposes only.
When testing radios, we often need dummy loads, but compact, low-power dummy loads with SMA connectors are relatively rare and typically cost over 40 yuan. Here's an open-source dummy load PCB design recommended for HAM radio enthusiasts who enjoy DIY or have a very limited budget (total cost approximately 5 yuan: resistors 3 yuan, SMA connector 2 yuan, PCB 0 yuan – assuming you can utilize free prototyping services).
https://bbs.elecfans.com/jishu_2296148_1_1.html
We can see that it essentially consists of 20 x 2512-sized, 1kΩ resistors connected in parallel, resulting in an equivalent impedance of 50Ω. Theoretically, these 2512-sized resistors can handle a power of up to 20W, which is more than sufficient for debugging QRP (low-power radio) equipment. The component size allows for soldering using either solder paste and a hot plate or a soldering iron with solder wire. Across the entire shortwave band, its SWR (Standing Wave Ratio) remains around 1.1.
If you're not confident in the cooling, you can even cut a piece of silicone thermal pad and attach it, along with copper heat sinks on the four corners of the PCB.
(For commercial use, dummy loads typically use ceramic resistors (which provide good heat dissipation), are soldered to a heatsink metal plate, or utilize finned heat sinks (as shown in the figure). High-power RF dummy loads also require internal filling with cooling oil.)
It's important to note that SMA connectors should be purchased in the "foot-mount" type (e.g., SMA-KHD, which has a SMA foot mount with an external thread and internal hole), so they can be securely soldered onto the three pads on the PCB board.
Additionally, at higher frequencies, both carbon film resistors and patch resistors exhibit significant parasitic effects that cannot be ignored. The PCB design must also consider the dielectric constant: longer trace lengths will cause impedance differences. Therefore, it is not recommended to use them in frequency bands above 30 MHz. If you require operation at higher frequencies, it is better to purchase commercially available products.
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