Replacing rtl-sdr coaxial connector with sma to improve sensitivity [en]

Some rtl-sdr dongles especially older ones are shipped with a coaxial connector (PAL). The coaxial connector is commonly used in home appliances and is typically used for 50 or 75 ohms characteristic impedance. The connector works fairly well considering its construction and production costs however most coaxial connectors suffer high signal loss at high frequencies. The exact characteristic plot for a given coaxial connector differs greatly by brand, type and even per unit. By replacing the rtl-sdr coaxial connector with a sma connector we can expect improvements for received signal strength at higher frequencies.
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Buying a proper SMA connector

There are a lot of SMA connectors available on the popular online shopping platform: ebay. Most of the connectors on ebay had plastic legs in stead of brass or other solid metals. Recognizing if you have a SMA connector with plastic or metal legs is rather hard but it can be done by the color and specularity of the material. If you have the opportunity to buy the connector at a local electronics store or other reputable online seller. Be Assured its probably in your best interest to pay slightly more and get a proper SMA connector.

fake versus real sma for rtl-sdr
Not covered is how to remove the coaxial (pal) connector from the rtl-sdr and soldering the SMA connector one. There are a ton of online tutorials both video and text out their covering how to solder dip and smd parts. Before replacing the connector a method of testing has to be determined. This testing method will show the differences in signal loss between the two connectors. In the end the signal to noise ratio for three different frequencies where tested. Both before and after replacing the connector.

  • Frequency – Protocol name – Protocol technique
  • 169.650 MHZ – FLEX – Binary FSK
  • 391.075 MHZ – TETRA – TDMA PSK
  • 930.650 MHZ – GSM(GPRS) – GMSK

Read more about these protocols

Measurements and results

Each measurement was made with a sample and hold approach where the highest values where kept for 30 seconds continuous and then stored in a table. A screenshot is made of the measurements for reference so you can see the difference between the noise floor and the signal. Sadly the screenshots of the measurements with coaxial connector where lost. All screenshots are those of measurements with the new sma connector.

169.650 Mhz
rtl-sdr sma sdrsharp 169mhz
391.075 Mhz
rtl-sdr sma sdrsharp 391mhz
930.650 Mhz
rtl-sdr sma sdrsharp 930mhz
Finally the measured results are put into a table which clearly shows the difference between the two connectors. At the first two measured frequencies the results are negligible. The final frequencies however shows a clear difference between the sma and coaxial connector.
rtl-sdr sma connector result table
In conclusion replacing the coaxial connector on a rtl-sdr has its purposes. It increases the sensitivity at high frequencies and you get a more common connector to attach your antenna’s with. Be sure to get a legitimate sma connector however as the performance of fake, plastic sma connectors can’t be guaranteed.

References

  1. Charateristic impedance
  2. SMA
  3. FLEX
  4. TETRA
  5. GPRS
  6. FSK
  7. TDMA
  8. PSK
  9. GMSK

Gallery

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