Tuesday, July 15, 2014

Testing 433Mhz RF antennas with RTL-SDR

A couple months ago I picked up a RTL2832U dongle to use with SDR#.  I've been testing 433Mhz RF modules, and wanted to figure out what kind of wire antenna works best.

Antenna theory is rather complicated, and designing an efficient antenna involves a number of factors including matching the output impedance of the transmitter.  Since I don't have detailed specs on the RF transmitter modules, I decided to try a couple different antenna designs, and use RTL-SDR to measure their performance.

I started with a ~16.5cm (6.5" for those who are stuck on imperial measurements) long piece of 24awg copper wire (from a spool of ethernet cable).  One-quarter wavelength would be 17.3cm (300m/433.9Mhz), however a resonant quarter-wave monopole antenna is supposed to be slightly shorter.  I started up SDR#, turned off AGC and set the gain fixed at 12.5dB.  The signal peaked at almost -10db:

The next thing I tried was coiling the antenna around a pen in order to make it a helical antenna.  This made the performance a lot (>10dB) worse:

I also tried a couple uncommon variations like a loop and bowtie antenna.  All were worse than the monopole.

The last thing I tried was a dipole, by adding another 16.5cm piece of wire soldered to the ground pin on the module.This gave the best performance of all, nearly 10dB better than the monopole.  An impedance-matched half-wave dipole is supposed to have about 3dB wrose gain than a quarter-wave monopole.  Given the improvement, I suspect the output impedance on the 433Mhz transmit modules is much closer to the ~70Ohm impedance of a half-wave dipole than it is to the ~35Ohm impedance of a quarter-wave monopole.

Have any other ideas on how to improve the antenna design?  Leave a comment.

Last minute update: I tried a 1/4-wave monopole wire antenna on the RTL dongle, and got 2-3dB better signal reception at 433Mhz than the stock antenna.  I tried a full-wave (69cm) wire antenna, and it performed better than the stock antenna, but slightly worse than the 1/4-wave monopole.
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4 comments:

  1. zoobabJuly 22, 2014 at 4:10 AM

    Try to build a yagi:
    http://boingboing.net/2003/06/07/floppy-disk-wifi-ant.html

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  2. UnknownDecember 8, 2015 at 11:36 PM

    This comment has been removed by a blog administrator.

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  3. Rob WilkinsonJanuary 14, 2016 at 11:32 AM

    Ralph

    Brilliant experiment. Thankyou for posting.
    Do you think that the gauge and number of strands of the wire have much effect?
    Did the board have an inductive coil at the base of the antenna, i.e. 3 turns of copper of 5mm diameter?
    The length of this coil presumably should be taken into account.

    HopeRF has a helpful primer on RF antennas
    http://www.hoperf.com/upload/rf/ANTENNAS_MODULE.pdf

    Rob

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    Replies
    1. Ralph DoncasterJanuary 14, 2016 at 1:35 PM

      The gauge and # of strands should make no difference for a low-power 433Mhz device.
      I have some receivers with the coil at the antenna base, and I have a few superhet receivers (no tuning required) that don't. Interesting question as to whether the coil impacts the antenna length. Maybe worth doing some more testing with different lengths (starting long and snipping off a few mm at a time).

      Delete

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