Impuestos

With the same hardware needed to characterize filters shown in the tutorial above, and an additional piece of hardware called a directional coupler the standing wave ratio (SWR) of antennas can also be measured. The SWR of an antenna determines where the antenna is resonant and is important for tuning it for the frequency you are interested in listening to. See the Antenna Guide section in this book for more information on SWR.

EQUIPMENT

For measuring the VSWR of an antenna you will need some of the same equipment needed for characterizing filters, as well as a directional coupler. BG7TBL Noise Source ($29.50 USD). Available on eBay and Aliexpress, just search for “DC12V 0.2A Noise Source for External Tracking Source” 12V Power Supply that can supply at least 0.2A for powering the noise source (~$5 USD)

RTL-SDR Dongle (~$10-$25 USD)

MiniCircuits ZFDC-20-5 Directional Coupler (~$38 USD). You can usually find a Minicircuits directional coupler on eBay by searching for “Minicircuits Directional Coupler”.

MiniCircuits directional couplers can often be found used, but in good condition on eBay for about half price. In our experiments we actually used a MiniCircuits ZFDC-15-5 coupler with SMA connectors, but they appear to be sold out on eBay at the time this was written. The ZFDC-20-5 should work just as well however as will similarly specced directional couplers. Try and find a directional coupler that works over the frequencies that you are interested in. A good video showing use of a directional coupler in a similar experiment and explaining a bit about how one works can be found here https://www.youtube.com/watch?v=iBK9ZIx9YaY .

MEASURING THE VSWR OF AN ANTENNA TUTORIAL

To measure the VSWR of an antenna with the noise source and RTL-SDR we will use the directional coupler in reverse. Here we connect the noise source to the output of the coupler, the antenna to the input of the coupler and the RTL- SDR dongle to the coupling (CPL) port of the coupler. This way power from the noise source should be reflected from the antenna into the RTL-SDR. The image below shows how to connect the noise source, directional coupler and RTL-SDR together.

Open the RTL-SDR Panorama GUI and enter the frequency range of which you’d like to check the VSWR of the antenna. Also, set the resolution to 1M and gain to 0.

First do a sweep with the antenna disconnected from the coupler. After the sweep is done, stop RTL-SDR Panorama and rename the scan.csv file which is found in the same folder as rtlpan.exe to swr.csv. Note that after clicking on stop in RTL-SDR Panorama you will need to wait for the current sweep to stop before you can rename the csv file.

Secondly do the same sweep but with the antenna connected. Now open the swr.csv file and the latest scan.csv files in Excel or any similar program and copy the power data (the last column) from scan.csv into an empty column in swr.csv.

Now subtract the values in the first power column (power with antenna disconnected) from the values in the second power column (power with antenna connected) that you have just copied over. This will give you the return loss. If you get any negative values in the return loss column you may need to adjust the values so that the negative values are forced to be a small positive value like 0.0001. In Excel this can be done by creating a new column and using the max function on the return loss column, such as “=max(0.0001, J2)”. Negative values sometimes appear because the measurement accuracy of this method is not perfect.

The VSWR can then be calculated from the return loss (RL) with the following equation.

Calculate the VSWR in another column. The image below shows what your completed Excel sheet might look like.

Now plot the SWR using a scatter plot with frequency on the x-axis (horizontal) and SWR on the y-axis (vertical). Change the vertical axis to a logarithmic scale for a better view.

Below we show the SWR plot of a monopole antenna that is 23 cm long placed on a 20 cm diameter cookie tin that is acting as a ground plane. A perfect ground plane antenna with a 23 cm long whip should be resonant at about 310 MHz. However, because the ground plane for this antenna is only 20 cm, the SWR minimum point is pushed up to around 355 MHz. At the minimum point the SWR is about 1.15. Using a larger cookie tin would cause the minimum SWR point to approach 310 MHz.

Here we compare our results with a SWR simulation of our 23 cm monopole antenna and 20 cm ground plane made in the antenna modeling software called 4NEC2. See the 4NEC2 tutorial section for more information on this Antenna modeling software. We can see that the real world SWR plot approximately matches the simulation. Unmodeled factors such as coax cable, the type of connectors used, the local environment as well as the RTL-SDR measurement accuracy can explain some of the discrepancies.

Many people incorrectly believe that the SWR value does not matter for receive only antennas. This misconception probably comes from the fact that a poor SWR value on a transmitting antenna could destroy the transmitter, but a poor SWR value on a receiving antenna poses no such threat. However, while it won’t destroy your radio, a poor SWR value will still significantly impact reception. By using the calculator available at

http://www.csgnetwork.com/vswrlosscalc.html , it is possible to determine the amount of signal loss you can expect with a particular SWR value. At a SWR of 1.5 we can expect a very small loss of about 0.177 dB, whilst an SWR value of 10 gives a 4.807 dB loss.

DOWNLOADS

Below are some example spreadsheets in .xls and .ods formats that we used in the above tutorial. You can use them as a starting point for your own measurements. Measuring Antenna SWR:

http://www.rtl-sdr.com/wp-content/uploads/2015/03/Measure_SWR.xls http://www.rtl-sdr.com/wp-content/uploads/2015/03/Measure_SWR.ods