The antennae available for LoRaWAN are typically ruby ducky/whip, ground plane, and fiberglass antenna. We decided to measure the SWRs using low-cost network analyzer with NWT4000-1 and then using low-cost RTL SDR from RTL-SDR.COM.
NWT4000-1 Network analyzer (you could find the instrument in aliexpress.com/ebay.com/taobao.com) is a good choice for a hobbyist with a tight budget(when it is difficult to afford a Vector network analyzer)to find SWR. An SWR bridge (a special type of directional coupler) is used in this experiment. One could purchase such SWR bridge from aliexpress.com/ ebay.com/taobao.com.
If you own an RTL SDR, you could measure SWR of the antenna up to upper L band using noise source/tracking generator and SWR bridge or directional coupler.
In India, the LORAWAN frequency range is 865 to 867 Mhz.
– Impedance: 50 Ohm
– Directivity: >30dB
– Frequency range: 2MHz-1500MHz
– Insertion Loss: 13dB
Specification of NWT4000-1(Spectrum/Network Analyzer)
– Model: NWT4000
– Frequency Range: 138MHz-4.4GHz (without interruption), stepping 1kHz
– RF Output power: -5dBm
– Maximum input signal strength: -5dBm
– Scan speed: 4 seconds 550 points
– Interface: USB, SMA
– Power Supply: DC12V / 0.5A
– Size: 118 * 64.5 * 23.5mm
– Weight: about 300 grams with all accessories
The Output of NWT4000-1 is connected RF IN of SWR bridge, the input of NWT4000-1 is connected to RF OUT of the SWR bridge and DUT (Device under test) is connected to the antenna connector.
The spectrum analyzer software could be downloaded from http://www.dl4jal.eu/
Rubber ducky Antenna, 210 mm
Lowest SWR 1.3 happens at 888 Mhz. At 868 MHz, SWR is around 1.98
|Maximum input power (W)||50|
|Operating temperature (℃)||-40～70|
|Maximum input power (W)||50|
|Lighting Protection||Direct Ground|
|Connector||TNC (or according to the customer)|
|Dimension (mm)||Ø115×225 mm|
|Length of Cable(m)||3.0|
|Reflector material||Stainless steel|
|Operating temperature (℃)||-50～70|
Another DIY way to calculate SWR
In this method, we used RTL SDR, Wideband Noise Generator, and SWR Bridge. The signal is provided by wideband noise generator and return power is received by the RTL SDR.
- Noise Source/Tracking Generator
- 10-12V Power Supply
- RTL-SDR Dongle(From RTL-SDR Blog)
- SWR Bridge (20-23$)
- Directional Coupler 20dB or 10dB (3$)
Noise Generator Specification
Supply voltage: + 12V
Current: > 500MA
Frequency: > 1MHz
Output signal coverage 1MHz ~ 3.5GHz
1.5G/48DB 2G/38DB 2.5G/30DB,3G/27DB,3.5G/20DB, reference ENR:0.1G/60DB 0.5G/55DB,1G/52DB,
1.5G/48DB 2G/38DB 2.5G/30DB,3G/27DB,3.5G/20DB,
We used rtl_power with the RTL-SDR Panorama (http://sourceforge.net/projects/guiforrtlpower/ ) GUI to record and graph a frequency sweep over the effective range of the antenna.
The SWR calculation is carried out by referring to excel sheet provided in https://www.rtl-sdr.com/rtl-sdr-tutorial-measuring-filter-characteristics-and-antenna-vswr-with-an-rtl-sdr-and-noise-source/
- Scan the complete range without connecting the antenna and measure the return loss.
- Scan the complete range connecting the antenna and measure the return loss.
Directional Coupler as SWR Bridge
This setup is the cheapest setup of all, one could get a directional coupler used mostly in GSM network as low as 3-12$, I got mine in 3$(approximately 220 Indian Rupees). These couplers are far better than TV tap and easier to find.
Specification of Directional Coupler used in this experiment
- Specification Frequency Range (MHz) 698 – 2700
- Coupling(dB) 20
- Insertion Loss(dB) ≤0.15
- In-Band Ripple ≤0.3
- VSWR ≤1.3
- Impedance 50 Ohms
- IMD (dBc) ≤-140 @ +43dBm×2
- Connectors N –Female
- Power Handling (Watts) 200 Peak Power(W) 1000
- Temperature (℃) -35 to +70
- Humidity 95%
- Environmental Specs indoor
Directional Coupler Output Port: Noise Generator/Tracking Generator
Directional Coupler Input Port: Antenna
Directional Coupler Coupling Port: RTL SDR Receiver
The directional coupler is needed to be used in reverse order to get the return power. In our experiment, we have used 20dB, however, 10 dB coupler could be a better choice.
10dB directional coupler
20dB directional coupler
The RTL SDR, along with noise source(11-25$) and direction coupler(3-15$) would be the cheapest setup to measure SWR of the different antennae.