LoRa/LoRaWAN tutorial 41: Dipole Antenna

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   / mobilefish  

This is part 41 of the LoRa/LoRaWAN tutorial.

In this tutorial I will explain what a dipole antenna is.

A dipole antenna is the simplest and most widely used class of antenna.
A dipole antenna consists of two identical conductive elements such as copper wires, rods or tubes.
The two elements contribute to the radiation.
If the total length of the dipole is ½ wavelength, than each element has a length of a ¼ wavelength.
When we speak of a ½λ dipole antenna, the total length of the antenna is a ½λ.

A nice animation where you can see the radiation pattern vs current distribution when you increase the dipole length:
   • 3D Dipole Radiation Pattern vs Current Dis...  

The equation to calculate the wavelength:
c = λ x f
c = speed of light = 299792458 m/s
λ = wavelength in m
f = frequency in Hz
If antenna f = 868 MHz:
λ = c / f = 299792458 / 868000000 = 0.34538 m = 345.38 mm

As explained earlier a ½λ dipole is a good antenna.
This means both elements of the antenna are 0.25 λ in length.
If f = 868 MHz and λ = 345.38 mm, L= 0.5 x 345.38 = 172.69 mm
Do not forget the velocity factor.
If the dipole is made of stainless steel (VF=0.9): l = 0.9 x 172.69 = 155 mm

A ½λ dipole antenna has a power gain of 1.64 (or 2.15 dBi) over an isotropic antenna (see tutorial 39).
At its feed point ½λ dipole antenna has an impedance consisting of 73 Ω resistance (R) and a reactance of 42.5 Ω (X).

4NEC2 card deck:
https://www.mobilefish.com/download/l...
Note:
Initially the length was set to 0.155 m then I used the 4NEC2 optimising functionality to improve the design.
The optimised length = 0.160 m.

The 4NEC2 model element length = 160 mm.
The real ½λ dipole antenna length = 146 mm (= 2 x 73 mm).
I used the N1201SA Vector Impedance Analyser to tune the antenna.
Why this discrepancy?
The real antenna is not 100% accurately modelled in the 4NEC2 program.
Think of the gap between the elements, terminal with screws, the type N female chassis.
All these influences the antenna behaviour.

Please be aware that the generated radiation patterns are merely a ROUGH indication how the real dipole antenna behaves.
As explained earlier the real ½λ dipole is not 100% accurately modelled.
If you want accurate radiation patterns of real antennas than the antenna radiation patterns measurements should be performed in an anechoic chamber.

The ½λ dipole antenna performance is compared with a sleeve dipole antenna.
More information about sleeve dipole antennas, see tutorial 43.
For this test I am using the end node and antenna C as demonstrated in tutorial 33.
More information about this end node, see:
https://www.mobilefish.com/developer/...
The end node uses the MCCI LoRaWAN LMIC Library:
https://github.com/mcci-catena/arduin...
The end node uses the following sketch:
https://www.mobilefish.com/download/l...

I have NOT modified the end node transmission power when using the ½λ dipole antenna.
In my area there are several gateways and I know that these gateways, which are connected to The Things Network, can receive my transmitted data.
The ½λ dipole antenna is attached to an end node at location A and transmitted data and I have done the same with a sleeve dipole antenna.
In both cases two messages per minute were transmitted.
The logged data can be found at:https://www.mobilefish.com/download/l...

Several nearby gateways were able to receive my transmitted sensor data, see:
https://drive.google.com/open?id=18SK...

I have conducted another test whereby the ½λ dipole antenna is directly connected to the end device. No coax cable is used.
Again the same tests were conducted using the same ½λ dipole antenna and sleeve dipole antenna at location A.
Two messages per minute were transmitted and both logged data can be found at:
https://www.mobilefish.com/download/l...
Note: The tests were conducted approximately 1.5 months later.

The RF coaxial cable with type N male plug right angle to SMA male connector is probably not working correctly.
I have replaced it with another setup.

Looking at the results I can conclude that my self build ½λ dipole antenna performs the same as the purchased sleeve dipole antenna.

Check out all my other LoRa/LoRaWAN tutorial videos:
   • LoRa/LoRaWAN tutorials  

Subscribe to my YouTube channel:
   / @mobilefish  

The presentation used in this video tutorial can be found at:
https://www.mobilefish.com/developer/...

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