LoRa/LoRaWAN tutorial 35: Coax Cables and Coaxial Connectors

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This is part 35 of the LoRa/LoRaWAN tutorial.

In this video series different topics will be explained which will help you to understand LoRa/LoRaWAN.
It is recommended to watch each video sequentially as I may refer to certain LoRa/LoRaWAN topics explained earlier.

In this tutorial I will explain:
- The different types of coaxial cables and connectors.
- What characteristic impedance is and how it is calculated.
- What the impact is of cable losses.

To transport radio frequency signals, a coaxial cable or coax cable is used.
Coax cables uses the RG ratings and RG stands for Radio Guide.
The number after RG refers to different cable specifications.
For example RG58, RG174, etc.

The higher the RG number, the thinner the central conductor core is.
The suffix U for Universal means for general utility use, for example RG 174/U.
A coax cable (or any type of transmission line) impedance is called characteristic impedance and usually written as Z0 and cannot be measured by an ohmmeter.
A transmission line characteristic impedance is measured by an instrument called the time domain reflectometer or an oscilloscope.
This impedance is a measure of resistance to the flow of electrical energy.

There are two main types of coaxial cables, ones with an impedance of 75 Ω and ones with an impedance of 50 Ω.
In general 50 Ω coax cables are used for data communications (LoRa, WiFi, etc.) or amateur radio.
In general 75 Ω coax cables are used for digital audio or video applications.

The coax cable impedance remains constant regardless of the length of the cable.

A signal travelling thru a coax cable losses power, also known as attenuation.
This signal power loss is measured in decibels per meter (dB/m).
These losses are mainly caused by:
- The conductor, in the form of ohmic losses.
Think of impurities in the conductor.
- The dielectric touching the conductor.
The dielectric absorb some of the energy transported by the conductor.

To avoid signal loss:
Always keep the coax cable as short as possible and preferably connect the antenna directly to the device.
Minimise the number of connectors.
As the gateway should be 50 Ω, always use 50 Ω coax cables otherwise this will result in impedance mismatch thus a bad VSWR.

Coaxial connectors are used to connect coax cables to other devices and maintain the cable’s shielding.
There are two coaxial connectors types: male and female.
A male connector (aka plug) has a metal pin which protrude from the center and a female connector (aka jack) has a recessed hole to receive the pin.
There are several coaxial connector types. In this tutorial only two types will be discussed: type N connectors and SMA connectors.

Type N connectors are threaded connectors and are larger, tougher and can withstand abuse compared to SMA connectors.
Type N connectors are available with 50 Ω and 75 Ω impedance.
When using a thicker coax cable, than type N connector is the best choice.
In general type N connectors are not waterproof.

SMA (SubMiniature version A) connectors are connector interfaces for coaxial cables with screw type coupling mechanism.
The connector has a 50 Ω impedance.
SMA connectors are smaller in size and are used together with smaller size coax cables.
In general SMA connectors are not waterproof.

There are two types of SMA connectors (technically they behave the same):
A standard polarity SMA male or SMA female connector: SMA male / SMA female
A Reverse Polarity SMA male or SMA female connector: RP SMA male / RP SMA female
To determine which is which is a two step process:
1. Barrel with a thread inside: SMA male
Barrel with a thread outside: SMA female
2. If SMA male has a centre sleeve (hole): RP SMA male
If SMA female has a centre pin: RP SMA female

A standard polarity SMA male connector has a center pin surrounded by barrel with inside threads, and the standard SMA female connector has a center sleeve surrounded by a barrel with outside threads.
A reversed-polarity SMA male connector has a center sleeve surrounded by barrel with inside threads, and the reversed-polarity SMA female connector has a center pin surrounded by a barrel with outside threads.

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[HeliumDownunder]

SUPER Thank you so much for your information.

[blake_ultrasmash]

Hello Mobilefish, I just reviewed your excellent video on Coax Cables and Coaxial Connectors. It was very helpful. I have a question about amplifiers for long Lora coaxial cables to aerial. My brother has a farm. On the farm is a decommissioned SES (State Emergency Services) radio tower. The tower is structurally sound. It is 40m in height. There are existing aerials mounted at the top and part way down the tower. The Lora frequency in Australia is 915MHz. We intend to either reuse existing coax cable or install new RG50 cable with a 12db aerial. The tower has an equipment room next to the base of the structure. The Lora receiver will be installed in the equipment room so total coaxial length is estimated at 50m. Obviously the attenuation will completely degrade the signal so I'm thinking of preamplification (active unit solar powered at top of tower next to aerial) and probably again at the receiver end. I have not yet done the calculation as shown in your video but I was thinking of using two 20db amplifiers type TQP3M9037-LNA. 5VDC supplied. One at the top and the other at the bottom. Do you think this approach is ok and can you recommend a cost effective alternative amplifier? Eagerly awaiting your reply. Best regards, Brian

[shanebekker]

Very informative a usual. Keep these informative videos coming :-)

[vinhdat]

Thanks a lot for informative video.

How to calculate loss if we use sma to n adapter? Will the adapter affect VSWR?

Thanks a lot.

[StigBSivertsen]

Really good video 👍
Do you have any suggestions on where to buy good quality coax cable?

[gavlogazakias]

great work, could you give me some help. I have an antenna that is 3 dbi in 868 mhz. Can you please tell me how many dBis will my antenna gain be if I use LMR 400 8meters vs LMR 600 8 meters? Thank you very much in advance!

[zanetamatus]

Can I connect my device that has Lora Male connector to normal TV antenna?