I have seen JB use this for purposes of testing antenna gain/range.
I am looking to build the same, I have all of the components except the middle piece.
Can someone please identify what component the green arrow is pointing to?  Also, where could I purchase this part?
It should be a female end on both ends if I am not mistaken.

Looks like an attenuator, usually they are sold in fixed values (3db, 6db, 8db....) and are meant for balancing a system. We use them a lot in RF cable TV systems where we need to pad the input to an amplifier so it's not overdriven. In this case it might be used in a similar way to not overdrive that meter. I've had sat meters in the past that if you didn't lower the input signal some you'd max out the meter which prevents you from properly peaking the dish because the meter is maxed out.

It's an attenuator.

Just search Goggle for "SMA Inline Attenuator" and you will hopefully find an appropriate vendor in your country (or in China).

Thanks for the prompt responses guys!
Being that when I search for them, I see numerous types - all with different fixed values (dB) available to purchase.
How do I know which is the appropriate type to buy?

I will be attaching this to the ImmersionRC RF PowerMeter v2, I have attached the tech specs of the RF PowerMeter if that helps.

The ImmersionRC RF Power Meter v2 has a built in attenuator but you still cannot feed more than 30 dBm into the meter otherwise you risk damaging it. If the device you are going to be testing outputs close to (or more than) 30 dBm you just need to use an appropriate attenuator to bring the level down below that, so it is all going to depend on what device you are actually measuring and what RF power level it is set to.

I'll be testing the output of the Radiomaster Ranger 2.4GHz ELRS Module in this instance; which will be connected to the back of my Radiomaster TX16S.
But I'd also just like to understand this process and have the appropriate tools in the event I'd like to conduct tests in the future.

https://www.radiomasterrc.com/collection...lrs-module

So the RadioMaster Ranger modules are capable of outputting 1W of power (30dBm) or up to 2W of power (33dBm) for the flagship module with the OLED screen on the back after applying the 2W power unlock hack.

A 3dB, 6dB, or 10dB attenuator should be suitable:

• A 3dB attenuator will divide the input power by 2. This will reduce a 30dBm input down to 15dBm.
  
• A 6dB attenuator will reduce the input power by 4. This will reduce a 30dBm input down to to 7.5dBm.
  
• A 10dB attenuator will reduce the input power by 10. This will reduce a 30dBm input down to to 3dBm.
  

The ImmersionRC RF Power Meter v2 has an external attenuation correction setting ("Attn") in its menu which enables you to tell it how much external attenuation you are using so it will then calculate the actual power of the device being tested by taking into account the external attenuation that has been applied.