So where do the Portable ADS-B solutions come into play? Bottom line is this... The purpose of the Portable Type solutions are to provide a means of using/displaying up to three (3) types of data (FIS-B Weather, ADS-B Air-to-Air traffic, BUT beware....understand what you are buying.
Points on the Subject:
To date, Portable solutions will ONLY provide you with two out of the three (FIS-B Weather and ADS-B Air-to-Air Traffic.) UNLESS your aircraft is otherwise certified ADS-B equipped. Why? See "How it Works" later in this article.
Air-to-Air Traffic will ONLY be available IF you are in contact with a Mode S w/ES Transponder or UAT ADS-B Out equipped aircraft.
Some portable systems reference traffic targets to GPS Altitude rater than Pressure Altitude. The industry standard for TIS-A, TIS-B, TAS, and TCAS is to reference target aircraft to pressure altitude. If your choice in a portable solution uses GPS altitude, this can translate into the altitude of your target aircraft being as much as 700ft. off. Future upgrades may resolve this issue. Caution: If your portable solution uses a built in sensor to determine pressure altitude, use of this device in a pressurized aircraft simply won’t work.
How it works
I found this illustration of Sporty's publication. It provides a good visual for illustrating the conversation though its not complete for the sake of conversation.
The illustration specifies a Stratus portable system. From a principle of operation, this is consistent with other portable systems such as the Garmin GDL39. It is important to remember that though each system operates under the same basic premise, there are other operational differences to consider for your application. Do your homework and be satisfied!
The key to understanding the illustrations is to notice the shaded "Hockey Puck" or as I like to call it "The Cylinder of Surveillance". What is it? This shaded area is the shield of data that is sent to an ADS-B Out equipped aircraft from a Ground Station. Note: You don't ever actually "See" the Cylinder; only the traffic within it.