The FAA explicitly prohibits the operation of portable ADS-B Out systems in any aircraft. This is for a variety of reasons.  Some of the major ones are listed below:

1. Use of internal antennas do not provide adequate view of both the sky for good GPS position data and the ground stations and other aircraft due to the airframe and fuselage blocking line of sight of ADS-B Out broadcasts.
2. The portable unit must use the same pressure altitude sensor used by the transponder and most portables do not connect to or extract the information from the transponder. This is necessary for the ground station to correlate a transponder target with the ADS-B Out target.
3. The transponder code must be manually entered into the portable ADS-B Out unit. This is a required item that must be broadcast by the ADS-B Out system and is used for target correlation.
4. Two parameters that must be set to zero in a portable and self identify the unit as a portable indicate to TSO Certified ADS-B In display systems that the targets are not trustworthy and may not be displayed to the pilot. This effectively makes these portable units a cloaking device and they are a hazard to all certified receivers according to the FAA.
5. Since these devices are portable and the user can enter any ID, an ID not matching the N-number can be wrong and not match the aircraft. A portable can be moved between various aircraft and it would be easy to forget to set the proper identifier to match the aircraft being flown. This can cause misidentification when ATC services are being used.

The FAA has announced they plan on changing the way that portables are treated by the ground stations starting late this year. The FAA currently provides these systems with “Client” services, which means that they wake up the ground station to broadcast TISB traffic around the location of the portable client. The client itself is not provided with a TISB for its location, after all the client should know where it is.  In October of 2015 the FAA plans to add a TISB for these types of clients so that they will no longer be invisible to certified systems. This will have the undesirable effect of presenting two targets for non certified receivers, that is the ADS-B Out target and the ground station generated TISB target. For aircraft that have one of these units they will see a ghost from the TISB.

Finally, the FAA will no longer provide this class of ADS-B Out emitters to receive client status after the beginning of 2016. This will have the effect of ignoring these ADS-B Out portables and eliminating the incentive to keep them broadcasting. The FAA may also request operators to cease and desist using these devices.