Arduino UNO voltage drop
You are doing something wrong.
The DC input on your UNO needs more than 5V.  There is a voltage regulator between the DC input jack and the 5V line that you are measuring.  That regulator requires more than 5V in so that it can provide 5V out.
When you plug in the USB cable, the 5V from the USB cable goes directly to 5V line of the UNO.  The regulator is then disconnected from the 5V line.
You need a power supply with more than 5V in order to use the DC input jack.
The Arduino UNO page say you need at least 7V, though no more than 12V in to the DC input jack.
The Arduino UNO uses anNCP1117 regulator.  The 5V output version of the NCP1117 has a minimum input requirement of 6.5V.