sed is a text tool. By default it outputs one line of output per one line of input. While it's easy to inject newlines into the output (making it more lines), it's not that easy to get rid of newlines from the input because they don't really belong to lines being processed, they separate them.
info sed (emphasis mine):
sed operates by performing the following cycle on each lines of
sed reads one line from the input stream, removes any
trailing newline, and places it in the pattern space. Then commands
are executed; […].
There is however
Add a newline to the pattern space, then append the next line of input to the pattern space. If there is no more input then
sed exits without processing any more commands.
This allows you to process lines in pairs, this is what you need. Include
N in your last
sed like this:
sed -e 'N;s/dBm\n/dBm /'
Note it will only work if the entire input logically consists of lines you want to process pair by pair, i.e. this logical structure is OK:
while this one is not:
In this case
sed will pair
paired 1 with
line 2 etc., against your logic. Another bad example:
excessive line will be paired with
line 2. This will break the logic of consecutive pairs.
So you have to be careful. On the other hand, if only the lines are paired right, you don't have to worry which newlines you're replacing, because you cannot alter ones separating pairs anyway. For this reason including
dBm in your pattern is now completely unnecessary. Your last
sed may as well be:
sed -e 'N;s/\n/ /'
If you ever need more complex logic, see this answer. It uses
sed to concatenate lines according to their content.