What Would be Best Way to Make Background Uniform?

No complicated masks or selections should be needed here (and most likely that will just end up looking fake).

Just a couple of Selective Color adjustments with soft gradients:

This is a 20% opacity gradient overlay of blue sky color 1A56B1. These gradient overlay settings will leave the horizon alone and lighten the high sky. Useful if the deep blue or gradient is distracting from the antenna.

I'd do that way:

1. from main menu choose select/color range and clic on the lighter area of the sky to have your selection. Adjust fuziness accordingly to select the lightest part, then pess OK.

2. from main menu choose layer/adjustment layer/curves, then in the Properties panel clic on the half of the diagonal and drag it down to let the middle tones darker.

Personally I quite like the photo as is.

However you asked a question so I suppose I should answer and be helpful.

This really depends how long you want to spend and how uniform it should be.

The quickest way would be similar to Bahaar Khan​ and use a layer mask and a separate layer with a uniform blue pasted on.

BUT then I would take this one step further and use some adjustment layers with layer masks to target the bottom of the sky and try to bring that blue closer to the uniform blue.

Particularly I would try using a hue/saturation layer as this is what your camera sensor created...

Again, I actually like the hue difference and the gradient it creates but this is how I would do it - to start with anyway.

Best,

EW