Why do we need a substrate like FR4 or Rogers RT Duroid?

Hello,

This is mostly because of production issues (price, good reproductivity). FR4 isn't the best material for patch antennas (it is a lossy dielectric). Many patch antennas would have better efficiency (Prad/Pinput) when there is air under it instead of FR4.

To reduce the size of a patch one may add dielectric material between the patch and ground. This is cheaper then folding the edges down and under the patch to get capacitive loading.

And...

As for my experience, when you just fab you antenna on Rogers duroid llike 5880, the surface might not be smooth, for the difference of warpage between you metal patch and substrate.

But for FR4, it is the cheapest and nearly hardest materials that can be found. So maybe, when applying the FR4, the surface of you antenna might be more soomth.

But why is dielectric needed?

Hello,

The dielectric constant is a material property, so when you use FR4 as a substrate (cheap, good available, every one can process it), you get about 4 as relative dielectric constant.

When you use a Teflon substrate (better and more consistent properties for microwave, but more expensive an mechanically weak), you get about 2.2 as relative dielectric constant. When you decide to use ceramic (GPS patch antennas), you get other electrical and mechanical properties.

For good antenna operation, it isn't needed as you can also make a patch in air where you only have a support in the middle (where the E-field strength has its minimum for a half wave resonant patch). When you put dielectric under the patch, you have to reduce the size to get it in resonance again.

Small detail: the additional displacement current that you get in the dielectric, does radiate (as it is real vibration of charge), so the dielectric does affect the radiation pattern under zero elevation (for a horizontal patch).