RF Transmitter/Receiver Distance

Thats a very good point.
If you can position the 'whip' above or below the tank.

I have redesigned the PCB to accept a 1/4 wave whip or a folded dipole. I will secure the antenna of choice to the component labeled "Ant".

1) When I try the folded dipole, how long should the length labeled "X" be?

2) Should 10 gauge copper wire work for making the folded whip?

**broken link removed**

Unlikely that your transmitter will accommodate loads from 70Ω (simple dipole) to 300Ω (folded dipole) without adjustment (or a matching section).

Put it another way: a transmitter designed to match a 50Ω antenna will be ok with a 70Ω dipole, but much less so trying to drive a 300Ω folded dipole.

I think it would still work, however the above is correct the impedance would make the range pants, unless you had a matching transformer or in this case an un/un which is getting silly, Nige's comments were spot on.