copy a formula down a column without changing a variable cell location

Hi,
I have a list of numbers in column A. I have a variable in Cell F1. I have created a formula in cell B1. I would like to drag and copy the formula in B1 down to match all the numbers in column A.

I know that I can select the outside lower right edge of the cell and holding CTRL down, drag to copy but when I do this the variable F1 gets changed to F2, F3 and so on in each of the cells of column B.
How can I make that variable stay as F1?

simple example:
A....B...................................... F
1....sum(A1 * F1).....................10
2....sum(A2 * F2)
3....sum(A3 * F3)
4....sum(A3 * F4)

all the F values in the above formulas should stay as F1.

I appreciate any help,

Bradley

Hi,

Just make F1 an absolute reference with $F$1

Rgds