Yes, the frequency is in mhz, and the formula is the same as for a 1/4 wave 
antenna, as that is exactly what you are building here anyway.

Thus, for a 40 meter counterpoise, divide 234 by 7.150, or whatever 
frequency within 40 meters you want.

A quarter wave is fairly broad, so cut for the middle of whatever band, and 
go with it.

In order to "tune" the counterpoise, you would need access to a meter which 
would measure rf current in the wire, and then trim for maximum current.

That's a bunch of trouble, especially for us. I've never known of anyone who 
found it necessary to tune one beyond the formula length. As I said in my 
first message, I found that if you are anywhere near the resonant length, 
you will notice the tuning improvement.




Mike Duke, K5XU
American Council of Blind Radio Amateurs