Topband: Measuring field strength

[Dan Zimmerman N3OX]

I've had lots of good responses regarding putting radials out to the
front yard.  I'm going to run radials around to the front of the
house, possibly by putting a perimeter wire around the house.

I'd like to make some field strength measurements while I'm working on
it; I'm not worried about absolute measurements; I just want to make
some relative ones.

There's a park across the road from me... I think I can get up to 5
wavelengths away from my antenna.  (I keep thinking that I should have
a north-pointed beverage along the river but I doubt the Maryland
National Capital Parks and Planning Commision would appreciate that)
My gut feeling is that 5 wavelengths is sufficient for good relative
comparisons.  I doubt I can get further away.  Closer would be more
convenient for me.  How far away is far enough?

Dan