[TowerTalk] Testing Yagi's Near Ground

[Bob Selbrede, K6ZZ]

Hello TT'ers,

How far above ground (poor to average quality ground) does
an HF Yagi need to be before ground effects no longer
significantly effect SWR measurements?  Is there a good rule
of thumb that works most of the time?  I assembled a
Cushcraft XM-240 this past weekend and would like to
check/verify the settings before installing it and thus
avoid repeated trips up and down the tram!  If I tune it
while mounted on a 30' tower trailer how much is the SWR
likely to change when I run it up to the 115' level?

As a data point, I set it for CW (7025 KHz) and checked it
on the saw horses.  The minimum SWR point was about 6390 KHz
which sounds about right from past experience.  I was able
to prop the driven element up at about 15' and the minimum
SWR point went up to about 6950 KHz.

TIA, Bob K6ZZ