[TowerTalk] 40 Meter Dipole Question

[WA3GIN in Alex. City, VA]

Question:

 

Installed a ½ wave 40 meter dipole 10ft above the penthouse roof which
is 410ft above sea level and 210ft above average terrain. The dipole is
almost perfectly horizontal.  

 

We get a dip on the antenna analyzer around 7Mhz but the SWR is 7.5:1 at
the dip and just goes up from there.  I presume there is something going
on with the roof which is concrete covered with 4” Styrofoam and a layer
of creek stones.  

 

Thinking about installing a lumped inductance across the feed-point and
using the center conductor of the 50 ohm coax to tap the coil where we
find a 1.1:1 SWR.

 

Looking for other ideas and suggestions as to how best to match the
input impedance.

 

73,

dave

wa3gin