Fri, 27 Apr 2001 21:10:12 +0100
Please could you offer advice as to why my 100 ft vertical =
doesn't seem to work.
It is self supporting made of steel pipe sections starting at six inch =
diameter at the base reducing down to half inch for the top section. The =
bottom forty inches are cemented into a plastic oil drum which is =
cemented in a holel in the ground. There are four small pipes welded at =
right angles to the forty inch section in the drum to add mechanical =
strength. The drum is filled with a sand cement mix. The mast is =
completely welded at each reduction in diameter. The insulation from =
ground of the mast is 200 meg Ohm. The earth system at the base is a =
stainless steel tank offering about seven square yards of surface area. =
At present there are ten half wave radials laying on the ground. The =
mast is fed by a soldered connection just above the concrete base, not =
forty inches below the concrete level right at the mast bottom.
On the MFJ analyser connected direct to the vertical and ground I get a =
very weak dip about 2.6 MHz.
With a copper long wire of the same length rising from the base to a =
tree at 40 ft I get a very pronounced dip at 2.0 MHz with a 1 to 1 swr.
The performance of the vertical is just about useless, with my Carolina =
windom at seventy feet I could work most of what I heard without too =
much problem. With the vertical I have called loads of stations with out =
I know the resistivity of copper and aluminium is lower but I still =
expected reasonable results.=20
Even though its D.C. resistance is 200 M ohm above ground, is the volume =
of concrete in the drum affecting things. Capacity to ground is not =
Or is it that Im feeding it at ground level and not forty inches below =
at the drum bottom.
Is it Skin affect, should I run a copper cable up the mast.
There is a flange just above ground level that I could insulate the mast =
Are shunt fed towers less efficient than a vertical that is completely =
above and insulated from ground.
With a stainless steel drum as described ( about 7 square yards ) buried =
at the base. How far away would I have to bury another drum to achieve =
the best decrease in resistance. Or is it best to keep to earth rods say =
2 yards long buried every 3 yards apart.
Many Thanks In advance
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