>From: Larry Burke WI5A <lburke@wt.net>
>
>"Is there a distance -- minimum, optimal or maximum --- that a vertical's
>feed point should be from the earth for a ground-mounted radial system?"
My tuning unit is in a protective box about 6' away from the base insulator
and tower base. 120 radials terminate to a heavy gauge 1 1/2" copper
strap that surrounds the concrete tower base. The tuning unit is connected
to the radial bus with more of the ground strap. The tower base is
connected to the tuning unit using a piece of #4 solid copper, which runs
parallel to the ground strap, about 2 feet above it. The coax line comes
into the protective box with the tuning unit components. I never have
decected any sign of rf in the ground side of any of the tuning components.
The 140' run of coax to the shack is buried.
I measured the base Z of the antenna at the tower and at tuning unit. There
was a measureable difference, so the 6' run of copper does have some effect
on antenna impedance as seen by the L-network, but to no noticeabe ill
effect. I double checked impedance measeruments using an rf ammeter and
known rf power level, and the readings were consistent.
My tuning unit box is sitting on concrete blocks which raise it about 6" off
the ground. I don't think it would have any effect if I raised it off the
ground and let the copper wire and strap slope down to the radial bus.
I have seen AM broadcast towers in swampy areas with the base insulator
mounted on a tower section to raise it 6-10' off the ground during times of
flooding, and they just ran ground strap down to the radias system while
placing the tuner at the same elevation as the base insulater. No doubt
this will work better with a near-perfect radial system, with at least 60
quarter-wave ground level radials.
An idea I have thought of in case someone had access to a tower that
happened to be too tall to make an effective low angle vertical radiator for
160, for example a tall broadcast or communications tower. Why not feed it
as a unipole down to a point 1/4 WL below the top of the tower, and attach
an elevated radial system to the tower at that point, and let the radials
slope towards the ground. You would need a large number of radials to make
an effective ground screen to isolate the active part of the tower from the
rest of the tower below the radial system. The idea would be for it to work
like a VHF ground plane, but scaled up to 160m. Just wonder if anyone has
ever tried this.
Don K4KYV
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