Topband: Vertical Antenna Feedpoint Above Ground - Part Deux

[Donald Chester]

>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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