I have a 50 foot aluminum mast on my property, guyed at 32 feet and
mounted on a 6" x 6" pressure treated pine post. I put a 15 foot
stinger of aluminum tubing at the top to give it a height of 65 feet
and hung an arm on it at 50 feet that extends out 3 feet.  This holds
the vertical part of my 160 m. inverted L,  which is positioned over a
ground system of 101 radials.  It's resonant frequency (where X is
minimal) is around 1840.  The resistive component at that point is
around 11 ohms.  This is with the mast grounded.  I can also put the
mast on a feedline using the wood post as a base insulator and use it
as a 1/4 w. vertical on 75 meters.  You can co-locate two verticals
with a shared ground system and it works okay.   I've always grounded
the 75 m. vertical  when using the inverted L on 160 because that
results in the low feedpoint impedance at the inverted L I'd expect to
see on 160.   It's been a long time and I don't clearly remember what
happens when I float the 75 m. vertical above ground except that the
160 m. impedance changes (goes up I think).  I should experiment with
someone within groundwave distance and see if grounding/floating the
support mast changes the efficiency of the inverted L.


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