[600MRG] Folded unipole antennas

[Rudy Severns]

The standard folded unipole antenna does not work when the height of the
antenna is substantially less than ¼-wave.  The reasons for this are
discussed in detail in chapter 4 (sections 4.2-4.6) in my antenna notes.
Even the presence of substantial top-loading does not fix the problem.  The
solution uses separate inductors in each downlead.  Again, this is all
carefully explained in my notes.

 

There have been a number of comments regarding sloping top-loading wires
reducing radiation resistance(Rr).  Sloping wires will have an opposing
vertical component of current which reduces the current moment of the
antenna, hence reducing Rr.  However, this effect is accompanied by a
reduction in tuning/loading inductor size and loss.  Initially the net
result is an improvement in efficiency as the sloping wires as made longer.
At some point, typically when 30-40% of the vertical is “shaded”, the
efficiency flattens out and then begins to decline as the wires are made
longer.  The flat region is very broad and not at all sensitive.  The
commercial boys worked all this out long ago.  On my web page there is an
article explaining this phenomena, look at my May/June 2013 QEX article,
Some Ideas For Short 160m Verticals.  The end of that article has an excerpt
from Carl Smith’s 1947 experimental exploration of this which clearly shows
the trade-offs at frequencies of interest to LF-MF operators.

 

GL and 73, Rudy N6LF