Long time readers of this reflector likely recall assertions that the
use of THHN wire (insulated "house wire") for wire antennas and radials
can cause increased losses and reduced antenna efficiency, especially as
the insulation degrades from UV exposure. ARRL Antenna Book and Handbook
contributor Rduy Severns, N6LF, recently published an extensive study of
this issue that combined both lab work and modeling. It's on his website
in the link below.
http://rudys.typepad.com/files/insulated-wire-and-antennas.pdf
The entire article is easy and fast reading, and, like all of Rudy's
work, is well worth the time. Key summary paragraphs are quoted below.
The issue with verticals with sparse radial systems (Conclusions
paragraph) is unequal current distribution between the radials. Graphs
show the difference to be less than 0.5 dB between THHN and bare copper.
Rudy has studied this issue extensively, and published a great 2-part
piece in QEX reporting on that work.
= = = = = = =
Mechanical issues
Leaving the insulation on the wire increases the weight of the wire and,
if there is icing, the increased diameter could lead to even more
weight. From a corrosion point of view insulated radials are very likely
to last longer than bare radials, especially for ground surface or
buried radials.
Conclusions
Looking at all these considerations it's clear that in general leaving
the insulation on the wire is pretty benign and loss due to the
insulation, either new or old, does not seem
to be significant. However, it was shown that in certain cases, mostly
related to GP verticals with sparse radial systems there can be a
substantial impact. However, that
really occurs only when very few radials are used. These problems tend
to go away as
the radial count is increased to twelve or more for elevated radials and
16-20 for
ground surface or buried radials.
= = = = = =
73, Jim K9YC
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