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[TowerTalk] N6LF Study of Losses in THHN Wire

To: towertalk reflector <towertalk@contesting.com>
Subject: [TowerTalk] N6LF Study of Losses in THHN Wire
From: Jim Brown <jim@audiosystemsgroup.com>
Reply-to: jim@audiosystemsgroup.com
Date: Tue, 19 Sep 2017 12:41:58 -0700
List-post: <mailto:towertalk@contesting.com>
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.

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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.

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73, Jim K9YC

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