Message-ID: <008f01bebe88$b387be40$b337f482@s-257519>
From: "Johan Bodin" <sm6lkm.jbeab@swipnet.se>
To: rsgb_lf_group@blacksheep.org
Subject: LF: SV: More on Rr
Date: Thu, 24 Jun 1999 23:28:56 +0200
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Hello,

Mike Dennison wrote:


>Further to my note on Radiation Resistance formulas, the highly 
>respected 160m expert ON4UN uses 1450 instead of "160 (or 60) 
>multiplied by pi squared" in his excellent book Low Band DXing. 
>This figure is lower than 160 x pi sq (1580) but nowhere near as low 
>as 60 x pi sq (592). I assume that he must have done some 
>practical measurements which gave him a better figure than that 
>calculated in the 1920s.


Most antennas for the 160m band are much bigger than typical
136 kHz antennas in terms of wavelength. The current distribution
is nearly sinusoidal but on 136 kHz only the last tiny bit of the "sine"
is actually present on the antenna wire so it can be approximated
as a linearly decreasing current (toward antenna end). This may
explain the difference.

The formula 1600 * L^2 / W^2 is valid but L is the so called effective
height, not the physical heigth of the wire. In case of a pure vertical
without top load, the effective height is physical height / 2 because
the average current along the wire is feed current / 2.

I agree with Rik Strobbe's explanation except for:

  >simplified for 136.75kHz this means that Ra (milli-Ohm) = 0.082 x l(meter)

I would suggest:

  Ra (milli-Ohm) = 0.082 x l ^2 (meter squared)

73 de Johan, SM6LKM