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From: "Dave Brown" <tractorb@ihug.co.nz>
To: <rsgb_lf_group@blacksheep.org>
References: <BAY124-F3900F6885CA03CA4608264CF250@phx.gbl> <000901c7aa26$ecfaff60$82147ad5@w4o8m9> <000d01c7aa8c$6d692860$7c0d7ad5@w4o8m9> <466AB1CB.2060907@telia.com> <001301c7acd6$14f492e0$15337ad5@w4o8m9>
Date: Tue, 12 Jun 2007 22:54:30 +1200
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Subject: LF: Re: Tree current
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Status: O

 Very interesting experiment, Jim.
Did you try checking the coil output with it positioned alnogside one 
of the trees, as opposed to actually wrapped around it?(i.e. wrapped 
round an imaginary tree trunk right next to the real one)
 I used Rogowski coils quite a lot, back when I was routinely 
measuring longitudinal noise currents in multi-core cables and cable 
sheaths etc-very useful devices. We made ours up using some special 
coaxial delay line cable we got in from the USA. It was rather 
expensive stuff back then.  They (Rogowski coils) are readily 
available commercially these days, so it's easier to go out and just 
buy one. Good to hear you did the homebrew thing successfully though. 
It certainly would have been cheaper than buying one!
73
 Dave, ZL3FJ


----- Original Message ----- 
From: "James Moritz" <james.moritz@btopenworld.com>
To: <rsgb_lf_group@blacksheep.org>
Sent: Tuesday, June 12, 2007 9:43 PM
Subject: LF: Tree current


> Dear LF Group,
>
> It has often been said that trees surrounding an LF vertical antenna 
> can be
> responsible for a substantial proportion of  the antenna loss, and 
> at my
> QTH there are several trees that have gradually been getting bigger 
> over the
> years, so I decided to try to measure how much RF current was 
> actually
> flowing in some trees.
>
> To measure the tree current, I used a Rogowski coil - this is like a
> toroidal RF current transformer, but without the magnetic core; 
> instead of
> measuring the secondary current, the open-circuit voltage across the
> secondary is measured, and is proportional to the total current 
> flowing
> through the area enclosed by  the coil, the number of turns, the 
> area of
> each turn, and the frequency. My Rogowski coil used 1.2m of rubber 
> hose as a
> former, with about 500 turns of insulated wire, and had a scale 
> factor of
> around 100mV out = 1A in. I measured the voltage with a SPM-3 
> selective
> voltmeter. The advantage of this type of current sensor is that the 
> loop of
> flexible hose former can be made large enough to fit round a tree, 
> and can
> be opened out, wrapped round the tree to be measured, and closed up 
> again. A
> practical difficulty is that the output is quite small compared to a 
> normal
> current transformer, and an aluminium foil electrostatic shield had 
> to be
> added in order to reduce the effect of pick-up of the intense 
> E-field
> directly under the antenna. This did not entirely eliminate the 
> problem, but
> reduced it to a reasonable level.
>
> I measured the current near ground level in the trunks of 8 trees - 
> these
> are scattered around within about 10m horizontally of the antenna, 
> with
> heights of about 5 - 10m (antenna height is around 9 - 11m). With an 
> antenna
> current around 4A, the currents ranged from 50mA to 190mA, with the 
> total
> for the 8 trees being 930mA. Generally, trees closer to the antenna 
> and with
> bigger areas of foliage had higher currents, as you would expect.
>
> The 8 measured are only a sample of the small trees and large bushes 
> in and
> around my garden, also at about 20 - 40m distance there are larger 
> trees all
> around my QTH as well. So it seems likely that a large fraction of 
> the
> electric flux of the antenna is intercepted by a tree at some point, 
> and a
> substantial proportion of the total antenna current is flowing to 
> ground
> through trees. Since wood is a poor conductor, it is not surprising 
> that
> trees close to the antenna increase the loss resistance. But also, 
> current
> flowing vertically in the trees will contribute to the overall 
> radiation of
> the antenna. However, since a current flowing "up" the antenna will 
> be
> flowing "down" the tree, and the spacing of tree and antenna is a 
> very small
> fraction of a wavelength, the effect of the tree current will be to 
> partly
> cancel the overall radiation, and so reduce  the radiation 
> resistance. If
> the current flowing in the trees is a large fraction of the total 
> antenna
> current, and the height of the trees is comparable with the antenna 
> height,
> one could expect a substantial reduction in radiation resistance and
> effective height of the antenna, as the field strength measurements 
> I made
> last week would seem to indicate.
>
> Cheers, Jim Moritz
> 73 de M0BMU
>
>
>
>
>
> -- 
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