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From: "Hugh_m0wye" <m0wye@radiouk.com>
To: <rsgb_lf_group@blacksheep.org>
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Date: Tue, 12 Jun 2007 11:39:07 +0100
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Subject: LF: Re: Tree current
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Hi Jim,
What an interesting experiment, and illuminating, although it told us what 
we already suspected, that lush foliage makes a nice garden, but rather 
spoils RF performance.

I have not come across the Rogowski coil before - does it work both ways - 
that is ,can you induce RF current into a tree trunk by applying an RF 
voltage to the coil ? Probably not very efficient, but you can probably see 
where I'm going with it !

The Rogowski coil seems to be rather like the signal clamps used to measure 
currents in power cables without actually connecting to them - but without 
the iron core.
Hugh M0WYE


----- Original Message ----- 
From: "James Moritz" <james.moritz@btopenworld.com>
To: <rsgb_lf_group@blacksheep.org>
Sent: Tuesday, June 12, 2007 10:43 AM
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
>
>
>