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Message-ID: <E8C908CC3E384F9DA38A59D6828A75F0@JimPC>
From: "James Moritz" <james.moritz@btopenworld.com>
To: <rsgb_lf_group@blacksheep.org>
References: <001b01ca6086$cd0966b0$0617aac0@jimdesk>
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Date: Sun, 8 Nov 2009 16:54:57 -0000
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Subject: LF: Re: Losses due to ant in tree?
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Dear Jim, LF Group,

I did some experiments a while back that compared the efficiency of a 
top-loaded vertical (inverted L about 10m high and 40m long) in an 
environment surrounded by trees, compared to the same antenna in the middle 
of an open field. On 500kHz the antenna surrounded by trees was only about 
1/6th as efficient as the field antenna, so trees certainly have a major 
effect. Part of the effect is due to increased loss resistance - the trees 
being a lossy dielectric in the capacitor formed by the antenna wire and the 
ground plane. Also, the radiation resistance is reduced (i.e. less field 
strength for the same antenna current), which I guess was due to induced RF 
current flowing in the tree in the opposite direction to the current in the 
vertical section, partly cancelling radiation from the antenna. It was 
possible to measure a significant fraction of the total antenna current 
actually flowing in the tree trunks. On 136k, the difference in efficiency 
was even greater.

So I expect the tree has a significant effect, especially since the wire is 
actually in contact. For my antenna, the wire was kept at least a few m away 
from any tree, but it was surrounded by several trees. You will probably 
find moving the antenna wire away from the tree reduces the loss resistance 
significantly, and it should also lead to an increase in effective 
height/radiation resistance.

Cheers, Jim Moritz
73 de M0BMU

----- Original Message ----- 
From: "James Cowburn" <james.cowburn@virgin.net>
To: <rsgb_lf_group@blacksheep.org>
Sent: Sunday, November 08, 2009 3:19 PM
Subject: LF: Losses due to ant in tree?


> Hi All,
>
>
>
> The vertical 5m section of my inverted L runs up into the branches of a
> silver birch tree.  The wire goes over a large bough at 5m or so and then
> comes out of the branches for some 22m down to a 4m high pole attached to
> the garage.
>
>
>
> Has anyone a guesstimation of the losses in signal caused by running the
> wire through the tree?