On 12/29/2011 1:45 AM, Jim Brown wrote:
> On 12/28/2011 6:18 PM, Jim Lux wrote:
>> For a given voltage at the feedpoint, the current in the tree will be
>> 1/30,000th that in wire. The power dissipated is that squared.. or, in
>> other words, negligible.
>>
>>
>> This is a fascinating observation...
> Indeed it is.
>
>> IN other places, we've discovered that trees are like soil (similar
>> conductivity and epsilon), and we know that laying an antenna on the
>> ground doesn't work all that well. However, that's the difference
>> between half of the space being occupied by the lossy medium and just a
>> tree's worth.
I often wonder if there is much difference in tree types up here in the
North.
I notice no difference between summer and winter as far as 160 and 75,
but the bands change so much any thing I see is purely anecdotal.
I do know that on 144 and 440 it makes a huge difference between winter
and when the trees are green in the warmer weather. My old tribander
worked very well at 100 feet which was well over the trees and much
higher than optimum. Did it work well because it was high, or because
it was above the trees. You can't just raise and lower the antenna as
the radiation angle changes. You'd need another ham with a fixed antenna
a couple miles away, a good attenuator, and a calibrated S meter. OTOH
I'd think that even at a couple miles ground reflections might play a
part whether shooting through the trees or over them.
> When thinking about attenuation in a forest, it should be remembered
> we're talking about a DENSE forest, a lot of big trees relatively close
> together, and some of those studies you've cited are talking about
> jungles. This is very different from hanging a wire in/near a single
> tree or a few trees. A very dense forest like mine, or the pine forests
> of the American South East, are somewhat analogous to the lossy soil.
>
>> To compare.. if I put two 8 foot rods into the soil 10 meters apart,
I was nearly electrocuted doing that. The two ground rods were safeties
for a 40 meter vertical about 50 to 75 feet West of the house at
Breckenridge. Due to an over generous solder joint on a Yaesu FT101B the
110VAC was tied to the chassis. The ground just outside was not enough
to pop the breaker. Some of the radials which were just pulled tight
and stuck in the ground had come up. I picked up one (the other end
hooked to an 8' ground rod and the coax shield). When I knelt down and
my knees touched the wet ground that wire grabbed me. I was balanced on
the balls of my feet and fortunately fell over backwards. When my knees
broke contact with the ground I threw that radial. BTW the electrical
service and ground was just half way between the station and the antenna.
> When I've measured resistance between two rods a few yards apart in the
> loamy soil of my redwood forest, the lowest DC R I've seen is on the
> order of 30 ohms. This is with a Simpson 260.
That's about what I saw out on the farm as well.
73
Roger (K8RI)
>
> Thanks for a VERY interesting post, and the digging that's behind it.
> I'll be chewing on it for a while, and I suspect others will too!
>
> 73, Jim K9YC
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