Short version: The trick is to turn the fence into a non-conductor at
160 meters to make it invisible to the beverage run above it.
Long version:
Strictly from a transmission line basis, the fence top of a fence with
NON-METALIC poles SHOULD be able to be used, IF attention is paid to
the fence below it. This does not work if one of the conductors is an
electric fence, but your text does not suggest that. Making the
assumption that it is YOUR fence and you can alter it as needed...
Principally the problem is the resonance of the fence below. Consider
the zone from one end of the beverage to the other. Go 75 to 100 feet
beyond either end and break the horizontal electrical continuity of
the fence wires at a selected post near, WITHOUT GROUNDING EITHER SIDE
OF THE BREAK. This may be as simple as attaching the wires separately
to a wooden post. A resistance of as low as a couple hundred ohms
between the separated conductors takes them out of the range of
effect.
Perform the same break at ROUGHLY every 125' feet for the run of the
beverage. Anything between 100 and 150 feet works well enough.
The optimal lengths of a beverage at 4 feet over the ground are
shorter because the velocity factor is changing. Using the prior
suggestion of bamboo (or ?? insulative) extenders is good to retain
commonly stated optimal beverage lengths.
73, Guy.
On Wed, Nov 3, 2010 at 5:00 AM, Greg - ZL3IX <zl3ix@inet.net.nz> wrote:
> Hi Brian,
>
> I can see two main problems with doing this. I haven't tried using the
> actual fence strand, but I did have a Beverage that ran close along the
> top of the fence.
>
> Problem 1.
>
> Being in close proximity to the other fence strands, coupling from them
> is significant, compromising the front to back ratio. I found that I
> improved the F/B ratio from about 8 dB to about 15 dB by increasing the
> spacing between the Beverage wire and the top of the fence, from 300mm
> to 1.8m.
>
> Problem 2.
>
> Unless the fence is very high, the top strand is probably quite close to
> the ground. The insertion loss of the transmission line formed between
> the strand and the ground will be significantly higher, than if the wire
> is raised up a couple of metres. This means that currents induced at
> the far end of the wire are significantly smaller in magnitude than
> currents induced in closer parts of the wire, even thoguh they come from
> the same wave. The spatial decorrelation effect of, say, a 300m wire,
> is then compromised, and the directivity is reduced. I found that the
> common mode inserton loss dropped from 8 dB to 4 dB, when I increased
> the height of the wire as described above.
>
> I achieved the height increase (and increased separation from the fence)
> by drilling holes in the tops of the fence posts, and inserting bamboo
> supports. These have now been in place for nearly a year with no sign
> of degradation.
>
> 73, Greg, ZL3IX
>
> On 2010-11-03 05:25, Brian Kassel wrote:
>> Can the top wire
>> of the fence be insulated and used? My 3 acre property shape is very long
>> and not very wide, sort of triangular shaped. I don't have enough room to
>> run a a wire across the property, only lengthwise, along the fence.
> _______________________________________________
> UR RST IS ... ... ..9 QSB QSB - hw? BK
>
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UR RST IS ... ... ..9 QSB QSB - hw? BK
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