The gain decrease is because of wire surface area. Conductivity of
aluminum is actually less than zinc, but the surface area of a #17 wire
is much smaller than the surface area of a 1/2 inch diameter cable.
The pattern formed by a long wire is very complex. There are large
numbers of lobes and nulls, both in the azimuth and elevation
directions. Most of these are very narrow. Because of this any small
phase change in the current, causes large changes in the pattern.
Visualize this: These phase changes at the source end, have several
wavelengths in which to accumulate a total phase change at the other
end. For this antenna, at low elevation angles on 160 meters, 16 lobes
are formed in the azmuith direction. These are created by the sum of
currents from all segments along the wire. A small phase shift and
instead of 16 lobes being formed, 18 or 14 lobes are formed. This makes
the position of these lobes change. This occurs both in the azimuth and
elevation directions. This is unlike a dipole where there are two big
lobes parallel to the wire, and requires huge phase changes to show any
change in the pattern.
Because these changes happen in the elevation direction as well as the
azmuith direction, there are frequencies where the antenna appears
omini-directional, and other frequencies at which the pattern is very
dependent on the azmuith direction. For some azmuith directions it may
have only 4 narrow lobes, and other in directions there may be a null
and the gain will be down many dB. The pattern is extremely complex.
Jerry, K4SAV
Barry wrote:
>Something else to think about...
>In my (limited) experience, if a model changes dramatically for a
>small change in length, height, etc., there is probably something
>wrong with the model.
>Barry, W2UP
>
>On 13 Oct 2005 Joe Giacobello wrote:
>
>
>
>>Jerry, I have not followed this thread very closely, but is the
>>reduction in gain with galvanized from resistive losses?
>>
>>73, Joe
>>
>>K4SAV wrote:
>>
>>
>>
>>>An update on the 800 meter long wire performance.
>>>
>>>Using 17 ga aluminum wire vs 0.5 inch galvanized cable I had first
>>>assumed, decreases the gain by about 3.7 dB.
>>>
>>>The antenna pattern is very sensitive to the exact overall length.
>>>Looking at it another way, it is sensitive to frequency change. A
>>>frequency change of 50 KHz changes what used to be a peak lobe into a
>>>null (looking mostly at the pattern off the end of the wire). For
>>>example, off the end of the wire, at 1.8 Mhz and an elevation angle
>>>of 10 degrees the gain may be -1 dB. Move to 1.85 MHz and the gain
>>>is -13 dB, same angle.
>>>
>>>The method of feeding the wire also effects the pattern. I assumed
>>>end fed with the shield dropping straight down into the salt water.
>>>This seems to produce the best pattern I could find.
>>>
>>>Implementing a vee beam with an 80 degree included angle seems to
>>>reduce the pattern into something that is very scrambled and with
>>>lower gain.
>>>
>>>I am normally unsure about simulator performance when wires are close
>>>to the ground, but in this case since this is an almost perfect
>>>ground, this seems to be within the guideline of the simulator. The
>>>simulator shows very little difference between using perfect ground
>>>and salt water except between 0 and 3 degrees elevation.
>>>
>>>Jerry, K4SAV
>>>
>>>
>>>
>>>
>>>
>>>>
>>>>
>>>>
>>>>
>>>_______________________________________________
>>>
>>>See: http://www.mscomputer.com for "Self Supporting Towers",
>>>"Wireless Weather Stations", and lot's more. Call Toll Free,
>>>1-800-333-9041 with any questions and ask for Sherman, W2FLA.
>>>
>>>_______________________________________________
>>>TowerTalk mailing list
>>>TowerTalk@contesting.com
>>>http://lists.contesting.com/mailman/listinfo/towertalk
>>>
>>>
>>>
>>>
>>>
>>_______________________________________________
>>
>>See: http://www.mscomputer.com for "Self Supporting Towers",
>>"Wireless Weather Stations", and lot's more. Call Toll Free,
>>1-800-333-9041 with any questions and ask for Sherman, W2FLA.
>>
>>_______________________________________________
>>TowerTalk mailing list
>>TowerTalk@contesting.com
>>http://lists.contesting.com/mailman/listinfo/towertalk
>>
>>
>
>--
>Barry Kutner, W2UP Internet: w2up@mindspring.com
>Newtown, PA Frankford Radio Club
>
>
>_______________________________________________
>
>See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless
>Weather Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any
>questions and ask for Sherman, W2FLA.
>
>_______________________________________________
>TowerTalk mailing list
>TowerTalk@contesting.com
>http://lists.contesting.com/mailman/listinfo/towertalk
>
>
>
_______________________________________________
See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather
Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions
and ask for Sherman, W2FLA.
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TowerTalk mailing list
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