August 30th 04, 07:28 AM
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On 29 Aug 2004 12:28:31 EDT, am (Gary V.
Deutschmann, Sr.) wrote:
Copper is GREAT for Antenna's!
It's only slightly heavier than Aluminum.
But their are MAJOR advantages to using copper.
Solid corrosion free joints is only one of them.
Due to the skin effect, RF currents only flow on the surface of the
conductor. At VHF frequencies, the skin depth in copper is only about
5 um, in which nearly all RF currents flow, thus the low resistivity
of this thin surface layer is critical. From RF point of view, it does
not matter much what is below this surface, so it might be empty (a
tube) or it might be something with lower conductivity, such as
stainless steel (with much greater mechanical strength).
However, if the antenna is not protected by a radome, the weather will
within a few years affect the copper surface, creating oxides and the
copper becomes green or dark brown. Now the question is, what is the
conductivity of these surface layers. If these layers are isolators,
the RF current will flow in the clean copper below, but if the layers
have some mediocre conductivity, part of the RF current will flow in
these oxide layers, increasing the total resistance.
This should not be areal problem with VHF antennas made of tubes,
since the length to surface ratio is quite small.
Paul OH3LWR
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