RadioBanter - View Single Post

December 12th 03, 09:16 PM

In article ,
says...
Is there a general rule as to the optimum way copper should be
distributed on a longwire antenna for maximum propagation? E.g.,
should the first half be heavier gauge than the second half? How much
heavier?

Ken KC2JDY

I looked into this a couple of years ago. There are two factors;
DC ohmic resistance and RF skin effect. I found three different
huristic formulas for RF skin effect in ARRL handbook, ITT Radio
Engineers Hndbk and a book I found in the Cornell Engineering Library
stacks. They agreed within a factor of 2^-2. The numbers I ran
indicated that for power 1 kw and Freq 30 mhz 16 guage AWG harddrawn
stranded would outperform 14 gauge solid or copperweld except for
tensile strength and was all the copper it would be economic to use.
Between 10 Mhz and 50 Mhz the use of tubing gives mechanical design
advantages but doesn't gain you a lot of electrical performance. The
higher the freq the more significant skin effect becomes. For long
antenna wires at low freq the tensile strength issue could be more
important than skin effect indicating it would be better to use quality
copperweld or solid harddrawn. 17 guage AWG copperweld electric fence
wire performs badly. It's very cheap, but the copper plating depth is
shallower than the skin effect depth and the steel core is grainy,
brittle, and rusts thru quickly at any stress fracture.
In theory the gauge of the wire could be heavier at current nodes
on the antenna, but in practice I doubt it would be worth the trouble to
join sections of different gauges. I certainly wouldn't want to bother
with a tapered wire. ;-] 73 KC2LVQ