On 4/21/2014 5:32 AM, Jim Lux wrote:
The real sticky issue when trying to calculate stuff is "what is the
voltage". And that's something that is hard to determine in a simple
way. One can do a numerical model (using NEC, for instance), but in
practice it's highly installation dependent.
YES! NEC will at least get you in the ballpark if the model you enter is
close to your reality (like feedline length). But antenna imbalance
increases the common mode voltage (and thus the common mode current),
and those imbalances can be hard to model, because they come from things
like ground slope, variations in soil under the antenna, surrounding
metal objects (wiring in your home, adjacent antennas and even adjacent
feedlines).
All of which are reasons why I've always taken a brute force "more is
better" approach to designing for high power. If you buy them right (in
quantity from vendors who specialize in Fair-Rite products, you should
be paying no more than about $5 for a #31 2.4-in o.d. toroid, tax and
shipping included). And all it takes to buy them right is a group
purchase with one or more local ham clubs. I recently did a very
successful group buy with Dexter Magnetics, near Chicago, and I've seen
good reports about Lodestone Pacific.
73, Jim K9YC
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