On 2/4/2018 7:18 AM, Richard (Rick) Karlquist wrote:
On 2/3/2018 8:59 PM, Jim Brown wrote:
than inductive loading. BTW -- there was an excellent piece of work
published 3-4 years ago studying the relative effectiveness of
bottom, center, and top loading of mobile antennas. The study
included extensive
These rules for mobile antennas don't apply to a vertical with a
substantial top hat. In that case, once the hat gets to a certain
size, the current in the vertical is uniform from top to bottom
and it doesn't matter where you put the coil. This is according
to W8JI.
Yes. An NEC model of my Tee with 100 ft vertical and 56 ft horizontal
clearly shows this, and it would be even more true in the shorter
antenna being discussed here. BTW --to increase SWR bandwidth, I'm using
two wires spaced about a foot for the vertical part of the antenna,
connected at both top and bottom. When I first did this about seven
years ago, that bandwidth approximately doubled. I wasn't concerned with
feedline loss -- I do this to make matching to the power amp easier.
Another technique that is often used for matching is to make the
wires much longer than resonant (that is, resonant around the top of
the BC band), so that the feedpoint Z is 50 +jX ohms (that is,
inductive) on 160M, and tune the capacitance out by adding a
capacitor of equal value in series. NEC can easily model this.
73, Jim K9YC
NEC will also tell you that putting this amount of top loading in place
at the heights Bill is talking about will cause the radiation resistance
to drop, so be careful.
As I understand it, radiation resistance is mostly a function of the
length of the vertical element. Sloping the loading wires downward from
the top (guy-wire style, as is being suggested) has the effect of
shortening the vertical element a bit, which would lower the radiation
resistance. This doesn't mean you shouldn't use the technique, but
simply that it should not be overdone.
Horizontal wires would not do that. My top loading wires are supported
from tall redwoods, so they slope downward to the antenna.
73, Jim K9YC
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