[Elecraft] OT antenna question

[Jim Brown]

On 5/28/2018 10:39 AM, John Oppenheimer wrote:
> An EZNEC model using 30 feet AGL and Q=200 for the coils, has a very
> close match the the reported values.

While SWR and driving Z values may be correct, there was an excellent 
piece 2-part piece in QEX 3-4 years ago showing that NEC does not 
correctly model phase shift through inductors, thus producing wrong 
answers for field strength (i.e., how well the antenna WORKS). This is a 
shortcoming of the NEC code, and is unrelated to the user interface like 
EZNEC/ELNEC and 4NEC2.

The antenna he was studying was a loaded mobile whip for HF. The first 
part of the article was a serious engineering description of the test 
setup, including the measurement system. The second described a 
carefully controlled study, using that measurement system, of inductive 
loading at the base, midway, and fairly high on the vertical radiator, 
as well as capacitive top loading. He compared measured results with the 
NEC model. As I recall, differences between modeled and measured data 
were as much as 10-14 dB for bottom loading.

In addition to exposing the limitation of NEC, the major conclusion from 
his study was that because radiation is produced by current and current 
is greatest near the feedpoint, a design that provides the greatest 
vertical height before the loading element(s) will generally be the most 
efficient (that is, produce the loudest signal). Those who have long 
been serious about HF mobile antennas have known this for years. At 
least as long as 20 years ago, W8JI has written a lot about this. In 
that time frame, my buddy K9IKZ told me about an annual conclave of HF 
mobile station builders convened in southern Indiana to do "shootouts" 
of their designs.

73, Jim K9YC