[TowerTalk] 40m 4el KLM - replacing linear loading with coils

[Wes]

I agree with Dave.  We had a similar discussion about this (using monopoles) 
nearly 20 years ago in the rec.radio.amateur.antenna forum.  I wrote a little 
paper at the time; I see it's available at: 
https://www.qsl.net/n7ws/Loaded_Antennas.pdf  Of note is my reference #1.

Wes  N7WS




On 4/28/2020 6:50 PM, David Gilbert wrote:
>
>
> Whether NEC accurately models the current isn't the point.  You don't need a 
> model to know that if you push the coil out away from the center the portion 
> of the current between the coil and the center is higher than it would be 
> between that same point and the center if the coil was at the center.
>
> And to a point, the further way from the center for the current distribution 
> the better the pattern.  Consider how colinear dipoles with a shared feed work 
> ... the total area under the curve for the current is that same as for a 
> single dipole except that the current is more widely separated.
>
> The problem with putting the coil TOO far from the center is that the current 
> there eventually becomes low enough that the coil needs to be really large.  
> As an extreme, a coil at the end of a dipole is useless for affecting the 
> tuning other than whatever capacitance it might have to the wire.  Coils need 
> current to have any effect.
>
> I don't understand why I have to be explaining this stuff.  It's pretty basic 
> and described in just about every antenna book that discusses current 
> distributions.
>
> 73,
> Dave   AB7E
>
>
>
> On 4/28/2020 4:18 PM, Jim Brown wrote:
>> On 4/28/2020 3:20 PM, David Gilbert wrote:
>>> There is some benefit, however, to use two loading coils on either side of 
>>> center for each element (instead of center coils) because that gives better 
>>> current distribution along the elements.  Most modern Shorty-40's do this, 
>>> and it's the same reason why some mobile vertical antennas use center 
>>> loading instead of base loading.
>>
>> There was an excellent 2-part piece in QEX 4-5 years ago showing by 
>> well-controlled measurements of real mobile antennas that NEC does not 
>> accurately model current distribution in an antenna with lumped inductors 
>> placed in a segment. The error is that it fails to account for current change 
>> through the inductor -- it models the current as the same on both sides of 
>> the inductor.
>>
>> NEC DOES, however, include an option to model inductors as a helix, which 
>> does account for the change in current and voltage through the inductor, but 
>> you need a version that allows a LOT of segments. I'm using a version of 
>> W7EL's EZNEC for which I paid about $500 ten years ago, and I've done that 
>> for a few portable designs that W6GJB and I were working on together.
>>
>> 73, Jim K9YC
>>
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