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

[David Gilbert]

Stretching out the current improves the field pattern ... i.e., "gain."  
I'm not talking resistive effects.  That's why "extended" dipoles with 
series capacitors (which makes the length longer for the same resonant 
frequency) gives some additional gain.  I could prove it to you with 
EZNEC except I'm too lazy at the moment.  It's not a dominant effect, 
but it's typically worth the trouble.

73,
Dave   AB7E



On 4/28/2020 4:05 PM, jimlux wrote:
> On 4/28/20 3:20 PM, David Gilbert wrote:
>>
>> Definitely true.
>>
>> 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.
>
> Different current distribution, but I'm not sure it's better.
>
> Sure you have close to a uniform current distribution instead of a 
> "cos(x)" distribution with a higher peak.  And since loss is I^2*R, 
> there might be slightly more loss with the center loaded.
>
> The pattern will be slightly different (compare an "infinitely short" 
> dipole which has uniform current and a half wave dipole which has 
> cos(x) distribution). So the 'element pattern' will be slightly higher 
> gain with the cos(x) distribution (2.15dBi), vs 1.7 dBi for uniform.
>
> If the radiation impedance is, say, 50 ohms at the center of the 
> element.  I calculate that the AC resistance of a 10 meter long 2" 
> diameter tube at 7MHz is 0.055 ohms.
>
> That's not even 0.1% percent of the radiation resistance, so whether 
> the I^2*R losses vary by a factor of 2 isn't going to make much 
> difference
>>
>> I suspect that using two coils for each element might even be 
>> stronger mechanically since the split elements have less cantilever 
>> weight where the two coils are positioned versus a single center one.
>>
>> 73,
>> Dave   AB7E
>>
>>
>>
>> On 4/28/2020 2:19 PM, jimlux wrote:
>>>
>>> One could also simplify the overall design - make all elements 
>>> exactly the same length, and use different values of the center 
>>> loading inductance to "tune" the elements.  There's nothing special 
>>> about the physical length of the elements, other than it creates the 
>>> right impedance at the right frequency, and you can get there with 
>>> loading reactance.
>>
>> _______________________________________________
>>
>>
>>
>> _______________________________________________
>> TowerTalk mailing list
>> TowerTalk at contesting.com
>> http://lists.contesting.com/mailman/listinfo/towertalk
>
> _______________________________________________
>
>
>
> _______________________________________________
> TowerTalk mailing list
> TowerTalk at contesting.com
> http://lists.contesting.com/mailman/listinfo/towertalk