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

[jimlux]

On 4/27/20 1:18 PM, Richard (Rick) Karlquist wrote:
> On 4/27/2020 10:38 AM, Simon Ravnič wrote:
>>
>> After 41 years of being on the tower it is time to upgrade the 
>> antenna. Has anyone made a modification of 40m 4el KLM - “Big sticker” 
>> and replaced linear loading rods with high Q coils?
>>
>> We are aware of M2 offer forf coil replacement kit, but it is a bit 
>> expensive so we would like to do it on our own.
>>
>> 73
>> Simon, S53ZO
> 
> The problem is that you don't know the required inductance
> of the coils.  It would be difficult to determine this
> for anyone but M2.  At one time, W6ANR used to sell
> coils for 40 meter Yagis.  Perhaps he has the values
> and would be willing to give them to you.
> 
> The recent TT thread about determining the resonant frequency
> of Yagi elements would help you.  You could find the 4
> resonant frequencies of the 4 elements of the unmodified
> Yagi, and then use trial and error to tune the inductors
> to get the same element frequencies.  Seems like a lot
> of work.
> 

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.