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

[Michael Tope]

Yeah, I was expecting the two-coil design to have better VSWR bandwidth. 
Did you compare the radiation resistance of the two designs? I would 
expect the one coil design to have a lower radiation resistance.

Perhaps the lack of difference in the VSWR bandwidth of the two designs 
explains why the Cushcraft 402CD/XM240 designs go to the trouble of 
using capacitance hats after the loading coils.

73, Mike W4EF..............

On 4/28/2020 4:26 PM, Brian Beezley wrote:
> I just tried a quick model to see if a single loading coil at the 
> center of a 40m dipole element was feasible. For simplicity I modeled 
> a 46-foot element with constant 0.75" diameter. I compared two loading 
> coils located halfway out each half-element with one loading coil at 
> the center. I was expecting the single-coil element to have much 
> narrower SWR bandwidth. But SWR for the two designs was remarkably 
> similar. I adjusted the inductances for resonance at about 7.15 MHz. 
> The SWR of two-coil design was about 2.7 at 7.0 and 7.3 MHz when 
> matched at 7.15 MHz while that of the single-coil design was 2.85. 
> Both had a load loss of 0.06 dB using coil construction similar to the 
> M2 coil (calculated Q about 965). The inductance of the single coil 
> was 8.0 uH while that of the two coils was 7.7 uH each. I quote SWR 
> values only to indicate the inherent Q of the elements. They are not 
> what a Yagi made from such elements would exhibit.
>
> In addition to simplicity, the advantage of using a single coil is 
> that when optimized for maximum Q (about 1230), the diameter increases 
> to about 6.9" with a length of about 4.7". Enclosing a coil of this 
> size to keep the Q from degrading when wet would create two large wind 
> loads halfway out each half-element. But the wind load would be no 
> problem when mounted at the boom. VE6WZ seems to get by without coil 
> enclosures, but these results are alarming:
>
> http://www.n3ox.net/tech/coilQ/
>
> After examining the M2 coil manual and making measurements on the coil 
> illustration, I estimate the coil diameter to be 3". The length is 
> 5.8125" according to the description (15.5 turns of 3/16" tubing 
> spaced the wire diameter). Lead length is 1.5" to the element 
> centerline. All dimensions are wire center to wire center. You can 
> model the coil inductance and automatically optimize Q with this:
>
> http://ham-radio.com/k6sti/coil.zip
>
> See README.TXT for installation instructions. I will be updating the 
> program tomorrow, but the current 3.89 version works fine.
>
> I used the RLC coil model in my antenna model. Inductance varies 
> somewhat over 7.0 to 7.3 MHz due to coil self-resonance so there is 
> some error when you use a simple RL model to cover the whole band. The 
> L and C of the RLC model are constant over 40m. I saw some difference 
> in SWR at the band edges between the RL and RLC models though nothing 
> great. If your antenna modeling program can handle RLC loads (R in 
> series with L, C in parallel with that series combination), use it for 
> best accuracy.
>
> Incidentally, Copperweld makes copper-clad aluminum wire if you want 
> to reduce coil weight when mounting coils halfway out each 
> half-element. AWG 5 wire (0.1815") with 10% copper by weight has a 
> copper thickness of 4.7 skin depths at 40m. That puts 99% of the 
> current in the copper. The M2 coil uses copper-clad aluminum.
>
> https://www.copperweld.com/application/files/6815/3833/2604/Welded_Copper-covered_Aluminum_CCA_10.pdf 
>
>
> I'm hoping someone verifies this loading comparison. The results are 
> surprising and I'm always suspicious of unexpected favorable results.
>
> Brian
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