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

To: towertalk@contesting.com
Subject: Re: [TowerTalk] 40m 4el KLM - replacing linear loading with coils
From: Michael Tope <W4EF@dellroy.com>
Date: Tue, 28 Apr 2020 19:08:31 -0700
List-post: <mailto:towertalk@contesting.com>
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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