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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