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.
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