Stretching out the current improves the field pattern ... i.e., "gain."
I'm not talking resistive effects. That's why "extended" dipoles with
series capacitors (which makes the length longer for the same resonant
frequency) gives some additional gain. I could prove it to you with
EZNEC except I'm too lazy at the moment. It's not a dominant effect,
but it's typically worth the trouble.
73,
Dave AB7E
On 4/28/2020 4:05 PM, jimlux wrote:
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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