I was wondering if I could get some help developing then modeling a
short dipole for 630m use.
I consider myself to have intermediate modeling skills and have modeled
a lot of antennas in EZNEC etc.
Here is what I would like to do. Take an existing 160m dipole (flat top)
at 70' and use a pair of inductors, one on each leg about 1-2 feet away
from the center insulator. I have already modeled this and so far things
seem normal. My model was simplistic and did not include any coax. For
the loads I simply increased the number of segments to 35 in each leg
such that the load ended up being within several % away from the
feedpoint. I had specified that the loads be at the ends (0%) but in
reality EZNEC put them several % out.
Now we all know given this tiny antenna for this band the feedpoint
impedance is going to be low.....in the ball-park of 12 - J30 is about
the best I could achieve for the lowest SWR at 474 kHz. Not quite
resonant but close enough.
Now, I seem to remember an old trick that I saw many years ago which I
think used to be called a gamma coil at the center of the dipole,
basically across the coax right at the feedpoint that will help bring
the feedpoint impedance up to 50 ohms. Similar to what many mobile whip
antennas have at their feedpoint.
So I guess I have several questions that I need help with. One is help
figuring out what this center inductor looks like in reality, what value
it should be, _and how is it to be connected to the dipole and the
coax_. And two, how to model such a beast along with, say, 150' of RG-8
coax.
In my basic model I simply used one segment for the center and is where
the feedpoint is within EZNEC, then I have two additional wires
connected off this short center wire, one on each side of that center
segment that makes up the actual dipole.
Because of the frequency each segment has to be nearly 2' long ! This
too is where I need help and that is in attaching not only my coax feed
but this center inductor in the model. I hope the above makes sense. TU
Gedas, W8BYA
Gallery at http://w8bya.com
Light travels faster than sound....
This is why some people appear bright until you hear them speak.
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