[Antennaware] Help with Short Dipole

[Gedas]

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
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