Topband: Bobtail curtain

[K4SAV]

Charles Bibb wrote:
> ......Here's the problem though. Every time I try to model a full-sized 
> bobtail using five wires everything looks good until I run an FF plot 
> in the vertical plane. Every time, the elevation angle is very high 
> (64 degrees, I think it was)......
>   
There are two possibilities, both of which will produce high angle 
signals like that.
1.  Are you perhaps looking at the end fire pattern?  That's about the 
angle you will have off the ends, although the gain should be way down.
2.  The other possibility is that the end vertical elements aren't 
really connected to the top wires.  Check the wire list to make sure 
they are connected.

Here is another problem that may crop up if you try to shorten the legs 
by using loading.  It is sometimes convenient to model vertical antennas 
using a Mininec ground, but Mininec ground doesn't give accurate answers 
if there are low horizontal wires in the model.  If you shorten the 
legs, the top wire will be low (compared to a wavelength on 160).  So if 
you use a Mininec ground expect the gain to be too high and the 
feedpoint impedance to be too low.

For the most accurate answer use a real ground.  However you can't 
connect a wire directly to a real ground.  So you either have to model 
radials, or use this trick.  Put the source at the top of the center 
wire. That will give the same answer.  You could actually feed it this 
way in the real world, if you could figure out how to get the feedline 
away from the antenna without messing up the pattern.

Oh, and don't forget to model the resistive losses of the loading coil 
used.  This will require a large coil and coil losses may be 
significant.  I'm not exactly sure how much since I haven't tried to 
arrive at a solution. Remember most of the resistive losses (ac 
resistance) comes from skin effect, coil capacitance, and coil shape.

Jerry, K4SAV