Talking about where the choke should go on an astroplane, I can model that.
The model I am using is not my own. I am using ghz24's model, actually I have modified it slightly, two of the elements were in backwards, and while this doesn't affect things like gain, impedance, and SWR, those wires did not show phase properly...
Anyway, taking the mast, and the discussion on its length, I can graph the change in mast length in 4nec2 and post the results here...
Lets start with SWR... SWR from 1 to 5 is charted on the left, and in number below the chart is the length of the mast before an isolation point.
As we can see, just above the mark labeled 102,right about where it would be 1/4 wavelength from the feed point, the SWR spikes up. How high does it spike? Lets see...
That is pretty high, it almost makes it all the way to 1300...
Looking back to the first SWR graph, any amount that is a little over 1/4 wavelength for the mast would be ideal. Even at the point of 3/4 wavelengths, the SWR on the model does not spike up to even 1.5 to 1.
Shall we look at other data? How about impedance next...
Here we can see why the SWR data above stayed so low over such a long area, once you get a little past the first 1/4 wavelength both X and R seem to be somewhat stable, especially R which is a little above what is should be.
And finally gain...
Here we see a pretty large variation in gain at the 1/4 wavelength point as it spikes both high and low. We see this again to a lesser extent at the 3/4 wavelength point. The area in between, however, is very stable.
Based on all of this information, I would suggest going a certain amount longer than 1/4 wavelength of mast/coax, length. Half wavelength is fine as it is in the middle of the stable region. Although some people may think it strange, the 1/2 wavelength mast/feed line has a current distribution that resembles a 1/4 wavelength mast/feed line, and I will show this below.
This phase shift is caused by the presence of the "basket area" (lower two wires and the hoop around the mast/feed line) of the antenna, namely the capacitance between the basket area and the mast/feed line, it causes a phase shift, much like a loading coil, although in this case in the opposite direction, making the element electrically shorter, thus requiring more physical length to compensate.
The DB
