[Antennaware] feedline modeling

Terry Conboy n6ry at arrl.net
Thu Sep 24 10:26:15 PDT 2009


It's usually much easier and often more accurate to use the built-in 
transmission line capability in EZNEC for modeling feed lines.  The 
exception is for cases, such as stubs in collinears or Sterba curtains, 
where the physical transmission line carries common mode current - in 
other words, if the line currents are not balanced (and often there is 
coupling to other parts of an antenna).  For coax fed antennas, you can 
use the technique outlined by W7EL in his diptl.ez sample model, to 
assess the impact of using an ideal balun, a practical balun, or no balun.

The built-in transmission line capability takes care of those "special 
cases", such as multiples of 1/4 wl or 1/2 wl, and all lengths in 
between or beyond.  It also calculates the effect of line loss, which 
can be very important for shorted or open lines, used for matching, or 
other lines which operate at high SWR.

Your approach to modeling transmission lines with parallel wires can 
work fine, but it may be necessary to use a high number of segments to 
get reasonable accuracy.  And as W7EL notes (thanks K9AY), the short 
wire segments for the source and load can exceed the minimum 
recommendations.  Be sure and check how well your model works by 
checking it at the frequencies where it is 1/4 wl or 1/2 wl long with 
loads equal, higher, and lower in Z than the line.  Then check that the 
impedance at the source end is what you expect.

73, Terry N6RY

On 2009-09-24 9:15 AM, Paul Decker wrote:
> Using EZ-NEC, is it reasonable to model an open wire feed line, for example a 600 ohm ladder line using two parallel wire segments shorted at each end by two other wire segments, one end being the source, the other being the load? 
> I know I can add a transmission line and change the impedance, but what about if I'm modeling the "special" transformation properties of specific lengths of transmission lines?  
> If not, how is this done?    
> thanks, 
> Paul (KG7HF)

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