Topband: EZNEC Model earth for Inv. L Re: EZNEC Shunt Feed Model Gain

[K4SAV]

G3PQA wrote:
> I would appreciate advice from Group which earth is recommended for 
> best accuracy when modelling an inverted L. Real-mininec-earth or real 
> high-accuracy-earth?  Am getting around 2dB difference between the two 
> methods (+1 and +3dBi).
> Also, should a loss resistance be put in series with real high 
> accuracy model radials to simulate earth impedance?
>  
>
Modeling shunt fed towers (if the shunt wire is included) and inverted 
Ls are two very difficult antennas to model in EZNEC (if you want an 
accurate answer) because you run up against rule limitations. 

With an inverted L for 160, invariably you end up with a low horizontal 
wire (compared to the wavelength).  That is a rule breaker when you use 
a Mininec ground.  If you do this, it usually results in a gain that is 
too high and a feedpoint impedance that is too low (usually but not 
always).  The longer the horizontal wire, the higher the error.  So you 
have to use a real ground (Sommerfield-Norton).  That means you have to 
model the radials.  Modeling radials is also tricky.  Since you have to 
put them above ground with NEC2, they will show resonance properties 
(unlike radials in the ground).  That means that if you model the exact 
radial system you have and that radial system only has a few radials, or 
radials that are not a quarter wavelength, you are likely to get some 
really strange results. The height of these radials above ground will 
also affect the antenna gain and feedpoint impedance, so you have to 
determine the proper height for the radials.  I also think EZNEC under 
estimates near field ground loss for low wires.  That is an opinion 
shared by several persons whom I would consider to be experts.

I will give you my method of reducing some of these errors (even though 
most people won't do it because it is too much trouble).  My solution is 
to model a very good radial system that has insignificant loss.  That 
requires many radials and some of different lengths to eliminate the 
resonant effects.  You can determine when you have arrived at a good 
radial model by using a vertical and comparing the response to a Mininec 
ground (which has no near field ground loss).  You will discover EZNEC 
arrives at an almost lossless radial system long before most tables for 
radial ground loss do.  NEC2 thinks that 16 low radials is within 0.03 
dB of perfect, however I use more than that to eliminate the 
resonances.  Then I go to that table of equivalent ground loss 
resistance for the radial system I want to model.  I then insert this 
resistance at the base of the vertical.  There is another problem here.  
Those tables were generated for quarter wavelength verticals in the 
broadcast band.  That may be close enough for a resonant inverted L, but 
it may not be accurate for an inverted L longer than a quarter 
wavelength or for frequencies far removed from the broadcast band.  You 
can then determine the proper height for the radial system by using a 
vertical and comparing the response between the radial system at various 
heights and a Mininec ground.  Then go back to the L model after you 
have determined a proper height for the radials.

So you see it isn't easy when you try to use work-arounds for NEC2 
limitations.  NEC4 also has some limitations but most people don't have 
that.  Using the method above I have been able to accurately calculate 
the feedpoint impedance for inverted Ls, but I have no way to validate 
the gain.  It does yield gain numbers that are CLOSER to measured data 
generated by others, than when not doing these work-arounds.  That's 
mainly because you will be replacing EZNEC's calculated near field 
ground loss with measured ground loss.  So I really don't know how 
accurate the gain numbers are, only that they are closer to real.

Be sure and always do the test for simulation errors using the "Average 
Gain" number.  There are a few other minor problems, but the stuff above 
should keep you busy for a while.

I don't think you will find any references to the techniques listed 
above since I made it up.  If you do find any please let me know.  
Comments appreciated.

Jerry, K4SAV