[antennaware] Modelling verticals with EZNEC

David Gould david.gould@btinternet.com
Sun, 19 Sep 1999 19:06:50 +0100

I have just started doing some antenna modelling using EZNEC V2,  
I am getting some results which seem to go against common sense.
I have been modelling some different types of loaded verticals and 
comparing them with a straight 1/4 wave vertical.  

The idea that I want to persue is using a centre-loaded vertical were the
coil is larger than is needed for resonance so that the feed impedance is
50 -jxx and then tune the reactive part out at the feed point with a series
capacitor.   (In the same way as a 3/8wave inverted L is often fed)

When I model this antenna the gain comes out HIGHER than a straight
1/4wave vertical.. This does not seem logical because of the losses of the 
loading coil.  I feel that I may be missing something or have made a  wrong 
assumption.    Any thoughts or advice would be appreciated.

The details of the antenna is as follows:-
Base part 56ft (tapering from 12inches to 6inches)
Join section 1ft (2inches) - holds centre loading coil where appropriate.
Whip section 30ft (tapering from 2inches to 0.1inch - like a fishing rod)
Freq 1.85MHz
I am using Real(Mininec) ground and adding 10ohms in series for ground loss
I am using a Q of 300 to derive the resistive losses of any coil.
I am using 1 segment per ft approx.

The results I get are as follows:-
                   GAIN    SOURCE IMP   COIL            COMMENTS
1) true 1/4 wave
                   0.4dbi      46 + j0           none        Reference antenna
2) base loaded  
                 -1.2dbi       21 + j0           -148        Feedline
tapped onto coil   
3) cntre loaded (resonant)   
                 0.15dbi       30 + j0          -540         Need further
matching at base
4) cntre loaded (resonant at lower freq abt 1.6MHz)  
                 0.65dbi       50 - j176        -710          Feed thru
series C(+176)

In antenna 4) the coil is modelled as 2.8 - j710  
I have used 710/300 = 2.3 plus 0.5 for connections = 2.8 resistive 
With the losses of this coil I cannot understand why the gain is better
than the
straight 1/4 wave which has no losses..  I would expect the gain to be several 
db down.

Any help/advice would be much appreciated..

73 Dave G3UEG


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