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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