At 12:09 PM 2008-05-06, Andrew Ikin wrote:
>I have a model for loop antenna. I have noticed that if I increase
>the wire diam. from 0.5mm to say 12mm for a 20m circumfrence wire
>loop antenna the 1.1MHz gain goes up from -28dBi to -13dBi.
>Increasing the diam. again to 100mm the gain is now -5dBi.
>Is the increase in gain due to lower wire resistance or a reduction
>in the loop inductance or both?
>Can I use say 300 Ohm ribbon ( wires in parallel ) to get the same
>results as 12mm diam wire. Also can I use two wires spaced 100mm to
>simulate a 100mm diam wire?
Andrew sent me a copy of his model, which is a non-terminated
triangular loop, apex up, fed in the center of the bottom wire, which
is 1 meter high. It does appear that the resistance of the wire is
the main determinant of the loop gain.
The radiation resistance of the loop at 1.1 MHz with zero loss
conductors, 0.5 mm diameter (~#24 gauge), is only 0.00342 ohms and
the 3D average gain over "MININEC" ground is -0.94 dBi. (Although it
would seem like a better idea to use "High Accuracy" ground, with
small loops using low resistance conductors, there are residual
errors that can make the calculated feedpoint resistance become
negative, at which point the model fails.)
Changing to a copper conductor of 0.5mm diameter raises the resistive
component at the feedpoint to 3.85 ohms and drops the average gain to
-31.45 dBi. So with a small conductor, the copper losses are over
1000 times the radiation resistance.
Doubling the conductor diameter to 1mm lowers the resistive component
to 1.81 ohms and raises the gain by 3.3 dB.
The loop inductance also decreases with increasing wire diameter, but
with zero loss conductors, the gain only changes by 0.03 dB going
from 0.5 mm to 32 mm diameter wire, while the inductive reactance
drops from 233 ohms to 120 ohms, so it would seem that the inductance
itself isn't an issue.
Using 300 ohm line will give roughly half the resistance of a single
conductor of the same diameter of each of those in the line, but will
have much more loss than a single conductor with a diameter equal to
the line spacing.
In the real world, you'll have to deal with impedance matching to
what looks like a high-Q inductor and this can have a big impact on
the system gain for receiving. (EZNEC essentially assumes no mismatch loss.)
73, Terry N6RY
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