Re: Antenna design question

K7ITM wrote:
On Oct 23, 10:35 am, richardharri...@xxxxxxxxx (Richard Harrison)
wrote:
Mike, N3LI wrote:

"I thought that the inductance tends donward as the diameter of the wire
increases. I can understand your calculation after the wavelength part,
but don`t quite get the increased inductance part."

Good observation.

Wire inductance decreases with the circumference increase as this
effectively places more parallel inductors in place along the surface of
the wire.

Wire capacitance increases proportionally with the square of the
circunference of the wire as it is proportional to the wire`s surface
area.

The fatter wire grows capacitance faster than it changes inductance.

Reactance along a wire antenna element varies quickly near resonant and
antiresonant points so is not uniformly distributed. This complicates
calculations and requires average values for some. Bailey says of surge
impedance: "Nevertheless, this variation in theoretical surge impedance
shall not deter us from setting uup practical "average" values of surge
impedance. Best regards, Richard Harrison, KB5WZI

I know we're talking about linear antennas here, but even in that
case, it's surely not true that capacitance increases as the square of
the wire diameter (or radius or circumference); nor inductance
proportional to 1/diameter. Consider that if both those were true,
doubling the wire diameter would quadruple the capacitance and halve
the inductance, and the propagation velocity along that wire would be
1/sqrt(4*0.5) or about .707 times as great as with the thinner wire.
Clearly things change much more gradually than that.

Trying to make a "readers Digest" version here....

If I'm following so far:

The lowered frequency of resonance is due to changes in the velocity factor.

The lowered vf is somewhat due to increased capacitance, and an increase in inductance - the latter part I'm still trying to grok. I think there is likely something more going on.

I'm still left with the increased bandwidth phenomenon. None of the above would seem to account for this.

I've been working with mobile antennas for the past several months, and I might be going astray, because I keep thinking about increased bandwidth as a partner of lowered efficiency. Not likely the case here.


Thanks to everyone for the help, while I'm happy to accept the obvious real results, It is even better if I can understand what is going on.


- 73 de Mike N3LI -
.

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