Jim,
Nope!
[13-j750] is the equivalent SERIES representation of the feedpoint
impedance, so the same 8.7 Amps flows through the resistive part AND the
reactive part; there is no 500 Amps flowing anywhere! The only effect of
the large reactance is to generate (or require) a large voltage across
it as a result of the 8.7 Amps flowing through it.
Here's another totally different real-life example to show that short
antennas generally require higher currents. Take a look at HI-Q Antennas
web page for their HI-Q-5/80 80m mobile whip - I think you would agree
it's a bit shorter than a full size 80m dipole. HI-Q quote it as having
a radiation resistance of 1.1 Ohms and an efficiency of 14% on 80m -
both believable figures.
Now, what's the current flowing into the base of the antenna when we
feed it with 1KW?
Well, it's 14% efficient so it must radiate 140 Watts. To dissipate 140
Watts in a Rrad of 1.1 Ohms requires a current of 11.3 Amps - that's
about 2.5 times the current into a half-wave dipole at the same power.
Need I go on?
73,
Steve G3TXQ
jimlux wrote:
> Taking your 13-j750 as the feedpoint impedance (including loss
> resistance within the antenna), if you're actually dissipating 1kW
> (either by radiation or IR loss), the inphase component of the current
> will be 8.7A, as you say, but the quadrature(reactive) current will be
> 8.7*750/13 or about 500A. The rms current will be sqrt(8.7^2+500^2)..
> fairly close to 500A.
>
> The source impedance driving this will be 13+j750. Typically, some sort
> of matching network would supply the reactive part, so there's a
> circulating current flowing between the reactive antenna and the
> reactive matching network.
>
> 500A is a healthy current to be flowing, even if it's not contributing
> to the radiation. A 0.1 ohm resistance in the wiring would dissipate
> 25kW. Fortunately, most matching network components have resistances
> much lower than 0.1 ohms (AWG 10 wire, ignoring skin effect, is 1mOhm/ft)
> _______________________________________________
>
>
>
> _______________________________________________
> TowerTalk mailing list
> TowerTalk@contesting.com
> http://lists.contesting.com/mailman/listinfo/towertalk
>
>
>
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|