Re: NEC-LIST: induced voltages

In response to dipole output voltage queries:

As far as I know you get 1 V in a 1 V/m field if you use a 1 m so
called "ideal dipole". This is a electrically short antenna with a
uniform current distribution. This will be the open circuit (high
impedance receiver) voltage.

A uniform current is in practice impossible on a short dipole unless
one uses capacitive plates on the ends. Without end loading the
current distribution is approximately triangular. This has a current
moment (area under the current/antenna position curve) of 0.5 compared
to uniform current distribution and hence you will get 0.5 V. If you
use a matched load rather than a high impedance then this will drop to
0.25 V. (see the values computed by earlier respondent)

So it is quite simple (for electrically short dipoles): The voltage
out (o/c voltage or measured by high impedance receiver) will be the
antenna length * the field * the current moment relative to an ideal
dipole current moment. The antenna length times the current moment
relative to ideal dipole is also called the antenna effective
length. Hence the field * effective length gives voltage out into high
impedance. Voltage into a conjugate matched receiver will be 0.5 the
o/c voltage by simple circuit theory.

Regards
Andre Fourie

PS In response to other comment: For maximum power match receiver to
antenna - For maximum voltage use high impedance receiver.

> Date: Tue, 23 Jun 1998 08:26:44 -0500
> From: Clifford Kraft <chkraft_at_lucent.com>
> To: nec-list_at_ee.ubc.ca
> Subject: Re: NEC-LIST: induced voltages
>
> All real antennas have something called an antenna factor which is the
> ratio of actual E field intensity (in a uniform field) to voltage at
> the terminals. In the real world, this is always greater than 1
> meaning that in a 1 volt/meter field, you read less than 1 volt at the
> terminals. The art of calibrating antennas involves determining this
> factor as a function of frequency.
>
> Cliff Kraft
> chkraft_at_lucent.com
>
>
> Grant Bingeman wrote:
> >
> > I have a dumb question. Assume you have a linear field of one volt
> > per meter. Now assume you have a wire one meter long aligned with
> > that field and loaded with an impedance at its center in such a way as
> > to maximize the voltage induced across that load. Is there a simple
> > relationship between that induced voltage and that impinging field,
> > and what restrictive circumstances must be applied? Must the load be
> > the complex conjugate of the wire's self impedance? Must the wire be
> > suspended in free space? Must the wire be infinitely thin? Must the
> > wire be very short compared to a wavelength? When, if ever, will the
> > induced voltage equal the field intensity?
> >
> > Grant Bingeman

Tel: Intl + 27 11 4030380
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Website: http://www.poynting.co.za
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Received on Thu Jul 09 1998 - 09:48:41 EDT