Re: NEC-LIST: calculating antenna bandwidth

Thanks for the Reply Jim.

I have interleaved my comments with your discussion below

----- Original Message -----
From: "Jim Lux" <James.P.Lux_at_jpl.nasa.gov>
To: "Wayne Shanks" <wshanks_at_matricsrfid.com>; <nec-list_at_gweep.ca>
Sent: Monday, April 01, 2002 7:04 PM
Subject: Re: NEC-LIST: calculating antenna bandwidth

> While an antenna is sort of like a tuned circuit, with a bandwidth, I
don't
> think the bandwidth is a function of stored/lost energy, as in a RLC
> circuit with a Q.

I still feal that stored energy is one part of a usefull definition of
antenna bandwidth

>
> Bandwidth is also a function of the source/driving impedance. Say my
> transmitter were a pure current source with infinite compliance. The
> antenna's radiated power will be, by and large over a limited frequency,
> proportional to the drive current(less any losses in surroundings and
> elements, of course). The reactive "power" circulating between antenna and
> transmitter will aggravate resistive loss in the feedline (or non-ideal
> Tx), but this is a problem that power distribution engineers face every
day
> when feeding generally inductive industrial loads like motors.

Yes... The use of Current sources (high or infinite source Z) complicates
such a discussion.

Currently (no Pun intended... maybe) I am using an applied E-field source
( EX card type 0)

In such a system the currents should be proportional to Z.

I assume the antenna load is real and is quala to the antenna port Z at
resonance.

>
> An antenna, driven from a resistive source (i.e. a 50 ohm transmitter),
> might, in fact, be an unmatched load, and reflect some power back, to be
> absorbed in the source, but is that really a "bandwidth" issue, or more
one
> of impedance matching.

Yes... I controll such issues...
For the this discussion, let us assume that the load is a perfect match for
the antenna at resonance.

Later on I can wory about my matching circuit and it's bandwidth.

>With an antenna tuner made up of fairly low loss Ls
> and Cs, acting as an impedance transformer, the apparent bandwidth (in
> terms of radiated power vs frequency, for constant voltage (!) on the
> idealized source) might be very narrow.

Hmmmm. None the less this is how any real antenna feed behaves

>
> But, the bandwidth of the antenna, in terms of power radiated vs power
> applied to the feed point, will probably be quite wide (although, the
> "bandwidth" (calculated this way), in a particular direction, might change
> dramatically).

Hmmmm.... you are refering to low antenna efficiency.

And just for clarity, let me not assign any directivity to the bandwidth....

Let us just consider the Impedance bandwidth (I am using simple dipoles)

>
> For a simple antenna, you could probably use one of the approximations for
> feedpoint impedance based on the sine and cosine integrals.

>Run NEC for a
> few points to "fit the curve" (i.e. find the parameters), and then
> interpolate. Another approach, based on the observation that simple
> antennas have Smith chart plots that are smooth curves, is to just run the
> calculation at a few points, fit a spline or other useful curve to the
> curve, in Smith Chart coordinates, and then look for the intercepts with
> the appropriate VSWR circle.

I have a numerical solution that is a varient of this aproach.

It works fine.... it is just un elegenat, and computationally expensive.

>
> For what it's worth, you can make an extremely broadband antenna (in the
> relatively constant feedpoint impedance sense), with low actual loss, with
> an appropriate geometry... Here, the apparent "Q" would be low, but there
> isn't any loss.
> Such an antenna might not meet some other criteria (simplicity,
> size). Equiangular spirals or other frequency independent configurations
> spring to mind. Horns and biconical antennas are also possibilities.

I will think about this a bit, Thanks for your input Jim

Wayne Shanks

>
> Jim Lux
> Spacecraft Telecommunications Equipment Section
> Jet Propulsion Laboratory
> 4800 Oak Grove Road, Mail Stop 161-213
> Pasadena CA 91109
>
> 818/354-2075, fax 818/393-6875
>
>

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