Fwd: NEC-LIST: calculating antenna bandwidth

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From: Fractenna_at_aol.com
Full-name: Fractenna
Message-ID: <115.f41da2c.29db97a9_at_aol.com>
Date: Tue, 2 Apr 2002 18:24:25 EST
Subject: Re: NEC-LIST: calculating antenna bandwidth
To: sbest_at_cushcraft.com
CC: nec-list-admin_at_gweep.ca
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In a message dated 4/2/02 5:02:52 PM Eastern Standard Time,
sbest_at_cushcraft.com writes:

> Hello All:
>
> Sorry that this post is off topic.
>
> This discussion regarding antenna bandwidth is very interesting as there
> are quite a number of different "antenna bandwidth" definitions used
> within the antenna community - both academic and industry. I think this
> is an important topic because there seems to be some discrepancy in the
> IEEE literature on this issue. There also seems to be some discrepancy
> in defining antenna bandwidth and what is acceptable in terms of real
> world antenna performance.
>
> In analyzing or comparing the bandwidth properties of different
> antennas, it would seem that we need to establish a set of antenna
> bandwidth definitions such as SWR bandwidth, or Gain bandwidth for
> example.
>
> First, most antenna designers don't have the luxury of knowing the
> system coaxial cable loss or the transmitter or receiver impedance so
> including these in any analysis of "antenna bandwidth" seems unnecessary
> when comparing or discussing antennas, in and of themselves. Certainly,
> they are important in understanding system bandwidth issues.
>
> The IEEE defines antenna bandwidth to be the range of frequencies over
> which an antenna meets some predefined performance standard (or set of
> performance standards). Many times this is sufficient because the only
> concern may be knowing the frequency bandwidth over which the antenna
> meets a certain gain or SWR specification. However, to compare antennas
> directly or, more importantly, to make claims about antennas being
> broadband or low Q, we need to define specific performance standards.
>
> In many cases, the antenna bandwidth is simply defined by the 2:1 SWR
> bandwidth determined with respect to a transmission line impedance,
> typically 50 ohms. This may be fine in many cases. Many times, it
> makes more sense to define the antenna's SWR bandwidth with respect to
> its resonant resistance. Also note that within the commercial segment
> of this industry, a 2:1 SWR is most times unacceptable and a 1.5:1 SWR
> (or lower) is required over any given operating band.
>
> The area where I have more of an issue is defining the antenna bandwidth
> from its -3 dB response points, especially if these are determined from
> impedance (SWR or Q). The 3 dB response in terms of impedance
> represents SWR limits of 6:1. Other than a very limited number of
> receive antennas, not many antennas are used where a 6:1 antenna SWR is
> acceptable. I recently read a Transactions article where the authors'
> were discussing a small antenna and claimed they had a small, broadband
> antenna (33%) with low Q, all determined from the -3 dB response points
> - the 6:1 SWR. Claiming to have an electrically small broadband antenna
> based on the 6:1 SWR bandwidth is a bit of a stretch in my opinion.
>
>
> Defining the -3 dB frequency bandwidth in terms of gain response (no
> impedance info), seems reasonable but again, there are not many
> commercial applications where the antenna gain is allowed to deviate or
> vary by 3 dB over the operating band. While this could be validly used
> in a relative antenna comparison, I don't think any general claims about
> the antenna being broadband or low Q could be made from this bandwidth
> calculation.
>
> Steve Best
>
>
>

Hi Steve,

I fear that we are treading on ancient ground, and that the 3 dB point is an
accepted convention. As a designer I see its limitations as a convention, but
we should accept it if stated as such.

The origin is from spectroscopy, where it became important (more than 100
years ago) to define a relevant quantity to characterize the spectral line
widths. The arbitrary choice of 'full width half maximum' (FWHM) caught on,
mostly in the physics/chemistry community. Its still a standard one there,
and one speaks of Gaussians with a given FWHM (and sometimes HWHM).

As antenna arrays (read: 'interferometers') migrated from the
physics/astronomy community, the FWHM notion migrated with it--hence the -3dB
bandwidth. I'm sure it also was used before that.

The SWR (2:1)bandwidth definition, as far as I can tell, has a more modern
urgency, since solid state PA's don't tolerate mismatch, and about 1 dB of
returned power was arbitrarily considered as the typical limit.

Depending on who I am talking with, I've just got used to switching from one
useage to another, just as I have to switch from 'beam' to 'array factor '
and ' UV coverage' to 'spatial frequency spectrum'.

Its a lot like saying: which is better-- time domain or frequency domain;-)?

Best,
Chip N1IR

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