RE: NEC-LIST: calculating antenna bandwidth

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

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