Re: NEC-LIST: Fano criteria

Doug:

Thanks for the comprehensive commentary/summary on broadbanding and
autotuners. I agree that the autotuners are working the "narrow band"
matching problem; whereas, the broadbanding problem is to achieve an
acceptable input impedance over a broader band of interest. And the
gain-bandwidth product being a constant for a given setup applies to
the broadbanding case. And the autotuner, if it does its job well,
should always win the narrow band race.

Some years ago I was involved in designing and testing broadband HF
antennas for manpack radios. In order to avoid the need for a tuner of
any kind, I went for antennas with a constant input impedance over the
2-30 MHz band (with a goal of VSWR <= 2:1). This was achieved using
travelling wave antennas, such as vertical half rhombics and sloping V
antennas with resistive terminations and feedpoint baluns as
required. The exact values of the resistors and the balun
transformation ratio were optimized, as were the required
elevation-plane patterns, using NEC. The result was a set of antennas
to cover different communications ranges which could handle fast
frequency hopping (FH) radios or direct-sequence spread spectrum
(DSSS) radios operating within the HF band by feeding the antenna(a)
from a 50-ohm port on the manpack transciever.

I was, in a sense, buying bandwidth with loss (the power dissipated in
the terminating resistors), which is fine if your link budget can
tolerate the loss.

All for now.

George
Received on Tue Feb 15 2000 - 02:50:00 EST