TopBand: Resonant verticals

[John Kaufmann]

Resonance is not a fundamental requirement for an antenna to radiate
or to receive.  As any antenna engineer can tell you, an infintesimally
short vertical monopole has a theoretical gain within a fraction of a 
dB of a "full size" quarter wave vertical.  Those of you with antenna
modeling programs like EZNEC can easily verify this.

However, there are some practical reasons to consider an antenna which
is resonant, or nearly so:

1) Nonresonant antennas which are electrically much shorter than 
a wavelength have very low radiation resistance which makes their
performance much more dependent on other resistive losses.  A very
short vertical will not produce as much gain as a quarter wave radiator
in the presence of significant resistive ground loss.

2)  Impedance matching tends to be simpler around resonance.  The 
absence of a reactive component to the impedance around resonance
makes it easier to obtain broadband matching, at least with simple 
networks.  Also, without  significant reactance, the voltages and/or
currents developed at the feedpoint tend to be less (sometimes much
less when compared to far-out-of resonance antennas like half-wave
verticals), which puts less stress on any feed network components at
the antenna.

3) I have also observed that impedance matching for non-resonant antennas
is more sensitive to environmental effects.  Example:  I load my 80-meter
quarter wave verticals as 1/8 waves on 160 meters and as 1/2 waves on 40.
In the wintertime when the ground is frozen and the leaves are off the trees,
the SWR curves on 160 and 40 always shift relative to the summer.  There is
virtually no detectable difference on 80 where the antenna is resonant,
however.
Similarly, when it rains, the SWR always goes up on 160 and 40 but not on 80.

As a side note,  I take great care to impedance match all my antennas to as
close
to 1:1 SWR as possible.  The justification for this is not any improvement
in real
performance compared to, say, 1.5:1 SWR.  Rather when something goes wrong
with the antenna (which inevitably happens at some point), it's very easy
to spot
the problem as a deviation from 1:1 SWR, and it's often easy to diagnose what 
the problem might be by observing what happens to the SWR curve.

73,
John W1FV 

***************************************
John Kaufmann
Optical Communications Technology Group
Room C-243
M.I.T. Lincoln Laboratory
Lexington, MA 02173

kaufmann@ll.mit.edu  (e-mail)
(617) 981-4041       (voice)
(617) 981-4129       (FAX)

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