This is an answer to an off reflector conversation, relating to a "too long"
electrical length over radials reducing performance. I am writing to the
list since the subject and it's objection occur in so many posted
conversations. Reduction of gain by too high current max has been touted by
some and called myth by others.
The reduction applies in fairly NARROW circumstances, which I think explains
the APPEARANCE of myth. Used in certain other circumstances, raising of the
current center on the vertical IMPROVES the results. The confusion comes
from trying to lump divergent circumstances together, to run the equation
with too few variables.
If the vertical run plus the T (or an L) exceeds an electrical quarter
wavelength, the current maximum will begin to rise away from the base of the
antenna. Some designs deliberately put the current max well up the
vertical. What is it that goes on?
Let us begin with a WAY overkill, "gold standard" 1/4 wave vertical with 120
uniformly spaced 125' elevated radials at eight feet. This will work very,
very well, little disagreement on that. There is deserved argument about
where LOSS of performance kicks in as the number of radials is reduced
and/or made non-uniform in length or spacing. And there is deserved
argument about the DEGREE of loss in such circumstances. The NECx series of
modeling programs underestimate that loss, how much depending on ground
factors (what kind of dirt) that are difficult to measure. In many cases
practically speaking these factors are unmeasurable unless one wants to dig
up the back yard to measure them.
The above gold-standard vertical will present a very predictable measured
gain. It does present the very best avoidance of induced current-in-dirt
losses, assuming the "dirt" is not a salt water marsh. So therefore one can
expect that only REDUCTION in efficiency is possible from here. This IS
assuming we are talking about overall efficiency, NOT measuring gain at a
SINGLE takeoff angle (pattern changes).
Part of the excellent performance of that antenna has to do with the
radials' cancellation of the RF fields headed toward the ground immediately
underneath. This is so because along the radials, the shape of the current
is a very even exact opposite of the current in the vertical wire. The
current maximum on BOTH the radials and the vertical wire are equal at the
feedpoint and reduce in equal proportion moving away from the feedpoint.
This means that the sum of the RF fields from vertical and radials are
hugely MINIMIZED below the radials, EVEN THOUGH in this configuration, the
current maximum in the vertical is as low, as close to the dirt, as it can
be. The cancellation effectively makes the lossy dirt under the radials
close to invisible, and forces the energy, that otherwise WOULD have been
dissipated as heat in the dirt underneath, to be spent as useful radiation
at other, more useful angles of radiation.
It also has the maximum circle of cancellation. An exercise for
illustration: elements are a dark room with a dark floor, an inch thick
book, a circular white dinner plate, and a small non-focusing flashlight
bulb. Put the book flat on the floor. Put the plate on top of the book.
Turn on the bulb and hold it just touching the center of the plate and note
the circle of darkness. Then raise the bulb and note the circle of darkness
shrinking. Vertical radiation aimed at a non-sea-water ground surface is
effectively lost.
The vertical, although this is a very imprecise analogy, has a similar issue
with raising the current center as the cancellation under the radials is
gradually lost as the current center is raised. There is no complete
vertical-radial field cancellation shadow as with the plate and flashlight,
but gradual deepening of shadow toward the center. The more shadow, the
less loss. Because of the inverse cube behavior of magnetic fields, the
degree of cancellation is reduced much more quickly, counter-intuitively, as
the current center is raised. AND THEN the RADIALS are now inducing fields
down at the dirt which are no longer balanced by the REDUCED opposite phase
field from the vertical. The performance *IS* reduced by the increasingly
UNCANCELLED current in the RADIALS inducing lossy current in the ground.
Now let us move away from our ideal to the much more common back yard that
has zero chance of supporting a DENSE and efficient radial system. If a
dense radial field cannot be done, then TO START WITH there was NOT the
degree of possible cancellation under the radials to UNDO, and at some
degree of sparse, or with no radials, RAISING the current center NOW
IMPROVES performance. This is because near magnetic fields return their
energy as the field collapses if they are not dissipated in close resistive
conductors within the field (e.g. dirt), allowing the energy to be
dissipated elsewhere.
Therefore, in sparse radial or no-radial situations like the 5/16 wavelength
single wire folded counterpoise, raising the current center and reducing
current at the base improves the performance significantly. This appears to
be one reason that an up 66', out 66' inverted L antenna does so very well
on 80 meters if one goes through the PITA to handle the very high Z feed
point. First, even the worst of grounds in series with the 2-4k ohm feed is
only a small PERCENTAGE loss, making a low Z counterpoise unnecessary.
Second, as inferred from above, there is proportionally very little E-field
at the ground to heat up the dirt because the current is mainly up at the
bend, and inverse cube factors have knocked it way down by the time it
reaches the ground.
For those with EZNEC, you can watch the fields at the ground by running near
field plots (NF Tab) with z variable (height, not impedance) = 0. For
vertical antennas, higher fields at the ground have higher loss. A ten
times increase of the integrated magnetic field times unit area at the
ground, results in a 20 dB increase in loss. Be prepared to be surprised by
the ugly if you run these calculations to see what happens.
I have not seen studies with dense radials where the radial length was
shortened to reoptimize the field cancellation as the current center was
moved up. Mostly I see pages and emails dedicated only to making the match
to 50 ohm coax as easy as possible. This is a degree of ignoring efficiency
that would never be tolerated with an amplifier, where every last watt is
bemoaned and begroaned. :>)
73, Guy.
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UR RST IS ... ... ..9 QSB QSB - hw? BK
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