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On 5/19/2012 7:35 AM, Billy Cox wrote:
However, to make postings claiming 12-18 db GAIN or that
elevated radials are "best" is a distortion of technical
truths and may cause others to incorrectly understand how
our antenna SYSTEMS really work with each QSO we make.
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HHmmm ...
After spending a lot of time reading the ARRL
Handbook, The ARRL Antenna Book, ON4UN's fine
treatise on Low Band DX-ing, Rudy Severn's fine
articles, and many more venerable sources,
including, without limitation, Orr and Cowan's
Antenna Handbook...
I came to the following conclusion, a Rule-of-Thumb,
as it were:
Four well-trimmed, tuned elevated radials is
roughly EQUIVALENT to having a substantial bed
of ground radials.
If so, that would make a few elevated radials approximate
the same effect as, say, 35 radials on the ground.
I admit this Rule-of-Thumb qualifies as "Ham Lore,"
but, after reading a lot on vertical antennas, and
after building and using them for the past couple
of years, I think it is fairly accurate - at least as a
generalized starting point.
THEREFORE,
I find it difficult to believe a 40 meter dipole at 45 feet will exhibit
12-18 dB gain over a vertical at the same location.
MUCH CONFUSION OCCURS AS A RESULT OF DIFFERENCES IN MEASUREMENT
PROCEDURES, AND SLOPPY USE OF STANDARD REFERENCE FIGURES.
Consider how some reference gain against an isotropic antenna, which has
0 dB gain. Others reference gain against a dipole IN FREE SPACE which
is said to have 2.15 dBi (i.e., gain over an isotropic antenna).
Still others, including W8JI, reference it against a dipole OVER REAL
GROUND which might have as much as 8 dBi because of the effects of
ground reflections. Verticals are sometimes claimed to not have
ground reflections in the near field (as Billy C suggests in his post),
but I thought there ARE ground reflections - at least in the far field
... something having to do with the Pseudo Brewster Angle...
Unfortunately, as Billy C suggests, various claims of gain or loss are
bandied about in a rather cavalier manner, which contributes to the
inaccuracy and fallacious nature of "Ham Lore." We MUST, as Billy
suggests, and Rudy Severns wrote in his Letters to the Magazine Editors,
and according to my favorite college logic Professor Sheridan
urged.... WE MUST BE RIGOROUS !! Especially rigorous in making sure
we are comparing apples to apples and not cherries to bowling balls.
The process of comparing antennas is a convoluted and difficult process,
because we are NOT comparing apples to apples in all respects, and yet
we must, and do, make comparisons.
Orr and Cowan, state:
"Strictly speaking, vertical and horizontal antennas cannot
be compared against one another as far as useful gain goes,
but such a comparison is meaningful in terms of results,
and provides the operator with a real-life comparison
between vertical and horizontal polarization."
Orr and Cowan, Radio Amateur Antenna Handbook, p. 23.
I believe much confusion has resulted from, and a significant quantity
of misleading Ham Lore has been promulgated, because proponents of one
antenna or another have failed to discern ALL the variables and
characteristic differences between the two or more antennas under
scrutiny. They often mix metaphors, as it were, by using the FREE
SPACE figure for one antenna with the OVER REAL GROUND figure from
another - either purposely to skew the argument, or accidentally
(ignorantly?) as a result of carelessness or haste.
You can compare two identical dipoles ... one as 2.15 dBi in free
space, with one that is 8 dBi over ground, and claim that one dipole is
better than the other. You can then claim the latter dipole is 8 dBi
better than the proverbial quarter wave ground plane antenna, which is
theoretically measured a .30 dBi.
AND YET REAL WORLD EXPERIENCE TELLS ME the vertical often outperforms
the dipole, and vice versa, depending on ALL THE OTHER VARIABLES of the
location. (Somebody else has also mentioned these situational and
circumstantial variables...) These include, as as been mentioned
previously, the fact the vertical generally has an omnidirectional
radiation pattern, while the dipole has a generally figure-8 pattern of
radiation, and, so, it stands to reason the vertical may perform better
than the dipole (which has more gain...) in some directions, especially
when the other station is located in the dipole's off-end null; but the
dipole may perform better than the vertical GP antenna, when the other
station is located exactly perpendicular to the dipole s axis.
And there is a myriad of variables that compound any comparison
(including those of geographic location, proximity to buildings, trees,
and other objects, the nature of those other objects, variations in
ground chemical composition, variations in water tables, and so forth.)
So - I guess I agree there is a lot of sloppy analysis being passed
around as science these days -- although, in a discussion forum such as
this one, one must resort to a certain amount of generalization in order
to make a point without consuming excess bandwidth, which leaves him
open to criticism for doing just that.
Nevertheless, WE MUST BE AS RIGOROUS AS SPACE AND TIME ALLOW... and the
more ... the better. That way, as Billy C suggests, we will not claim
more from any given piece than the author or the data can support.
So... as with so much else in life... you just can't win !
All in all,... I conclude a few elevated radials
renders your vertical roughly equal to a ground-mounted
vertical with a substantial radial field. It is not, however,
substantially - certainly not 12-18 dB better - than a
ground mounted vertical with a good radial field.
----------------------------- JHR --------------------------------
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