Re: 40-meter Loops: which way is up?

---------

From: L. B. Cebik (cebik@utkux.utcc.utk.edu)
Date: Fri Nov 29 1996 - 17:21:06 EST


In the various exchanges about full-wave loop antennas, replies often omit
the crucial fact: whether they are referring to a horizontally oriented
loop or to a vertically oriented loop. The difference makes a great
difference in performance and construction ease.

Among vertically oriented loops are the delta and the quad (usally square
rather than diamond for the low bands. Square loops or quad loops of 1 wl
generally show a lower take-off angle than dipoles set at the height of
the lower quad wire--simply because the upper wire contributes to that
angle. And they have a little gain over the dipole, but the exact amount
depends on comparative heights between dipoles and quad loops.
 
Vertically oriented deltas give the operator two options. Fed up one leg
(precisely where depends on the nature of the triangle), they become
phased verticals with a very low take-off angle, but lesser gain. Fed at
the bottom or top center, they become dominantly horizontally polarized
antennas similar in performance to the square quad loop--allowing for
differences in how much wire is high and how much is low.

Horizontally-oriented delta and quad loops perform much like dipoles at
the same height, with essentially the same take-off angles and only a
little gain (or broader lobes, depending upon actual layout). Just as
with a dipole, the key word for improvement is height, the higher, the
better, as the take-off angle (or lobe of maximum radiation) gets lower.
I have modeled dozens of configurations of horizontally-oriented 1 wl
loops and can find no especial good reason to prefer one layout (square,
triangle, hex, octagon, etc.) over another.

Unless one can get a non-vertically polarized antenna at least one-half wl
up, expect a high angle of maximum radiation, good for short skip paths.
If you are stuck below the half-wl level for the horizontal, consider one
of the vertical antenna options for a second, low take-off angle antenna
for longer skip paths on the lower HF bands. Among such antennas are the
inverted L, 1/4-wl vertical, vertical dipole, the sloper (fed at the
bottom or the top of the wire, the delta (equilateral or right
angle) fed as phased verticals, and the half square.

All models presume flat, uncluttered terrain. Peculiarities of local
terrain can modify antenna properties and make an antenna work well (or
poorly) beyond its modeled performance. However, the peculiarities of
terrain are not transferrable to other locations. Equally
non-transferrable (but teachable) is operator performance--some of us can
work the world while loading up a single human hair; some of us have
difficulties working the guy next door with rhombics at both locations.
When passing along ideas about what works and what does not with antennas
for the lower HF bands, we should all try to sort out what is generally
true and thus forms the basis for good expectations on the part of
others--and what is true because of the peculiarities of my own situation
and thus might not work as well elsewhere.

Case in point with another type of antenna. I have just finished a study
of off-center-fed (OCF) dipoles (1/2 wl). They perform just as OCFs
should perform, but about 80% of what has been said of them in print
(especially ads for kits) is simply false or misleading. A lot of OCF
"data" has come from individual "experience" and wishfully simplified
formulas, but little of it prepares the OCF builder with the right
expectations.

Where antennas can be vertically or horizontally oriented, I hope we can
keep our reference points straight in the exchanges so that we the readers
and learners can apply the right information to the right antenna.

-73-

LB, W4RNL


Search QRP-L Archives

[ QRP-L Archive | ]
[ 1993 | 1994 | 1995 | 1996 | 1997 | 1998 | 1999 | 2000 ]

---------

This archive was generated by hypermail 2b29 on Fri Jun 02 2000 - 11:31:27 EDT

kd4ab@kd4ab.org