MISCONCEPTS ON VERTICALS
i4jmy at uugate.wa7slg.ampr.org
i4jmy at uugate.wa7slg.ampr.org
Sun Sep 22 15:08:36 EDT 1996
There is a lot of confusion in the matter of vertical antennas, ground
planes, elevated radials and counterpoises.
This will remain until the concept of NEAR and FAR FIELD won't be taken
in consideration and understood as it would deserve.
In a classical Ground plane antenna, disregarding the other parameters,
important but not related to the actual topic, unless the radial lenght
is ENORMOUS, what is done varying their number, size and position, just
under the antenna, is not worth to affect substantially the type of Far
This is the reason why the best set of radials will bring to have the best
possible antenna efficiency, but without changing SOIL and AREA type, the
inherent performance won't change ever.
The most efficient Ground Plane will be a poor DX radiator when on a desert
area, a "poor" Ground Plane will be an outstanding antenna over sea water.
A real Ground plane antenna then is that placed at ground level with ground
electrical connection to the soil, where the ground works as the "missing"
half of itself and for the current return path to the generator.
Return current that's increased with the aid of a number of radials.
The antenna radials will work for ONLY for increasing efficiency and NOT
for other purposes.
There is infact no need or electrical reason to make them quarter wave long.
If the ground were perfecly conducting the radiation resistance,in case
of a quarter wave radiator, would be exactly the half of a dipole and the
maximum radiation in the vertical plane would occur at an hypothetical 0
This situation is nearly reached (Brewster angle is always present) over
A Ground Plane elevated from ground is then a different antenna than the
ground plane in the strict sense of the word.
The radials now are an electrical COUNTERPOISE, the term ground plane is
then largely improper.
A quarter wave elements antenna of this type, with its center just more
than 1/4 lambda from ground, performs like an half wave vertical dipole
with the center at the same height.
One million of radials in such an antenna will'not make it more effective.
They can be only 2 (but opposite to cancel their radiation) or they can be
collapsed straight down from the feed point toward the ground, in the way
that the antenna is transformed to a vertical dipole.
Of course if fed point height is much less than 1/4 wave, there is no space
for a full size vertical quarter wave element and the need of a counterpoise
is the obvious consequence.
Counterpoise has to be bigger (in number of horizontal radials) as much as
the ground level is approached, reaching the classical requirements at zero
It is not worth for HF communications to elevate a vertical antenna a lot
over the ground. Unless the antenna will be several wavelenght from the
ground, there will be two lobes, the first at low angles but small(er) and
the second, reinforced one, but too high for effective DXing.
There are existing a number of PC programs for Antenna Analysis, if well
used they can show what really happens even to people who is not involved
in the antenna & communication field and the complex math of the antenna
What's commonly assumed is often not true, or better, what's read on books
or spoken about is often misunderstood or applied where it shouldn't be.
E-mail: i4jmy uugate.ampr.org
More information about the CQ-Contest