"JB" wrote in message
...


The 1/4 wave ground plane has a useless pattern. Main lobe at about 30
degrees above horizon, but broad enough to be usable and simple to build.


The quarter-wave antenna's height above ground has much more to do with the
elevation angle than the fact that the antenna is a quarter-wave ground
plane. Using EZNEC, I see that a quarter wave antenna situated 3 wavelengths
above real/high accuracy ground of medium characteristics has a main lobe 4
degrees above the horizon. At that angle, the gain is 5.1 dBi. You can
confirm this if you have a copy of EZNEC.

John

JB wrote:

The 1/4 wave ground plane has a useless pattern. Main lobe at about 30
degrees above horizon, but broad enough to be usable and simple to build. . .

J-poles, vertical dipoles have a more useful pattern below horizon. They
provide useful performance without being too large. . .

A quarter wave ground plane (with or without sloping radials), a J-pole,
and a vertical dipole all have virtually identical patterns provided
that the outside of the feedline can be adequately decoupled. In free
space, the maximum is directed to the horizon. In a real installation,
height above ground and reflections from other objects will modify the
elevation pattern, in the same manner for all those antenna types.

The patterns of all these antennas can be affected by current conducted
to or induced on outside of the feedline. Those with EZNEC or other
modeling program might find it interesting to attach a vertical wire to
the "ground" side of the feedpoint and extending downward to represent
the outside of a feedline. An effective current (choke) balun can be
simulated with a 1k ohm resistive load inserted in the wire. You'll find
considerable current can occur on the wire when it has particular
lengths, the lengths depending on whether the bottom end is grounded or
open, and placement of "baluns". When the current on the wire is high,
considerable pattern distortion can result. I've always supposed that
this is the cause of widely differing reports of the effectiveness of a
J-Pole -- some people get luckier than others with feedline length. A
pair of current baluns, one at or near the feedpoint and another about a
quarter wavelength below, are usually enough to suppress the current on
the outside of the feedline to a low enough value to prevent pattern
distortion.

Roy Lewallen, W7EL

On May 29, 9:31*am, "JB" wrote:


The 1/4 wave ground plane has a useless pattern. *Main lobe at about 30
degrees above horizon, but broad enough to be usable and simple to build.

The 1/4 GP with sloping radials and the 1/2 wave are so close in
performance to almost be unnoticeable if both are mounted in
the air say at 30 ft. The two antennas will have almost the same
pattern at almost the same exact angle. When modeling both
at 30 ft, the GP's maximum is at 2.7 degrees, vs 2.6 degrees
for the 1/2 wave. "145 MHZ".
The GP's performance will vary a bit due to the number of
radials, but in general the difference between the two antennas
will amount to about .3 to .5 DB. Not much.
This does not take into account common mode currents.
In some cases, I bet it's possible for a 1/4 GP to outperform
a J-pole if the J pole has no decoupling from the line.
Myself, I've never used a J-pole. I prefer a gamma loop
type match if I build a base fed 1/2 wave. I don't like
the matching device to be parallel with the radiating element.
Most of my "simple" 2m verticals are 1/4 wave GP's.. :/
I have one in the attic hung from the rafters. It has either
6 or 8 radials.. I forgot which.. Been a long time since I've
been up there. If you use more than 3-4 radials which is
the norm, you will see an increase in performance.
I think more due to better decoupling of the line rather
than less ground loss. At 30 ft, ground loss is not much
of an issue as long as the antenna is complete. Just one
radial will make for a 1/2 wave vertical dipole of sorts, and
ground loss should not be much of an issue at several
wavelengths in the air. So... I think the increase in
performance is more due to better decoupling from the line.
Either type can use further decoupling techniques for
improved performance. The GP will usually use a 2nd
set of radials 1/4 wave below the main radial set.
The same scheme can be used for the 1/2 wave if
common mode currents are a problem and skewing
the pattern up off the horizon.
In many cases, decoupling of the line is more important
to gain at a low angle than element length.
Does no good to use a longer element if line currents
skew the pattern upwards.

Richard Clark wrote:

More efficient than a rubber ducky was allowed (the need for that
efficiency has been skirted, however), so the claim of efficiency
appeals to vanity when the difference was already noted. More
efficient than a rubber ducky giving full quietening? What does that
matter if not to suit vanity?

Is it vanity to think that since the repeater is full quieting on their
end with their rubber ducky, then their signal must be full quieting
into the repeater, or vanity to wish such people would use a j-pole?

ac6xg