John KD5YI wrote:
"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

If you're just looking for a portable vertical antenna, just a wire on
the end of the coax, and hanging it up, works pretty well. The shield of
the coax (outside surface) serves as the other half of the dipole.
Heck, it will have a bizarre pattern, and couple RF everywhere, but
you're talking about an antenna you're hanging out of a hotel window or
something.. You're not doing earth-venus-earth radar tests... you're
just getting the antenna away from where you are sitting to "outside"..

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

"Rich Griffiths" wrote in message
communications...
On Thu, 28 May 2009 11:15:52 -0400, Bob D. wrote:

I decided to install a 2m FM rig in my garage to stay in touch with the
local repeaters while working on the car.

snip

A J-pole for outdoor use can be easy to build, easy to mount, and very
rugged. For indoor use, you might want to settle for a simple twin-lead
J-pole, which is even easier to build and can be easy to mount.

snip

I'd go with whatever seems easiest to make and easiest to mount in your
particular situation.

Amen. I have long been a fan of the copper pipe J-pole.. I've built about
two dozen of them over the years, 6m, 2m, 220 and 440. We have three of
mine on the local fire station for RACES. We go up on the roof and retape
the connectors every few years -- that's it.

My only deviation from the many Internet plans has been the inclusion of a
100 pF cap inline with the center conductor at the feed point if the VSWR
minimum wasn't 1:1 somewhere in the band after the first couple of tries.

Yes, you should choke the feedline, if not with ferrites, then with a few
tight turns of the coax.

The copper pipe j-pole can mount to almost anything; there's no significant
RF current in the lowest pipe section (below the J).

"Sal"

"Jim Lux" wrote in message
...
John KD5YI wrote:
"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

If you're just looking for a portable vertical antenna, just a wire on
the end of the coax, and hanging it up, works pretty well. The shield of
the coax (outside surface) serves as the other half of the dipole.
Heck, it will have a bizarre pattern, and couple RF everywhere, but
you're talking about an antenna you're hanging out of a hotel window or
something.. You're not doing earth-venus-earth radar tests... you're
just getting the antenna away from where you are sitting to "outside"..

If you are using such construction, why not just build a coax type of
colinear? It isn't so difficult to add a section of outer shield for
decoupling. Have you priced PA hybrids lately?

BTW any degree uptilt is usually worthless unless you are deliberately
limiting coverage to the horizon. and anything significantly higher in
elevation is line of sight anyway. For ground level base station or
repeater that covers a localized area, a J-pole is attractive because of its
broad pattern that allows numerous reflections through and between the
buildings. I've been told that an upside down ground plane ought to work
better, but it doesn't

"Jim Lux" wrote in message
...
John KD5YI wrote:
"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

If you're just looking for a portable vertical antenna, just a wire on the
end of the coax, and hanging it up, works pretty well. The shield of the
coax (outside surface) serves as the other half of the dipole. Heck, it
will have a bizarre pattern, and couple RF everywhere, but you're talking
about an antenna you're hanging out of a hotel window or something..
You're not doing earth-venus-earth radar tests... you're just getting the
antenna away from where you are sitting to "outside"..


Bob D. -

Please don't take my comments as being negative about a J-Pole. I happen to
like them for other reasons. They can be made very rugged. The few I made
started out as a 30-foot mast. 3/4 wavelengths down from the top I mounted a
horizontal metal bracket. To that I mounted a vertical 1/4 wave tube. I then
used automotive hose clamps to connect a 1/2 wavelength piece of coax near
the bottom just above the horizontal bracket. I adjusted the connection
point of the coax to find the lowest SWR point and then replaced the 1/2
wave piece of coax with my longer lead-in. I put some weatherproofing on the
coax.

I ran 10 gauge wire from my nearby ground rod over to the bottom of the
mast. What I like about this arrangement is that the entire assembly is
grounded. I once had a nearby lightning strike cause my IC2AT to block
signals for several seconds while I was listening to a local repeater with a
home-made ground plane. Charge buildup, I guess. That never happened with my
J-Pole. That thing worked flawlessly for several years until I replaced it
with a commercial dual-band (expensive) antenna.

Have fun.

73,
John