wrote:
I happen to like them as they can be made from common materials with
excellent ruggedness. As to the other claims of fantastic gain and
all, it's bogus. It's a vertical halfwave endfed and the gain is
2.4ish DB over a dipole and that's all. If executed correctly it
will be better than a 1/4wl groundplane but not by any fanstastic
amount. For many applications that is a "good enough" antenna.

Allison

What's the mechanism which gives this half wave antenna 2.4 dB gain over
a half wave dipole?

Modeling shows the gain to be the same as a dipole, if you can manage to
keep the current on the outside of the feedline to a small value. If you
can't, the gain in the horizontal direction typically becomes less than
a dipole.

Roy Lewallen, W7EL

wrote:

I happen to like them as they can be made from common materials with
excellent ruggedness.

A 1/4WL ground plane takes one SO-239 and five pieces of
19 inch copper wire (or stingers). :-)
--
73, Cecil http://www.qsl.net/w5dxp

Roy Lewallen wrote:
What's the mechanism which gives this half wave antenna 2.4 dB gain over
a half wave dipole?

A J-Pole over ground obviously has gain over a half wave
dipole in free space? :-)
--
73, Cecil http://www.qsl.net/w5dxp

wrote:

Cecil Moore wrote:
A 1/4WL ground plane takes one SO-239 and five pieces of
19 inch copper wire (or stingers). :-)

Now drive over one with a Chevy Pickup, the Jpole survived that
at field day one year.

Well, I just did that with a GMC pickup (hope that counts).
Nothing broke. I just straightened out the wires and it
still works just fine. If, Heaven forbid, a wire broke, it
would only take a butt connector and 20 seconds to repair it.
--
73, Cecil http://www.qsl.net/w5dxp

Same here..in the attic...works great...even at 50w.

73 de KB9BVN


--
=========================================
Radio Amateurs - Fill your junk box, from my junkbox!
http://www.hamradparts.com
73 de KB9BVN
=========================================


"Buck" wrote in message
...
On 25 Apr 2006 16:43:11 -0700, wrote:


jimbo wrote:
SNIP
jimbo - AJ7IM

Hi Jimbo, it has been my experience, and everybody else's that an
indoor or attic J-pole is unpredictable. A high impedance end fed
antenna in an attic environment will be hard to predict. Moving it one
foot can effect things.
You might consider the SO-239 ground plane. It can be built and
tuned in 30 minutes, is 50 ohms, so it will not be so concerned about
its environment. Likely you could not tell a difference in performance
from a J-pole especially if they are both in the attic.
Oh yes, the standard-if the SWR is below 3.0, don't worry about it.
Gary N4AST


I started building dipole antennas. they can be mounted horizontal or
vertical and easily moved around. I use cpvc with a dipole wire
inside and a coax-choke balun. They work quite well.

Just another option.
--
73 for now
Buck
N4PGW

wrote:

Verticle polarization has less power headed toward space (up for a
H-dipole) and still benefits from ground effects. The model for free
space nets the same gain as dipole. The problem with free space is
my backyard has dirt in it.

It appears you're comparing a half wave horizontal dipole with a (half
wave) J pole. Ground does have a major impact on the performance of the
antennas, but the reflection results in loss of a lot of signal power if
the wave is at a low angle and vertically polarized. Horizontally
polarized waves, in contrast, reflect with very little loss at low
angles. I haven't tried modeling it, but would guess that in a typical
line-of-sight path where there can be one ground reflection in addition
to the direct path, the signal would go through fades and reinforcements
as you varied the antenna height. Because of the lower loss of
horizontally polarized reflections, the fades would be deeper and the
reinforcements stronger with horizontal polarization. On the average,
the horizontal would be better (that is, the average signal for all
heights would be stronger) because no significant loss is incurred in
the reflection. But getting a decent signal would be much more of a gamble.

Those looking at modeling results from EZNEC or similar programs should
realize that the far field analysis isn't a good simulation of this
situation. Probably the best way to model it would be to model two
antennas, one at each location, over ground. Put a source in one antenna
(the transmitting antenna) and a conjugately matched load at the
feedpoint of the other (the receiving antenna). Then look at the power
delivered to the load for various receiving antennas. There's a
practical limit to how far apart the antennas can be before numerical
limitations are reached, but with double precision calculations they can
be quite a distance apart.

The conventional J pole with half wavelength radiator has the same gain
as a vertical half wave dipole, again assuming you can keep the current
on the outside of the feedline to a low value for both antennas.


However for free space groundplane compared to free space verticle
halfwave there is gain. Better radiation angle as well.

A quarter wavelength vertical ground plane antenna has the same gain and
pattern as a half wavelength vertical dipole, and the same as a J pole
(if you can keep the current on the outside of the J pole feedline to a
reasonably small value). This is true whether in free space or over
ground, and it can be demonstrated by theory, measurement, and modeling.

Roy Lewallen, W7EL

Bob wrote:
snip

My point is, why not make a simple ground plane for the attic that may
not get bothered by surroundings as much as the J-pole.
Bob


I made one of those antennas right after I got my Tech license. Hit
the local repeater with no problems. I guess I forgot or thought it
was too simple for the current application.

jimbo

"Bob" wrote in message
. com...
The J-pole crowd seems to tout fantastic claims about this antenna and I
have never been impressed. Many people have tested the J-pole against
other easy to make antennas and the end fed half wave J-pole usually
does not stand up to even it’s brother, the center fed half wave dipole.
I assume the decrease in performance is due to mistakes in assembling
and tuning the matching section. Here is a link to a group that tested
some J-poles against other antennas in the CA desert and the J-pole lost
to even simple 1/4 wave ground plane types.
http://www.geocities.com/SiliconVall...5/anttest.html

snip

A quick look at the numbers doesn't make the j-pole look so all-fired bad.
"Losing" by 1 dB or so isn't a serious loss. Ham radio isn't exactly the
Kentucky Derby.

I don't know what "fantastic claims" you've heard, but my affinity for the
j-pole comes from its simplicity and durability. It's a halfwave vertical
that requires no ground plane and can often be matched to 1:1 at the sweet
spot. Mounting is a dream -- it even works if you drop the low end into a
plumbing vent pipe. (How would I know that? ;-)

I have never built a center-fed halfwave, but we use a bunch of them in the
Navy -- call them "stovepipes" because of the way the fiberglass housings
look -- and they work very, very well. The elements are large metal
cylinders, tending to make them quite broadband. I do not know how they're
fed ... balun or what.

John , KD6VKW

I happen to like them as they can be made from common materials with
excellent ruggedness.
A 1/4WL ground plane takes one SO-239 and five pieces of
19 inch copper wire (or stingers). :-)

AND if you make it out of stiff wire (say #12 or so), it can be folded
to fit in a grab-n-go box and then unfolded when needed! The slight bends
caused by folding/unfolding may not look neat, but it's mighty handy!

--
--Myron A. Calhoun.
Five boxes preserve our freedoms: soap, ballot, witness, jury, and cartridge
PhD EE (retired). "Barbershop" tenor. CDL(PTXS). W0PBV. (785) 539-4448
NRA Life Member and Certified Instructor (Home Firearm Safety, Rifle, Pistol)

The J-pole crowd seems to tout fantastic claims about this antenna and I
....[snip]....

... my affinity for the j-pole comes from its simplicity and durability.
... Mounting is a dream -- it even works if you drop the low end into a
plumbing vent pipe. (How would I know that? ;-)
....[snip]....

I once made a J-pole from 300-ohm twinlead and tweaked it until it worked
OK while hanging from the ceiling, but when I moved it over next to a
wood-framed picture window (hung it from the window casing), the SWR went
to pot. Did I do something wrong?

(I concluded the J-pole must be VERY sensitive to its surroundings.)

--
--Myron A. Calhoun.
Five boxes preserve our freedoms: soap, ballot, witness, jury, and cartridge
PhD EE (retired). "Barbershop" tenor. CDL(PTXS). W0PBV. (785) 539-4448
NRA Life Member and Certified Instructor (Home Firearm Safety, Rifle, Pistol)