"Cecil Moore" schreef in bericht
t...
michel wrote:
Should it be that simple? I understood from other posters that it will
not work with the 1/4 wave stub..

A 1/2WL + (1/4WL series stub) is a resonant Zepp.
When the stub is shorted at the bottom and tapped
for 50 ohms, it becomes a typical J-Pole.

A 5/8WL + (1/5WL series stub) is a resonant Extended
Zepp. There is only one resonant point and it may
not be at 50 ohms but it may be close enough. You
won't know till you try it. The feedpoint impedance
can be varied by varying the spacing between the
stub elements.

A 5/8WL + (1/4WL series stub) is non-resonant.
Anywhere you tap on the stub will result in some
reactance. There's really no reason to build
this inferior design.
--

Cecil,

That is some clear information!! The 5/8 wave version needs a shorted stub
as well?

michel wrote:
That is some clear information!! The 5/8 wave version needs a shorted stub
as well?

What we are looking for is the length of a series
matching stub section that will bring the system
to resonance, i.e. we are looking for the current
maximum point.

The graph at http://www.w5dxp.com/majic.gif
gives the length of the series matching stub
section for dipoles of various lengths from
1/2WL to 1.5WL. The graph can be used for
monopoles by doubling the monopole length.

For instance, doubling the 5/8WL monopole length
gives a 10/8WL dipole length which is 1.25WL.
A 1.25WL dipole needs a series matching stub section
of 0.19WL, i.e. the bottom stub on a 5/8WL J-Pole
needs to be 0.19WL. Make it a little too long and
trim for resonance.

If you double the 1/2WL monopole length of the
J-Pole you get a 1.0WL dipole. From the graph,
a 1.0WL dipole needs a 0.25WL series matching
stub section, i.e. the bottom stub on a 1/2WL
J-Pole needs to be 0.25WL. Of course, that is
the standard J-Pole design.

If you happened to want to design a 0.35WL J-Pole,
the series matching stub would need to be 0.33WL
long. 2(0.35WL) = 0.7WL

Let me know if you understand the graph.
--
73, Cecil http://www.w5dxp.com

michel wrote:
That is some clear information!! The 5/8 wave version needs a shorted stub
as well?

After my first cup of coffee, I canceled my previous
posting. The matching method I described works well
for balanced dipoles but is probably not applicable
to monopoles with no ground plane because the current
in the other leg of the series matching section has
no place to flow in a J-Pole configuration. So a 0.2WL
matching stub is not a good feed design for a 5/8WL
monopole and will generate common-mode problems unless
there is a ground plane into which the current can flow.
Best to stick with the standard 1/2WL J-Pole design. I
apologize for my fuzzy thinking - it made sense until
I woke up. And it would work for a 5/8WL monopole if
it already had ground plane radials.
--
73, Cecil http://www.w5dxp.com

On Oct 3, 9:59 am, Cecil Moore wrote:
So a 0.2WL
matching stub is not a good feed design for a 5/8WL
monopole and will generate common-mode problems unless
there is a ground plane into which the current can flow.
Best to stick with the standard 1/2WL J-Pole design. I
apologize for my fuzzy thinking - it made sense until
I woke up. And it would work for a 5/8WL monopole if
it already had ground plane radials.
--
73, Cecil http://www.w5dxp.com

It's not the matching scheme that is the real problem..
A single 5/8 radiator over no radials is going to be a real
dog no matter how you match it.
And if you do use radials and a 5/8 radiator, you might
as well feed it with a simple series loading coil.
I've never been a great fan of J-poles, but if I were to
build one, it would be the standard 1/2 wave version.
If one is going to build a copper J pole using 5/8
elements, they should use two and build it as a
collinear. And even in that case, there should be a
decoupling section added for the best performance.
My favorite "cheap and easy" antenna for VHF is
not the j pole.. It's the 1/4 wave ground plane with
sloping radials. It's easier to build, needs no matching,
and the gain should be very close to most 1/2 wave
j poles.
MK

On Oct 4, 7:40 am, wrote:
On Oct 3, 9:59 am, Cecil Moore wrote:
So a 0.2WL

matching stub is not a good feed design for a 5/8WL
monopole and will generate common-mode problems unless
there is a ground plane into which the current can flow.
Best to stick with the standard 1/2WL J-Pole design. I
apologize for my fuzzy thinking - it made sense until
I woke up. And it would work for a 5/8WL monopole if
it already had ground plane radials.
--
73, Cecil http://www.w5dxp.com

It's not the matching scheme that is the real problem..
A single 5/8 radiator over no radials is going to be a real
dog no matter how you match it.
And if you do use radials and a 5/8 radiator, you might
as well feed it with a simple series loading coil.
I've never been a great fan of J-poles, but if I were to
build one, it would be the standard 1/2 wave version.
If one is going to build a copper J pole using 5/8
elements, they should use two and build it as a
collinear. And even in that case, there should be a
decoupling section added for the best performance.
My favorite "cheap and easy" antenna for VHF is
not the j pole.. It's the 1/4 wave ground plane with
sloping radials. It's easier to build, needs no matching,
and the gain should be very close to most 1/2 wave
j poles.
MK

Amen, bro. Another advantage of the ground plane: the radials
decouple the feedline from the antenna. In a J-pole installation,
either you put some effort into decoupling the feedline, or you accept
that the feedline is going to radiate (and change the pattern). (I
suppose yet another option is to deny the fact that feedlines can
radiate...)

It's easy to build a "quick-and-dirty" 146MHz ground plane with an
SO-239, some 4-40 screws, washers and nuts, and three lengths of 12 or
14 AWG copper wire. A couple half-wave lengths attach to the flange
of the SO-239 with the 4-40 hardware (or just solder them) so there
are 4 1/4 wave wires sticking out from the flange; a 1/4 wave piece
solders into the center pin. The radials can be bent down a bit to
get a match to 50 ohms. You can put a little loop in the top of the
radiator and hoist it into a tree with fishing line or the like.
There are several ways you can attach it to the top of a mast; it
doesn't care a lot about what you do below it.

Cheers,
Tom

Hi Tom,

K7ITM wrote:

Amen, bro. Another advantage of the ground plane: the radials
decouple the feedline from the antenna. . .

Here's a fun experiment with EZNEC.

1. Open the VHFGP.EZ example file. Click View Ant to open the View
Antenna display. In the View Antenna display control section, click
Center Ant Image so you can see the antenna better.
2. Add the following wi

End 1: 0, 0, 5 (wavelengths)
End 2: 0, 0, 4.727 (wavelengths)
Diameter: 0.25 (inches)
Segments: 6

This represents the outside of a feedline connected to the feedpoint.
3. Click the Currents button. Look at the display and, in the Currents
box, compare the current on the outside of the "feedline" (Wire 6,
Segment 1) to the main radiator current (Wire 5, Segment 1).
4. Change Plot Type to Elevation. Click FF Plot to see the 2D elevation
pattern.
5. Reconsider the statement about decoupling. . .

This is admittedly contrived to show a particularly extreme case. But
try different lengths of "feedline" either open or connected to ground
and you'll find other cases where the feedline current is high and the
pattern distorted. You'll also find cases where inserting a "balun"
(high impedance load) in the "feedline" will actually increase the
feedline current due to changing the current distribution to a value
more favorable for the particular feedline length.

Roy Lewallen, W7EL

On Oct 4, 11:33 pm, Roy Lewallen wrote:


Here's a fun experiment with EZNEC.

1. Open the VHFGP.EZ example file. Click View Ant to open the View
Antenna display. In the View Antenna display control section, click
Center Ant Image so you can see the antenna better.
2. Add the following wi

End 1: 0, 0, 5 (wavelengths)
End 2: 0, 0, 4.727 (wavelengths)
Diameter: 0.25 (inches)
Segments: 6

This represents the outside of a feedline connected to the feedpoint.
3. Click the Currents button. Look at the display and, in the Currents
box, compare the current on the outside of the "feedline" (Wire 6,
Segment 1) to the main radiator current (Wire 5, Segment 1).
4. Change Plot Type to Elevation. Click FF Plot to see the 2D elevation
pattern.
5. Reconsider the statement about decoupling. . .

This is admittedly contrived to show a particularly extreme case. But
try different lengths of "feedline" either open or connected to ground
and you'll find other cases where the feedline current is high and the
pattern distorted. You'll also find cases where inserting a "balun"
(high impedance load) in the "feedline" will actually increase the
feedline current due to changing the current distribution to a value
more favorable for the particular feedline length.

Roy Lewallen, W7EL

I'd be curious to see what you get doing the same with a 1/2 wave
whip.
IE: usual j-pole, 1/2 wave whip...
I tried doing a test using the demo version. Being it was limited
segments,
I tried to keep it even, and used twice the segments for the 1/2 wave
radiator vs the 1/4 wave, but used the same amount of segments for
the "feedline" .
I also used "real ground", and the elevation plot so I could more
easily see the appx real world plots.
With the GP, I notice heavy current when the feedline is appx 1/4
wave, but not so bad when it's longer. In some cases I saw a gain
where the feed currents seem to be in phase with the antenna
currents.
When trying the 1/2 wave, I didn't see the problem too much using a
short 1/4 wave feed, but the longer lengths were much worse than
the plots for the GP.

1/4 WL GP

6.58 dbi at 3 degrees -no feedline
5.26 dbi at 42 degrees -feedline .28 wl
7.39 dbi at 3 degrees -feedline 1 wl
6.09 dbi at 3 degrees - feedline 2 wl
6.77 dbi at 3 degrees - feedline 3 wl
7.05 dbi at 3 degrees - feedline 4 wl
7.87 dbi at 3 degrees- feedline 5 wl and grounded at "0"

1/2 WL whip

6.43 dbi at 3 degrees -no feedline
6.84 dbi at 3 degrees -feedline .28 wl
6.23 dbi at 3 degrees -feedline 1 wl
6.67 dbi at 55 degrees - feedline 2 wl
14.82 dbi at 68 degrees - feedline 3 wl
21.42 dbi at 67 degrees - feedline 4 wl
6.68 dbi at 3 degrees- feedline 5 wl and grounded at "0"

Anyway, I may have had problems running this
test with the limited segments, but it seems to
show the 1/2 wave as having the worse problems
of the two overall. Really bad at 3-4 waves length
of line.
So while the decoupling for the GP is not always the
greatest, I think it's still probably less a problem
than the usual 1/2 wave whip with no decoupling.
I've never used chokes or baluns per say to decouple
a VHF vertical. I always use radial sets, cones, sleeves,
etc..
The usual ground plane really needs two radial sets to
decouple the line well. Most good sleeve dipoles will
use an extra sleeve for decoupling the line.
You might get a bit different results using unlimited segments.
But I betting the trend will still be fairly close, with the
non-decoupled 1/2 wave being the worst overall at the longer
line lengths.
MK

On Oct 5, 8:39 am, wrote:

BTW, you may notice the no feedline 1/2 wave shows
a higher gain than the sloping radial GP.
I'm not sure if this is right or not...
But I didn't tweak the 1/2 radiator.. It's exactly .50 wl
long..
MK

wrote in message
ups.com...
snip

I've never been a great fan of J-poles, but if I were to
build one, it would be the standard 1/2 wave version.
If one is going to build a copper J pole using 5/8
elements, they should use two and build it as a
collinear. And even in that case, there should be a
decoupling section added for the best performance.
My favorite "cheap and easy" antenna for VHF is
not the j pole.. It's the 1/4 wave ground plane with
sloping radials.

Having made a couple of whip+radials antennas out of old coathangers, I
am not at all hostile to your views.

However, as a fan of the j-pole, myself, I offer their ruggedness as a
distinct advantage. My first copper pipe j-pole (from the early 1990's) is
still on my roof. It has turned a slightly darker color but is otherwise as
good as new. I made three for our Fire House RACES station (two 2m & a 6m)
and they will outlive me.

Just my $0.02.

73,
"Sal"
(KD6VKW)