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

On Oct 5, 6:45 am, wrote:
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

Higher?? You posted 6.34dBi for the half wave, and 6.58dBi for the
quarter wave GP ... Am I misreading something, or was that a typo
about which has higher gain, or what?

Thanks for taking the time to make the simulations and posting the
results, Mark. And thanks to Roy for pointing out that the GP doesn't
fully isolate the feedline from antenna currents. I should say
"pointing out once again" as I know he's posted it before, now that
I'm reminded about it. I don't have time at the moment, but put it on
the list to play with in simulations when I have some time.

Still, the 1/4 wave GP is extremely easy to hack together in an
emergency -- you can even strip a quarter wave of braid off coax and
twist copper wire or coat hangers around the top of the braid for the
GP -- and should perform comparably with other more complicated
antennas. A plus is that you only need to remember (or figure out)
how long 1/4 wave is to know how to cut it to get a decent, if not
perfect, match.

Cheers,
Tom

On Oct 5, 11:49 am, K7ITM wrote:
On Oct 5, 6:45 am, wrote:


Higher?? You posted 6.34dBi for the half wave, and 6.58dBi for the
quarter wave GP ... Am I misreading something, or was that a typo
about which has higher gain, or what?

Dang.. I guess I had it backwards in the 2nd post...
Normally, I would think the 1/2 whip would show slightly higher,
but dunno.. Quiver in the force I guess.
I have no real problems with either type. A 1/2 wave whip is fine
if it's decoupled.
But few J pole users seem to add decoupling sections.
Most of the ringos sold for VHF lack decoupling also..
One note.. If I build a 1/2 wave, I usually prefer to feed as a ringo,
vs as a J pole..
But I still mostly use the GP's as they are simple. I've got one up
in the attic hanging from the rafters as an emergency antenna
when T-storms are in the area and I'm chicken to use my outside
antennas.
MK

K7ITM wrote in news:1191602989.414910.321210
@r29g2000hsg.googlegroups.com:

....
results, Mark. And thanks to Roy for pointing out that the GP doesn't
fully isolate the feedline from antenna currents. I should say
"pointing out once again" as I know he's posted it before, now that
I'm reminded about it. I don't have time at the moment, but put it on
the list to play with in simulations when I have some time.

It is an interesting topic for modelling.

I wrote some notes up after modelling an unloaded elevated vertical as a
multi-band HF antenna. I was particularly interested in the effectiveness
of decoupling of the mast / feedline and the loss implications of mast
current flowing to ground. The article is at
http://www.vk1od.net/multibandunload...ical/13mEV.htm . The spikes in
loss in Fig 3 and 4 are mainly due to loss in the simulated resistance of
the earth connection at the lower end of the mast.

So, the issue is not just about pattern distortion, efficiency may be
impacted significantly.

One of the popular antennas amongst our new six hour hams is to use their
StationMaster on 40m and 80m The StationMaster is a half wave vertical
for 27MHz base fed with a parallel tuned circuit and the 50 ohm coax is
tapped onto the coil of the tuned circuit. (A 27MHz version of the once
popular ham ringo... but the coil is multi turn.)

When modelled on a 10m high mast on 40m and 80m, most of the current
moment contribution is from the support mast and the efficiency is low
due to losses in mast current flowing to lossy ground. Additionally,
there a huge transmission line losses. It is a top fed dummy load... but
with the magic of an ATU, it has a VSWR of 1:1!

But, when their investment in ham radio is 6 hours of learning, they
aren't well equipped to appreciate that the StationMaster isn't an
efficient antenna on 40m and 80m.

Owen

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)