RadioBanter - j-pole 5/8 wave

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K7ITM wrote:
On Oct 2, 12:23 pm, Jim Kelley wrote:

michel wrote:

I don't know what construction technique you have in mind, but I prefer to
build them from 1/2" copper pipe. If that's what you intend to do, then
just cut the vertical radiator 5/8ths instead of 1/2 lambda. Build the
rest the same (1/4 wave stub). Attach your coax to the stub section using
hose clamps, and tune by sliding the clamps up or down until you achieve
best SWR. Works great.

ac6xg

Should it be that simple?

It can be.

I understood from other posters that it will not
work with the 1/4 wave stub..

For some posters, a view of the forest is obscured by trees. A j-pole
is a quarter wave stub with a 1/2 wave (or longer) radiator attached
to one side of the open end. The feed is connected nearer to the
shorted end of the stub. Build it and they (the QSOs) will come. Get
it up as high above the roof as you can.

ac6xg

I suppose that since there will be an antenna current on the stub, and
the 5/8 section would show a reactive feedpoint if fed against a
ground plane, things are a bit more complicated than just a resistive
matching section (the 1/4 wave stub). I would expect that (1) the
antenna won't behave quite like a 5/8 wave fed against ground, nor
like a 5/4 wave center fed doublet, and (2) the stub will have to be
adjusted in length as well as in transformation ratio to get a
"perfect" match. In addition, if the feedline is not decoupled from
the antenna, the antenna current on the feedline will change both the
pattern and the feedpoint impedance (match) from what it would be if
the stub+radiator were in freespace.

I'd do some NEC simulating to get an idea of a starting point AND an
idea if the pattern was really an improvement over the normal half-
wave over a quarter-wave stub, before trying to build one; and I'd put
some effort into decoupling the antenna from other nearby metal
(including the feedline)--or at least include other elements in the
simulation. Even with simulating, I'd expect to have to do some fine
tuning (of stub spacing or stub tap point, and possibly of stub
length) if I really cared about a good match.

Cheers,
Tom

Hi Tom -

I've built several of them and achieved a good (1.5:1) match in the
middle of the band.

73, ac6xg

K7ITM wrote:

On Oct 2, 12:23 pm, Jim Kelley wrote:

michel wrote:

I don't know what construction technique you have in mind, but I prefer to
build them from 1/2" copper pipe. If that's what you intend to do, then
just cut the vertical radiator 5/8ths instead of 1/2 lambda. Build the
rest the same (1/4 wave stub). Attach your coax to the stub section using
hose clamps, and tune by sliding the clamps up or down until you achieve
best SWR. Works great.

ac6xg

Should it be that simple?

It can be.

I understood from other posters that it will not
work with the 1/4 wave stub..

For some posters, a view of the forest is obscured by trees. A j-pole
is a quarter wave stub with a 1/2 wave (or longer) radiator attached
to one side of the open end. The feed is connected nearer to the
shorted end of the stub. Build it and they (the QSOs) will come. Get
it up as high above the roof as you can.

ac6xg

I suppose that since there will be an antenna current on the stub, and
the 5/8 section would show a reactive feedpoint if fed against a
ground plane, things are a bit more complicated than just a resistive
matching section (the 1/4 wave stub). I would expect that (1) the
antenna won't behave quite like a 5/8 wave fed against ground, nor
like a 5/4 wave center fed doublet, and (2) the stub will have to be
adjusted in length as well as in transformation ratio to get a
"perfect" match. In addition, if the feedline is not decoupled from
the antenna, the antenna current on the feedline will change both the
pattern and the feedpoint impedance (match) from what it would be if
the stub+radiator were in freespace.

I'd do some NEC simulating to get an idea of a starting point AND an
idea if the pattern was really an improvement over the normal half-
wave over a quarter-wave stub, before trying to build one; and I'd put
some effort into decoupling the antenna from other nearby metal
(including the feedline)--or at least include other elements in the
simulation. Even with simulating, I'd expect to have to do some fine
tuning (of stub spacing or stub tap point, and possibly of stub
length) if I really cared about a good match.

Cheers,
Tom

I found them to work quite well, though I fine tuned them on the mast
- not in the garage next to the transmitter.

73, jk

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

On Oct 4, 11:01 am, "michel" wrote:

In my search on vertical antennas I also found a option to add on a 1/4 wave
element a 5/8 element. This needs a phasing coil?
I also found a 5/8 element mounted over a 1/4 innerelement.. ?

But wat is the best for homebrew, and a few dB gain?

With VHF verticals, half the battle is decoupling the feedline.
It's no use worrying about extended verticals, unless you also
consider decoupling the line. Radiation from the line will skew
the pattern upwards off the horizon, and any gain from extended
radiators will be useless.
If you are going to use a 5/8 radiator, I would go whole hog and
build a copper collinear with dual 5/8 elements. I wouldn't waste time
with other versions. And even the dual 5/8 collinear needs a
decoupling section of some type.
One reason I like the 1/4 wave ground planes is that they naturally
decouple from the feedline fairly well. The more radials you use,
the better the decoupling, and if you add a 2nd set that is 1/4
below, you will have very good decoupling.
That method can be used to decouple a dual 5/8 vertical.
I would worry more about how the line will be decoupled, than
I would element length. It's more important.
MK

Gary #203 wrote:
WELL, I WOULD DO A 3/4 WAVE BY A 1/4 WAVE.

A 3/4WL vertical monopole has a take-off-angle of 47 degrees.
5/8WL is the length limit for good monopole DX performance.
--
73, Cecil http://www.w5dxp.com

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)

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