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

wrote:
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





Yes, BUT, the 5/8th wave radiator will put more of the signal toward the
horizon, instead of launching it at a 40 degree plus angle away from the
horizon. So while one configuration can have higher dbi ratings, it
doesn't count unless the signal goes where it will be most effective.

I have a friend (engineer) that designs and builds his boats (some
rather large sailboats) with everything quantized mathematically. I
showed up one day with a model sailboat sitting on a carry stand that I
had made. He asked me how I calculated the angles needed to accommodate
the hull accurately. He said that this problem had been bugging him for
a while. I was surprised because this guy is really smart.

I grabbed two rulers and put each one along the side of the boat and
then clamped them at that angle. I then transferred the angle of the two
rulers to a sheet of paper by simply drawing lines along the inside of
the v that was created. His jaw dropped in surprise. He was amazed at
how easy the process was and he realized that the same process would
work with his full size boats. No math required.

While EZNEC is a fantastic program, it is no better than the programmer
that wrote it. No one person can take absolutely every variable into
consideration because many of them are very, very complex and nearly
impossible to quantize.

I suggested a simple 1/2 wave J-pole antenna earlier that was easy to
make, super easy to tune and one that worked very effectively. Yet
everyone is beating their brains out trying to come up with the best
5/8th wave J-pole, even though this design will require lossy matching
devices to get the impedance down to a manageable/acceptable level.
What's up wid dat?

I do realize that figuring out such a design is fun in and of itself and
may be the real purpose of the exercise. Still, I'll betcha no one on
the receiving end of the OP's signal could tell if he was using the
5/8th wavelength J-pole or the 1/2 wavelength J-pole.


Ed, NM2K

On Oct 5, 2:42 pm, Ed Cregger wrote:
wrote:
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

Yes, BUT, the 5/8th wave radiator will put more of the signal toward the
horizon, instead of launching it at a 40 degree plus angle away from the
horizon. So while one configuration can have higher dbi ratings, it
doesn't count unless the signal goes where it will be most effective.

Dunno.. You sure you ain't got it backwards? Unless the 5/8 is on a
large
ground plane, etc, it's usually the one with the higher avg launch
angles
vs the 1/2 wave.
The 5/8 with no radials should be pretty bad at that.. Even a set of
1/4 wave radials under a 5/8 will give a fairly poor pattern.
If I were to build a j pole, it would be a 1/2 wave.
If I run a 5/8, I'd have two elements as a collinear, or at least
have 5/8 or 3/4 wave radials. I'd never run just a single 5/8 wave
radiator
on it's own. It's not a "complete" antenna like a 1/2 wave j pole is.
Or to my way of qualifying anyway...
MK

Ed Cregger wrote:

Yes, BUT, the 5/8th wave radiator will put more of the signal toward the
horizon, instead of launching it at a 40 degree plus angle away from the
horizon. So while one configuration can have higher dbi ratings, it
doesn't count unless the signal goes where it will be most effective.

That is born out he

http://www.cebik.com/gp/58.html

I have a friend (engineer) that designs and builds his boats (some
rather large sailboats) with everything quantized mathematically. I
showed up one day with a model sailboat sitting on a carry stand that I
had made. He asked me how I calculated the angles needed to accommodate
the hull accurately. He said that this problem had been bugging him for
a while. I was surprised because this guy is really smart.

I grabbed two rulers and put each one along the side of the boat and
then clamped them at that angle. I then transferred the angle of the two
rulers to a sheet of paper by simply drawing lines along the inside of
the v that was created. His jaw dropped in surprise. He was amazed at
how easy the process was and he realized that the same process would
work with his full size boats. No math required.

While EZNEC is a fantastic program, it is no better than the programmer
that wrote it. No one person can take absolutely every variable into
consideration because many of them are very, very complex and nearly
impossible to quantize.

I suggested a simple 1/2 wave J-pole antenna earlier that was easy to
make, super easy to tune and one that worked very effectively. Yet
everyone is beating their brains out trying to come up with the best
5/8th wave J-pole, even though this design will require lossy matching
devices to get the impedance down to a manageable/acceptable level.
What's up wid dat?

I do realize that figuring out such a design is fun in and of itself and
may be the real purpose of the exercise. Still, I'll betcha no one on
the receiving end of the OP's signal could tell if he was using the
5/8th wavelength J-pole or the 1/2 wavelength J-pole.

W4RNL may have provided the definitive work on the subject.

http://www.cebik.com/vhf/jp4.html


Ed, NM2K

73, ac6xg

Ed Cregger wrote:

...
I suggested a simple 1/2 wave J-pole antenna earlier that was easy to
make, super easy to tune and one that worked very effectively. Yet
everyone is beating their brains out trying to come up with the best
5/8th wave J-pole, even though this design will require lossy matching
devices to get the impedance down to a manageable/acceptable level.
What's up wid dat?
...
Ed, NM2K

Until recently, I owned an all fiberglass houseboat--beautiful rig.
However, on fresh water it offered no counterpoise properties whatsoever.

A continuously loaded 1/2 wave end fed vertical utilizing a modified
gamma match feed ended up the solution, and one which required a minimal
counterpoise.

Regards,
JS

Yeh, Larson makes an antenna that is designed for(insulated) vehicles
(thinkit is the "OS" series, 1/2 wave) , and also, it is possible to use
regular 1/4, and 5/8 wave antennas on such, useing Burglar alarm, or
"Waterproofing Repair Tape" ( 2 Inch aluminium tape), available at
Home Depot, or Lowes. Tho NOT at a 45 degree angle, should make a
adaquate counterpoise for your antenna, and can be on either side of
the roof! Give it a try! Jim NN7K


John Smith wrote:

Until recently, I owned an all fiberglass houseboat--beautiful rig.
However, on fresh water it offered no counterpoise properties whatsoever.

A continuously loaded 1/2 wave end fed vertical utilizing a modified
gamma match feed ended up the solution, and one which required a minimal
counterpoise.

Regards,
JS

Ed Cregger wrote:

Yes, BUT, the 5/8th wave radiator will put more of the signal toward the
horizon, instead of launching it at a 40 degree plus angle away from the
horizon. So while one configuration can have higher dbi ratings, it
doesn't count unless the signal goes where it will be most effective.
. . .

When mounted on a perfect ground plane of infinite extent, any ground
mounted vertical monopole higher than 1/2 wavelength will have one or
more high angle lobes. As the height increases above 1/2 wavelength, the
gain at the horizon increases even though a high lobe appears at around
60 degrees above the horizon. The gain at the horizon peaks out at about
5/8 wavelength, where the high lobe is about 9 dB weaker than the main
lobe. As the antenna gets longer than 5/8 wavelength, the power going
into the upper lobe starts reducing the gain at the horizon (and the
lobe's elevation angle slowly drops) until at one wavelength, all the
power goes to the upper lobe and there's no radiation at the horizon at all.

The gain increase of 1/2 or 5/8 wavelength antennas over shorter
monopoles comes about by a narrowing of the lobe pointing toward the
horizon. Unfortunately, though, radiation at the low angles is severely
attenuated by reflection from real ground. And this is just where most
of the power from longer verticals is going. So a 5/8 wave HF vertical
usually won't exhibit the gain over a shorter antenna you see with a
perfect ground simulation. Likewise, a finite ground plane like a car
roof impacts low angle radiation, so it has more of an effect on a 1/2
or 5/8 wave radiator than a shorter one, and once again you often won't
see the gain you might expect.

A few minutes with the demo version of EZNEC or a similar program shows
the effect of finite ground on various antenna heights very clearly. Use
MININEC-type ground to eliminate the separate effect of ground system
resistive loss. The full EZNEC program will let you model an antenna on
a car top (by using a wire grid to simulate the car top).

Roy Lewallen, W7EL