Which antenna is better: 5/8 wavelength vertical or a J pole?

Frequency of operation is 145 MHz = 2 metres.

The 5/8 wavelength vertical has a loading coil. There are losses in the
coil.

The J pole has a quarterwave matching stub. The matching stub provides an
out of phase current which means that there is a cancelling field close to
the radiating element. Also the J pole is end fed, which means the
transmitter is not connected directly to a maximum current point.

Does the J pole have a disadvantage because of the cancelling field from
the matching stub and the fact that it is end fed?

Also consider gain and angle of radiation.

"David" nospam@nospam wrote in
:


Which antenna is better: 5/8 wavelength vertical or a J pole?

Better for what? For example, a 5/8 whip would usually be better than a J
Pole for mobile applications on a car roof, but that doesn't make it
better for all applications.


Frequency of operation is 145 MHz = 2 metres.

The 5/8 wavelength vertical has a loading coil. There are losses in
the coil.

It is certainly popular to talk down an antenna with loading coils or
traps because they are "lossy".

Everthing in a real world antenna has loss, the issue is the magnitude of
the loss, and the impact of that on system performance for the intended
application.

You might find it hard to believe that some antenna systems incorporate
loss elements in order to reduce feed line loss by more than that in the
introduced elements.

Whilst you have chosen to raise the loss in the coil, you haven't raised
the issue that a J Pole has currents flowing in lossy conductors,
components of which that do not directly contribute to radiation.


The J pole has a quarterwave matching stub. The matching stub provides
an out of phase current which means that there is a cancelling field
close to the radiating element. Also the J pole is end fed, which

This is not a very good way of analysing the J Pole. The U section can be
thought of as carrying currents that have differential and common mode
components. The common mode components contribute directly to radiation
field. You should also consider common mode current on the supporting
structure and feedline.

The 5/8 wave vertical also has potential for significant common mode
current on the supporting structure and feedline, you need to look at the
effectiveness of the decoupling method employed (usually a radial set).

means the transmitter is not connected directly to a maximum current
point.

What does that matter? BTW, neither is the base fed 5/8.

There is a maxim in ham radio that antennas should always be fed at a
current maximum. You could subscribe to that, but you would limit
yourself by excluding a range of good solutions, and with no guarantee
that a current fed antenna is optimum.


Does the J pole have a disadvantage because of the cancelling field

There is not perfect cancellation at all points on the U section.

from the matching stub and the fact that it is end fed?

"End fed", as opposed to a centre fed dipole perhaps. But isn't the 5/8
"end fed"?


Also consider gain and angle of radiation.

The three dimensional gain distribution is important, but dependent on
the common mode issue mentioned above (amongst other things).

It is fair to say that J Poles are very popular, and that they are overly
represented in problems discussed in online fora.

On the other hand, the 5/8 which was once very popular for mobile work in
this area, is long lost, replaced by two, three and four band antennas
where VSWR is more important than any other performance parameters. I use
a 5/8 vertical on my car, and regularly work repeaters mobile at
distances well over 100km. The antenna is more than thirty years old, and
has never required repair despite hitting low trees, carpark roofs etc
lots of times. The modern multiband antennas are not that robust. I
wouldn't even think of a J Pole in this application.

So, "best" can be a quite complex requirement.

Owen

On Nov 7, 1:08*pm, "David" nospam@nospam wrote:
Which antenna is better: 5/8 wavelength vertical or a J pole?

Frequency of operation is 145 MHz *= 2 metres.

The 5/8 wavelength vertical has a loading coil. There are losses in the
coil.

The J pole has a quarterwave matching stub. The matching stub provides an
out of phase current which means that there is a cancelling field close to
the radiating element. Also the J pole is end fed, which means the
transmitter is not connected directly to a maximum current point.

Does the J pole have a disadvantage because of *the cancelling field from
the matching stub and the fact that it is end fed?

Also consider gain and angle of radiation.

They are both "problem" antennas.. The usual 5/8 GP
is a problem antenna because years ago some designer
decided to use 1/4 wave radials instead of 5/8 or even
3/4 wave.
And most ignore common mode issues.

The J Pole often suffers because most tend to ignore
common mode issues, which leads back to lame
incomplete antenna design. :/

Both can be greatly helped by adding decoupling
sections at and below the feed point.
The 5/8 antenna should be run as a collinear
with dual 5/8 elements.
And with a lower decoupling section.
The dual 5/8 collinear is more effective than any
half wave vertical, even if the half wave is decoupled
from the feed line.

BTW, I have no real issues feeding from a high voltage
point. And the loss in a 5/8 loading coil is probably
not enough to even measure for most people.
Both are non issues.
The real issue is lack of decoupling, and using a
perverted 5/8 over 1/4 wave element scheme in
the case of the usual 5/8 ground plane.

The perverted antenna design actually ruins the
pattern of what would be a decent antenna if it
were designed correctly with dual 5/8 elements.
Even running 3/4 wave sloping radials is much
better than using the usual 1/4 wave radials.

Of course, mobile 5/8 whips are only the upper
half of the antenna. They don't supply the lower
half in that case. For those, the vehicle is the
lower part of the antenna and performance can
vary from good to horrible depending on where
it's mounted.

Which is better?
As already said, just depends on what you are looking for. One thing
about these two antennas is that one is a 1/2 wave, the other a 5/8
wave. They have different radiation patterns/characteristic. So,
which of those characteristics would 'fit' your requirements the
best? That's the one that's best for -your- situation.
- 'Doc

"David" nospam@nospam wrote in message
...

Which antenna is better: 5/8 wavelength vertical or a J pole?

Frequency of operation is 145 MHz = 2 metres.

The 5/8 wavelength vertical has a loading coil. There are losses in the
coil.

The J pole has a quarterwave matching stub. The matching stub provides an
out of phase current which means that there is a cancelling field close to
the radiating element. Also the J pole is end fed, which means the
transmitter is not connected directly to a maximum current point.

Does the J pole have a disadvantage because of the cancelling field from
the matching stub and the fact that it is end fed?

Also consider gain and angle of radiation.





I'm currently building a 5/8 wave ground plan for 2mx as a tower based
antenna, I have used J pole and slim jim type antennas in the past for the
same purpose with success. I would think that the 5/8 radiator may have
slight gain advantage, but believe that the J pole may have a slightly lower
angle of radiation. The J pole is certainly easier to construct as there is
a bit of mucking about with 1/8wave loading coil on the 5/8 antenna.
There has been much discussion about common mode currents produced on the
feed line for the J pole that can create an unpredictable radiation pattern
which is partly why I'm constructing the 5/8 and partly I'm building a 5/8
ground plan as I have never built one before.

Build both and compare.

If the antenna is for mobile I would certainly use the 5/8 antenna.


--
Peter VK6YSF

http://members.optushome.com.au/vk6ysf/vk6ysf/main.htm

"David" nospam@nospam wrote in message
...

Which antenna is better: 5/8 wavelength vertical or a J pole?

Frequency of operation is 145 MHz = 2 metres.

The 5/8 wavelength vertical has a loading coil. There are losses in the
coil.

The J pole has a quarterwave matching stub. The matching stub provides an
out of phase current which means that there is a cancelling field close to
the radiating element. Also the J pole is end fed, which means the
transmitter is not connected directly to a maximum current point.

Does the J pole have a disadvantage because of the cancelling field from
the matching stub and the fact that it is end fed?

Also consider gain and angle of radiation.


I would not say any of the simple veticals are better. While I have not
tried them from fixed locations, expirimenting with several differant mobile
antennas over the years it seems that one type is not really that much beter
than another.

Depending on the direction and height of the repeaters almost any antenna
can be better going into one repeater and worse going to another. Several
of us got together and put several antennas (one at a time) on the same
mount of a car. Depending on the particular repeater, there was not one
overall winner. The car also had a 40 meter loaded whip that we tried and
it was actually better into some of the repeaters.

About the only antenna overall not suited was a colinear about 6 feet long.
It worked well enough while parked, but at highway speeds it whipped around
so much the mobile flutter made it almost unusable.

"Peter" wrote in
. au:

....
construct as there is a bit of mucking about with 1/8wave loading coil
on the 5/8 antenna.

Why do you call it a 1/8 wave loading coil? It wouldn't be along the
lines of the flawed "loading coil replaces the missing degrees" concept
would it?

A 5/8 monopole's performance is quite senstive to the ground plane
implementation. The behavior of a 5/8 monopole over a perfect ground is
not replicated over real radial systems or car roofs, yet people compare
antennas based on the perfect ground plane environment.

As the length of the radiator is increased beyone a half wave, low angle
gain increaeses until about 0.6 wavelengths when power is shifted into a
developing upper lobe. The optimum length over a perfect ground is
probably just a little less than 5/8, and less still over practical
ground planes.

The other dimension is feedpoint impedance. For a simple series L
matching arrangement, R is a little high and the optimum length is
typically longer than 5/8.

So, for optimum pattern, and low VSWR, a better solution is a tapped base
coil with 0.6 wavelength vertical... but that doesn't play well with the
simplest of mobile antenna bases that provide only one connection to the
screw on antenna.

Owen

On Nov 9, 3:02*pm, Owen Duffy wrote:
Why do you call it a 1/8 wave loading coil? It wouldn't be along the
lines of the flawed "loading coil replaces the missing degrees" concept
would it?

Of course, the loading coil doesn't replace all of the missing
degrees, but it does replace some of the missing degrees. The
following inductance calculator will give the Z0 and axial propagation
factor of a coil from which the VF of the coil can be calculated. When
one knows the Z0 and VF of the coil, it can simply be treated as a
transmission line.

http://hamwaves.com/antennas/inductance.html

It is obvious that the loading coil at the bottom of a 5/8WL antenna
somehow causes the antenna to be electrically 3/4WL (270 degrees) long
because that's the only way the reflected wave can arrive back at the
feedpoint in phase with the forward wave in order to give a resistive
feedpoint impedance. So we need to answer the question of exactly
where those delays and phase shifts occur. Here's a conceptual model
of the 5/8WL base-loaded antenna. (The 50 ohm tapped point on the coil
has been ignored to simplify the problem.)

FP-//////////-----------5/8WL-------------------

(1) The coil occupies a certain number of degrees of the antenna. (2)
Since the Z0 of the coil and the Z0 of the whip are different, there
is a phase shift at the junction of the coil and the whip that can be
easily calculated. (3) The 5/8WL whip obviously occupies 225 degrees
of the antenna. All we have to do is figure out what the phase shift
is at the coil/whip junction and how many degrees the coil occupies.

degrees of coil = 270 - 225 - coil/whip phase shift

There's no magical faster-than-light propagation through the coil as
predicted by the lumped-circuit model. Coils are known to cause a
delay and the Hamwaves inductance calculator provides us an easy way
of calculating that delay through the loading coil. I can provide an
example if necessary.
--
73, Cecil, w5dxp.com

On Nov 9, 6:39*pm, Cecil Moore wrote:
... the Hamwaves inductance calculator provides us an easy way
of calculating that delay through the loading coil.

http://hamwaves.com/antennas/inductance.html

Assume a 10" long, 100 turn coil with a diameter of 2" wound with #18
wire. In metric, that's 254 mm long, 50.8 mm diameter, and 1.024 mm
wire. At 4 MHz, the above calculator indicates that the axial
propagation factor is 2.122 rad/m which we can convert to degrees/inch
by multiplying by 1.4554 which yields 3.088 degrees per inch. The coil
is 10 inches long so the number of degrees occupied by the coil at 4
MHz is 30.9 degrees.

If this coil is used as a base loading coil in a 4 MHz mobile antenna,
it occupies ~30.9 degrees of the antenna. A 7 foot whip occupies ~10.2
degrees at 4 MHz. The antenna, at resonance, is known to be 90 degrees
long. So the phase shift at the coil to whip junction has to be ~48.9
degrees assuming resonance at 4 MHz.

For an electrical 1/4WL base-loaded antenna, e.g. an HF mobile
antenna, there exist three phase shifts that add up to 90 degrees. The
phase shift through the coil plus the coil to whip junction phase
shift plus the phase shift through the whip have to add up to 90
degrees. For a center-loaded antenna, there are four phase shifts that
must add up to 90 degrees. The phase shift at the base to bottom of
loading coil junction is negative. That's why we need more inductance,
i.e. more phase shift, in the center-loading coil than we do in the
base loading coil.

If we are dealing with a 5/8WL (225 deg) antenna, the phase shift
through the base coil plus the phase shift at the coil to whip
junction must add up to 45 degrees such that 225 deg + 45 deg = 270
deg = 6/8WL.
--
73, Cecil, w5dxp.com

On Nov 7, 4:18*pm, Owen Duffy wrote:
"David" nospam@nospam wrote :



Which antenna is better: 5/8 wavelength vertical or a J pole?

Better for what? For example, a 5/8 whip would usually be better than a J
Pole for mobile applications on a car roof, but that doesn't make it
better for all applications.



Frequency of operation is 145 MHz *= 2 metres.

The 5/8 wavelength vertical has a loading coil. There are losses in
the coil.

It is certainly popular to talk down an antenna with loading coils or
traps because they are "lossy".

Everthing in a real world antenna has loss, the issue is the magnitude of
the loss, and the impact of that on system performance for the intended
application.

You might find it hard to believe that some antenna systems incorporate
loss elements in order to reduce feed line loss by more than that in the
introduced elements.

Whilst you have chosen to raise the loss in the coil, you haven't raised
the issue that a J Pole has currents flowing in lossy conductors,
components of which that do not directly contribute to radiation.



The J pole has a quarterwave matching stub. The matching stub provides
an out of phase current which means that there is a cancelling field
close to the radiating element. Also the J pole is end fed, which

This is not a very good way of analysing the J Pole. The U section can be
thought of as carrying currents that have differential and common mode
components. The common mode components contribute directly to radiation
field. You should also consider common mode current on the supporting
structure and feedline.

The 5/8 wave vertical also has potential for significant common mode
current on the supporting structure and feedline, you need to look at the
effectiveness of the decoupling method employed (usually a radial set).

means the transmitter is not connected directly to a maximum current
point.

What does that matter? BTW, neither is the base fed 5/8.

There is a maxim in ham radio that antennas should always be fed at a
current maximum. You could subscribe to that, but you would limit
yourself by excluding a range of good solutions, and with no guarantee
that a current fed antenna is optimum.



Does the J pole have a disadvantage because of *the cancelling field

There is not perfect cancellation at all points on the U section.

from the matching stub and the fact that it is end fed?

"End fed", as opposed to a centre fed dipole perhaps. But isn't the 5/8
"end fed"?



Also consider gain and angle of radiation.

The three dimensional gain distribution is important, but dependent on
the common mode issue mentioned above (amongst other things).

It is fair to say that J Poles are very popular, and that they are overly
represented in problems discussed in online fora.

On the other hand, the 5/8 which was once very popular for mobile work in
this area, is long lost, replaced by two, three and four band antennas
where VSWR is more important than any other performance parameters. I use
a 5/8 vertical on my car, and regularly work repeaters mobile at
distances well over 100km. The antenna is more than thirty years old, and
has never required repair despite hitting low trees, carpark roofs etc
lots of times. The modern multiband antennas are not that robust. I
wouldn't even think of a J Pole in this application.

So, "best" can be a quite complex requirement.

Owen

Owen, I think the popularity of VHF mobile 5/8 antenna lies in it has
more gain than a 1/4 wl antenna and is easier to match to 50 ohms than
a .5 wl antenna. I had certainly rather DIY a 2M 5/8 mobile antenna
than a .5 wl version. From information I have seen the 5/8 often
touted for its low angle of radiation may actually have a
significantly higher angle of radiation than the .5 wl antenna used in
a similar situation. This is not to say either antenna would not be
equally useful.

Jimmie