On Nov 10, 3:46*pm, JIMMIE wrote:
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

It depends on the vehicle and mount location.
I think often a vehicle provides a better lower section
than the usual ground plane with 1/4 WL elements.
I've seen 5/8 whips do quite well on vehicles if they
are mounted in a good location, the best being the
center of the roof. They beat 1/4 WL whips in comparison
tests, and often showed less "picket fencing".
I imagine a 1/2 wave would work well, but I've never
actually tried a 1/2 wave whip on a vehicle due to
it generally being more complex to build and match.
And in the end, I think the 5/8 would probably beat it
anyway.

But in another comparison on 10m, the elevated
5/8 GP's beat the elevated 1/2 wave's I tried.
And all were decoupled from the feed line.
The comparisons were done locally, using the
space/ground wave which is low angle critical,
and the 5/8's always won to stations that were 30-40
miles away vs the 1/2 wave's.

Anyway, I'm not nearly as negative about 5/8 wave
antennas as many people are.
But like I say, I think the 5/8 over 1/4 wave GP scheme
is perverted and can be greatly improved using more
sane designs. :/

The maximum gain for a single element is .64 wave.
And that gain is usually considered appx 3 db better
than a 1/4 wave. But if I remember right, the dual 5/8
collinear is usually rated at about 3 db better than a
vertical 1/2 wave. "appx 5.1 dbi".
For VHF/UHF use, the old AEA Isopoles were one
of the best commercial verticals built as far as gain
and decoupling of the feed line. Those were dual 5/8
designs with lower decoupling cones.
It was the superior decoupling that really made them
shine at low angles.

On Wed, 10 Nov 2010 13:46:12 -0800 (PST), JIMMIE
wrote:

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
snip


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
In the late 1970's my work car was a tiny Ford Fiesta (AKA "Fiasco"
It was fitted with a 5/8 wave on a fender mount. The transciever was a
Heath 2036 at 5 watts. The antenna did perform very well as far as
distance was concerned.

However, it was tall enough to ping the light fixtures in the many
parking garages that were essential to my job.

The biggest advantage of a J-Pole is that it is not dependent on a
good ground.

John Ferrell W8CCW

On Nov 11, 11:42*am, John Ferrell wrote:
On Wed, 10 Nov 2010 13:46:12 -0800 (PST), JIMMIE

wrote:
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

snip



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

In the late 1970's my work car was a tiny Ford Fiesta (AKA "Fiasco"
It was fitted with a 5/8 wave on a fender mount. The transciever was a
Heath 2036 at 5 watts. The antenna did perform very well as far as
distance was concerned.

However, it was tall enough to ping the light fixtures in the many
parking garages that were essential to my job.

The biggest advantage of a J-Pole is that it is not dependent on a
good ground.

John Ferrell W8CCW

I ve had both .5 and .64 wl antennas on my big Chevy Van. I cant tell
any significant difference in performance except for a couple of .5
antennas that were really poor performers, I think this may have been
due to matching network design.. Except for this cause I dont see how
there could be any perceptable difference in the two antennas short of
careful measurements on an antenna range..


Jimmie

On Thu, 11 Nov 2010 14:08:39 -0800 (PST), JIMMIE
wrote:


In the late 1970's my work car was a tiny Ford Fiesta (AKA "Fiasco"
It was fitted with a 5/8 wave on a fender mount. The transciever was a
Heath 2036 at 5 watts. The antenna did perform very well as far as
distance was concerned.

However, it was tall enough to ping the light fixtures in the many
parking garages that were essential to my job.

The biggest advantage of a J-Pole is that it is not dependent on a
good ground.

John Ferrell W8CCW

I ve had both .5 and .64 wl antennas on my big Chevy Van. I cant tell
any significant difference in performance except for a couple of .5
antennas that were really poor performers, I think this may have been
due to matching network design.. Except for this cause I dont see how
there could be any perceptable difference in the two antennas short of
careful measurements on an antenna range..


Jimmie

After I could not stand the Fiesta any longer I ordered a new Cadillac
Cimarron. I did not want to drill any holes in the new Caddy so I
bought one of the thru the glass end fed whips. Its performance was OK
and it did not draw attention to the vehicle so I lived with it.
130,000 miles later it went to our son radio, antenna and all! It only
made sense when he was licensed. If I were doing it again I would have
drilled the appropriated hole in the roof above the dome light and
installed a Larsen 2m & 400 antenna. That was the last time I
hesitated about drilling a hole where I needed it.

Now that I am in a 2008 Chrysler Minivan I have taken the lazy way out
and placed a mag mount quarter wave on the luggage rack (no scratching
there) with the coax ty-wrapped to the rack so there is not enough
slack to allow real damage if it gets knocked off. An itty-bitty Yaesu
FT-90 sits on a sticky pad on the dash and plugs into one of the
vehicles many12v power outlets. The power outlets are not like the
cigarette lighters of the past. They are well installed and fused at
20 amps. As long as you purchase a plug to handle the power and trim
the cable to what is necessary all is well.

The FT-90 gets too hot to handle and shuts down on lengthy rag chews
at full power, but does fine at lower power settings.

You can waste a lot of time and money over killing a mobile antenna
for repeater operation. If you are out in the wide open spaces, maybe
it is worth it!
John Ferrell W8CCW

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?

I referred to the 1/8 wave loading coil without really thinking about it. I
was unsure of the loading coil dimensions, so I simple tried a 1/8 wave
length wire formed into a coil. This is for the simple series arrangement
5/8 radiator. This created a load coil that appeared to have a little too
much L so I have removed one turn, seems to load up ok after a little
trimming of the radiator. Keen to hear how too determine the
value/dimensions for the loading coil.

Having said that I'm not sure what so wrong with missing degrees" concept.


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.

My current 5/8 wave ground plan project is simply to get something on air,
however I plans to construct an improved version with the tapped coil
approach.

I may be looking in the wrong places, but I have been surprised at how
little information there is on the net regarding 5/8 wave ground plan.

Thanks Owen for the above over view of the 5/8 wave ground plan.

Cheers

Peter VK6YSF

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

On Nov 13, 7:11*pm, "Peter" wrote:
Having said that I'm not sure what so wrong with missing degrees" concept..

:-) Because there are two sides of the argument each at the two
extremes. The technical truth is that the coil does replace a certain
number of degrees of the missing part of the antenna but not all of
the missing degrees. I am preparing a technical article that explains
the details. Please stand by.
--
73, Cecil, w5dxp.com

On Sun, 14 Nov 2010 09:11:22 +0800, "Peter" wrote:

Having said that I'm not sure what so wrong with missing degrees" concept.

Hi Peter,

Drives some up the wall. Your terming it as a concept is perfect, but
it will still be argued for being a corrupt teaching of a literal
equivalence.

73's
Richard Clark, KB7QHC

"Peter" wrote in
:

....
I referred to the 1/8 wave loading coil without really thinking about
it. I was unsure of the loading coil dimensions, so I simple tried a
1/8 wave length wire formed into a coil. This is for the simple series
arrangement 5/8 radiator. This created a load coil that appeared to
have a little too much L so I have removed one turn, seems to load up
ok after a little trimming of the radiator. Keen to hear how too
determine the value/dimensions for the loading coil.


Hi Peter,

One theoretical method is to model the antenna, and find the feedpoint
impedance. The R component decreases as length increases from a half wave
to three quarter wave, and X increases towards zero.

Best pattern is closer to a half wave, but R is very high. At 0.7
wavelengths, R is low enough for an acceptable match by using a series
inductor, and reactance will be a few hundred ohms give or take depending
on length.

Having said that I'm not sure what so wrong with missing degrees"
concept.

For one thing, if a certain inductance is required, the quantity of wire
needed depends on several underlying coil parameters.


....

My current 5/8 wave ground plan project is simply to get something on
air, however I plans to construct an improved version with the tapped
coil approach.

That lets you shorten it for a bit more gain, and a good match.

If you cut the vertical for 0.6 wavelengths, you should think of starting
with an inductor with reactance towards 1000 ohms.


I may be looking in the wrong places, but I have been surprised at how
little information there is on the net regarding 5/8 wave ground plan.

Probably displaced by OTS 4 band verticals.

Have fun.

Owen

Owen Duffy wrote in
:

"Peter" wrote in
:
....

My current 5/8 wave ground plan project is simply to get something on
air, however I plans to construct an improved version with the tapped
coil approach.

That lets you shorten it for a bit more gain, and a good match.

If you cut the vertical for 0.6 wavelengths, you should think of
starting with an inductor with reactance towards 1000 ohms.


I meant to elaborate on this a bit more. (Did I hear someone groan?)

If for example, the feedpoint Z of a 0.6 wave vertical over four quarter
wave radials was 150-j500, your tapped coil matching network can be
designed using bulk standard circuit theory to transform 150-j500 to 50
+j0, and nowhere do you use the missing 54° in those calcs. That might
suggest that the "missing degrees" are some kind of explanatory crutch
(or ham speak) that is not directly related to solving the problem.

Owen

On Sun, 14 Nov 2010 02:25:50 GMT, Owen Duffy wrote:

I meant to elaborate on this a bit more. (Did I hear someone groan?)

If for example, the feedpoint Z of a 0.6 wave vertical over four quarter
wave radials...

I'll bite (or groan as the expectation demands) - why "quarter wave"
radials? A rule of thumb?

* * * Rhetorical questions follow * * * *

Quarter wave in physical length?

Quarter wave in electrical length?

Elevated Quarter wave radials?

If elevated, Quarter wave drooped radials?

* * * Philosophical ponderings follow * * * *

The discussion of radials usually attends ground mounted studies in
the 100M band in the 1930s. Those studies sought to reduce loss while
mimicking a conductive ground of infinite extent. Radiators taller
than Quarter wave were treated to feedpoint loading (such as found in
the current topic, albeit with the possibility of it being elevated
and thus muddying the philosophy here). All such historical (and
current AM band engineering) feedpoint loading presumed, basically, a
non-resonant ground system. As Quarter wave long radials imply
resonance (at least in the first read), this would suggest that,
perhaps, this "tuning" should be further examined in light of
feedpoint loading. The conclusion, to my mind, would be that
significant reduction in feedpoint loading could be accomplished by
tailoring radial length (much less drooping that is already part of
the lore).

At first blush, it would seem that the radials would be shorter than
Quarter wave (forgive me for not first confirming this astonishing
leap of faith).

Of course, there is every chance some reactance will remain to be
"tuned" away (returning us once again to loading) - if the mismatch is
deemed significant.

If such is the case, and returning to the original design, what
problem is the Quarter wave length radial rule of thumb responding to?

* * * * Alternative analysis * * * *

Or to put it into the light of other antenna topological discussions,
and in this regard the off-center fed dipole. Here we have an
off-center feed (we rarely go on to describe all such installations as
"vertical dipoles"). We can fully expect that, as such, we are
transforming the expected 70±j0 Ohms into some other value. Quite
frequently in an OCF design, it is much higher - and variable by the
degree of offset. However, for a fixed frequency, this is better
understood and can be anticipated. The proximity to ground and the
geometry (the radials certainly disturb the shape of an OCF dipole,
even if vertical) further change things, but conceptually the monopole
with resonant radials still constitutes an OCF design that is "on
center fed" for the vertical element when it, too, is a Quarter wave
in length.

For many prospective feed points along the length of the OCF dipole,
the only consideration needed is for a ratio transformation, not
tuning. This is usually resolved in a BalUn. Hence "loading" is
removed from the picture through careful consideration of the whole
antenna, the degree of offset, and not through arbitrary assignment of
Quarter wave length radials to all vertical designs.

* * * * Conclusion * * * * *

The concept of a loading coil where its length of wire "replaces" the
missing length of radiator wire is a commonplace for technologists. It
serves the discussion quite well at that level.

The value of this length of wire's inductance is going to vary by
significant value for the many coil form variables available to the
technician. Hence the exactness of this "replacement" is questionable
on the face of it at the engineering level of discussion. This
equivalence "replacement" is forced further into unresolved exactness
if we move the same coil up into the radiator (without changing the
radiator's length).

The same could be said with the treatment of Quarter wave length
radials, which, after all, are a special and not general solution.

73's
Richard Clark, KB7QHC