As an alternative to using a choke on the feedline, what sort of
results might one get with a standard J-pole by using a half-wave
coaxial balun and tapping up a bit further on the elements?

Still don't think that would clean up spillover currents...
If I were to use a J pole, and wanted decoupling, I would
add a 1/4 wave coax section below the feed, and then
have a set of radials at that point. They can be grounded
to the mast, or left free, as long as they are connected to
the shield. The 1/4 wave of coax is physical length, not
electrical. But I've found in testing, it's not all that critical.
Of course, he wouldn't be able to claim "no radials" at that
point, but it would be pretty well decoupled. This is the same
basic design cushcraft used with the ringo ranger. In that case,
the coax length was 50 inches long, to a set of 20 inch radials.
They used the longer length due to the dual 5/8 design.
They actually seem to claim that 50 inches of coax as a 3rd
radiating element, but I don't quite see it that way. If that were
truly the case, it would beat the isopole.
The comparison between the isopole and the ringo ranger 2
show how important decoupling is. They are both appx dual
5/8 designs. No real difference in element length.
The isopoles improved decoupling is what make it the winner
when you compare the two head to head.
A sleeve would also be easy to use with a J pole...
Actually, I sort of prefer the center fed 1/2 wave "sleeve"
dipole vertical, with a 2nd lower decoupling sleeve, over the
usual J pole design. MK

Ian, not wishing to be classed amongst the old wives you have very
carefully avoided talking about "power radiated from feedlines". You
have shifted to using clamp-on ammeters.

But people who DO discuss things in such terms are unable to justify
the use of chokes by quantifying the power actually radiated and
setting limits on what power level is acceptable or is not acceptable.
If they can't measure or calculate the power level then they know
nothing about what they are are talking.

Refer to what Lord Kelvin said about measurements.

Can you suggest an acceptable level of amps as measured on a clamp-on
ammeter?
----
Reg, G4FGQ

----------------------------------------------------------------------
---------


"Ian White GM3SEK" wrote in message
...
Reg Edwards wrote:

They who discuss "power radiated from the feedline", yet are unable
to
measure (in watts) or calculate (in watts) the MAGNITUDE of the
effect, belong to a set of waffling old wives.


Those who take no precautions to prevent their feedline from
becoming
part of the antenna, belong to a set of people who don't even know
what
their antenna IS.

There's a part you call "the antenna", and another part you call
"the
feedline". Wishful thinking will not stop RF current from flowing
directly from one to the other.

How is it possible to decide whether or not a choke or balun is
needed, and where to locate it, unless the magnitude of what one
MIGHT
wish to prevent is known.

Try a clamp-on RF current meter, a little modeling... or even a
little
common sense.

There's a place called "the feedpoint" where the antenna and the
feedline are connected directly together. Might that be a good
location
for a choke to keep them separate? Yes, it almost certainly would.

Chokes may also be needed at other locations, but it's hard to
justify
anywhere else as your *first* choice. (The exception is the Carolina
Windom and similar antennas where part of the feedline is intended
to
radiate. But even there, they put a good choke at the point where
they
want RF currents on the feedline to stop.)

Queen Elizabeth 1 of England had the good sense to take a bath every
six
months "whether I need it or not". If you don't know whether your
feedline smells of RF, then follow her excellent advice and use a
choke.


--
73 from Ian GM3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek

Dave Platt wrote:
In article ,
Ian White GM3SEK wrote:

The version of the "non-standard J-pole" which Cebik models is fed at
bottom center, in a dimensionally-symmetrical way. His current plot
seems to show equal currents at this feedpoint, and this would seem to
make the use of a choke or balun on the feedline somewhat less
significant than with a standard J-pole.

An antenna model without a feedline will *force* equal and opposite
currents at the feedpoint - it is always fed through the perfect balun!

Cebik's plot of currents in the "standard" J-pole shows unequal
currents all the way down to the bottom, and he notes this in his text
and asserts the need for a choke.

He doesn't say whether his models do or do not include a feedline.

If they did, the configuration of the feedline would become another
important variable which he couldn't fail to mention. Also the drawings
show no feedline.

Add an un-choked feedline to the model (another thick wire, representing
the coax shield) at either side of the feedpoint, and see where the
current goes now. If you don't use a choke on a real-life antenna,
there's nothing to stop the current going wherever it likes.

Agreed. The results are likely to be quite variable depending on the
feedline distance to the nearest ground. Seems to me that the worst
case would result from a small integral multiple of 1/2 wavelength, no?

That's right. At the point where the coax shield connects to the
antenna, the current will divide three ways, between the antenna, the
inside of the shield, and the outside of the shield. The split will
depend on the ratio of the impedances in each of those three directions.

If the impedance for current flowing down the outside of the shield is
low (which any multiple of 1/2 wavelength grounded at the bottom will
achieve) then away the current will go - there's nothing to stop it.

A feedline choke creates a high impedance against current flow down the
outside, so the current from the inside of the shield flows almost
exclusively into the antenna.

Even if you choke a J-pole at the feedpoint, there will also be induced
currents further down the feedline because the antenna and the feedline
are usually installed in a straight line. But that doesn't override the
need to choke the feedline at the most obvious place.

My guess is that in most simple J-pole installations, the feedline
radiation and the resulting disturbance of the antenna's omni pattern
are probably not going to be worth worrying about too much. Nearby
buildings, trees, etc. are likely to result in larger differences
in the far-field pattern than any quirks in the antenna's own pattern.

Most people using J-poles won't worry, that is true... but that's mostly
because J-poles are used in relatively undemanding applications where
you either hit the repeater or packet node, or you don't.

In defence of Al, if the J-pole is mounted directly on a car roof, then
there's no point in attempting to choke the feedpoint. But if it's
mounted on a mast, the mast and feedline will radiate. How much will
depend on the exact installation, and is pretty well unknown unless you
can measure the actual RF currents.


As an alternative to using a choke on the feedline, what sort of
results might one get with a standard J-pole by using a half-wave
coaxial balun and tapping up a bit further on the elements?

That is a workable feed method, but the half-wave balun is a voltage
balun. The antenna is asymmetrical, so the balun is acting as
center-tapped voltage source which is trying to push equal currents into
the unequal impedances on either side of the feedpoint. That will never
quite succeed, so there will always be some out-of-balance current left
over.

Like all voltage baluns, the half-wave coax type doesn't do anything
directly to *prevent* the out-of-balance current from flowing away on
the outside of the feedline. On the contrary, there is a hard-wired
connection that will *allow* such currents to flow.



--
73 from Ian GM3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek

Reg Edwards wrote:
Ian, not wishing to be classed amongst the old wives you have very
carefully avoided talking about "power radiated from feedlines". You
have shifted to using clamp-on ammeters.

But people who DO discuss things in such terms are unable to justify
the use of chokes by quantifying the power actually radiated and
setting limits on what power level is acceptable or is not acceptable.
If they can't measure or calculate the power level then they know
nothing about what they are are talking.

That's not really quite true. Can you or anyone else tell how many watts
is radiated from each element of a Yagi? How? Then how do you know which
elements are important an which are not?

Roy Lewallen, W7EL

Reg Edwards wrote:

Ian, not wishing to be classed amongst the old wives you have very
carefully avoided talking about "power radiated from feedlines". You
have shifted to using clamp-on ammeters.

But people who DO discuss things in such terms are unable to justify
the use of chokes by quantifying the power actually radiated and
setting limits on what power level is acceptable or is not acceptable.
If they can't measure or calculate the power level then they know
nothing about what they are are talking.

Refer to what Lord Kelvin said about measurements.

Can you suggest an acceptable level of amps as measured on a clamp-on
ammeter?

No. Reg knows perfectly well that the RF current is only one part of a
much bigger picture.

An acceptable level is one that:

1. Does NOT make the microphone bite your lips (or does not leave
lasting scars :-)

2. Does NOT cause your transmitter to act up because there's too much RF
current flowing through your station, trying to find "ground"

3. Does NOT cause RFI to your family and neighbours

4. Does NOT cause unpredictable changes in transmitter loading

5. Does NOT lead to unacceptable pickup of interference when you're
trying to receive.

So that "acceptable level" depends entirely on each individual's
particular station layout, how they operate, where they live, what kinds
of consumer electronics the family and neighbours use, how they are
installed... and how much that individual ham cares about getting along
with the family and the neighbours.

Every case is totally individual. That is why every individual needs to
do his own thinking and make his own decisions.

The only "old wives' tale" is that somebody else can do it for you, or
tell you from 5000 miles away what does or doesn't matter.

You don't actually need to measure amps in order to make those
decisions. Basically it's all about simple practical things like the
list above.

Where the RF current meter really helps is if you decide you do have an
RFI problem. Then it lets you *see* how well you're managing to fix it.
Too much of RFI investigation work is like groping in the dark. The RF
current meter is like taking a blindfold off.


--
73 from Ian GM3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek

On 13 Jun 2005 23:29:21 -0700, wrote:

what is never GOOD is having another tell one WHAT IS GOOD for
them... if you do that in normal life, expect to get told off as often
as here...

What is good is talking to people that don't
have their heads up their rearend.
All I'm doing is stating fairly simple facts.
If you choose to ignore them, fine with me...
But don't start with your goofball behavior pattern
just because I'm trying to clarify that *no* type of feed on an
elevated vertical is capable of fully decoupling the antenna
on it's own.


I quote from Al's first post....
"One think you may have missed, the original post was about a Arrow
Antenna J-Pole. This is an Open Stub type J-Pole, Not a Closed Stub
type like the copper pipe ones. The OSJ does not need a choke, it does
not have a problem with feedline radiation or a problem with Common
Mode Currents. "

That statement is *false* if you want the best performance.
That type of antenna is most certainly able to suffer from
feedline radiation. Sure, it may be better decoupled from
the line using his method vs another, but that does not deal
with spillover currents. There must be a 2nd decoupling device to
deal with those currents. The antenna does include that as
sold.

If you think I really give a hoot about Arrow antennas, or Al,
or whoever, you are sadly mistaken. He would have never
heard from me if he hadn't said that his J pole can't suffer from
common mode problems.
I'm a dumbass compared to many of these people, and even I
know thats a silly statement to make, unless you like holes in your
toes.
Oh...And I can't ever recall ever being "told off" on any post, on
any NG...I don't have the behavior patterns, hyperactivity, etc,
that you seem to exhibit. So I don't attract the attention that people
like you do. The only people that ever get me going are smartasses.
I have run across 4-5 real® smartasses on my journey through the
internet.
They tried to tell me off, but I quickly showed them that that was an
effort in futility. I'm rated a black belt when it comes to
smartasses.
You wanna be #6 on the list? Go ahead...Make my day....
At this point, I don't consider you a full fledged smartass...
You are like a 9 year old that is hyperactive from gorging on
chocolate, that turned out to actually be coated preluden pills...
So I cut you a bit of slack...So far.....Don't push your luck...

You would have never heard from me, except that your post struck
me as overall BS...As usual....
Note...

This room is less about antennas than it is about egos, hero worship,
and the "good ole boys club."

Only people that are defensive, or are trying to cover something up,
or trying to claim some silly nonsense would feel this way.
All people have ego's to varying degrees.
Best to ignore....
I would never worship a man.
I don't belong to any clubs, but I reckon if I did, I would
prefer a bunch of good ole boys, over a bunch of anal
retentive "girly men" who constantly feel victimized
by the world in general...
MK


Well said.

That's not really quite true. Can you or anyone else tell how many
watts
is radiated from each element of a Yagi? How? Then how do you know
which elements are important an which are not?

Roy Lewallen, W7EL

================================

Neither you, I or anybody else knows. But, as usual, you have put
your finger on the source of the trouble. It's a descriptive language
problem.

To speak in terms which most people understand, when Cecil acquired
his burn the injury was just as likely to come from the antenna as it
was from the feedline.
Actually, it came from the PA via the tuner.

It is not entirely unrelated to the confusion about standing waves
caused by referring to the so-called SWR meter as an SWR meter, when
it doesn't do anything of the sort and, in any case, there is no line
in a position on which SWR can be measured.
----
Reg, G4FGQ

Ian White GM3SEK wrote:
Where the RF current meter really helps is if you decide you do have an
RFI problem. Then it lets you *see* how well you're managing to fix it.
Too much of RFI investigation work is like groping in the dark. The RF
current meter is like taking a blindfold off.

And the "RF current meter" doesn't have to be expensive or
complicated. A before/after relative current value may be
all one needs. I use a toroid with ten turns feeding a
1N34A curcuit and a DC voltmeter with very short leads. It
was "calibrated" using forward current into a 50 ohm load.

Simple measurements like this are covered in, "Baluns,
What They Do And How They Do It", by Roy, W7EL, in the
ARRL Antenna Compendium, Vol 1, article available at:

http://www.eznec.com/Amateur/Articles/
--
73, Cecil http://www.qsl.net/w5dxp


----== Posted via Newsfeeds.Com - Unlimited-Uncensored-Secure Usenet News==----
http://www.newsfeeds.com The #1 Newsgroup Service in the World! 100,000 Newsgroups
---= East/West-Coast Server Farms - Total Privacy via Encryption =---

.... my gawd, you go into far too much verbiage for far too simple of
explanation(s) of your personal, views, beliefs and offerings... I do
choose to ignore them and toss 'me into "what nm5k likes" bucket--which
frankly, "charlotte--I don't give a damn."

.... if the only time you hear these complaints are from me, no need to
worry, if others point out your nature is less than desirable (and new
members--not the good ole boys here who are into the "mutual admiration
societies", "legends of their own mind(s) clubs) then you may want to
use it to your advantage--or not...

John

wrote in message
oups.com...
what is never GOOD is having another tell one WHAT IS GOOD for
them... if you do that in normal life, expect to get told off as often
as here...

What is good is talking to people that don't
have their heads up their rearend.
All I'm doing is stating fairly simple facts.
If you choose to ignore them, fine with me...
But don't start with your goofball behavior pattern
just because I'm trying to clarify that *no* type of feed on an
elevated vertical is capable of fully decoupling the antenna
on it's own.


I quote from Al's first post....
"One think you may have missed, the original post was about a Arrow
Antenna J-Pole. This is an Open Stub type J-Pole, Not a Closed Stub
type like the copper pipe ones. The OSJ does not need a choke, it does
not have a problem with feedline radiation or a problem with Common
Mode Currents. "

That statement is *false* if you want the best performance.
That type of antenna is most certainly able to suffer from
feedline radiation. Sure, it may be better decoupled from
the line using his method vs another, but that does not deal
with spillover currents. There must be a 2nd decoupling device to
deal with those currents. The antenna does include that as
sold.

If you think I really give a hoot about Arrow antennas, or Al,
or whoever, you are sadly mistaken. He would have never
heard from me if he hadn't said that his J pole can't suffer from
common mode problems.
I'm a dumbass compared to many of these people, and even I
know thats a silly statement to make, unless you like holes in your
toes.
Oh...And I can't ever recall ever being "told off" on any post, on
any NG...I don't have the behavior patterns, hyperactivity, etc,
that you seem to exhibit. So I don't attract the attention that people
like you do. The only people that ever get me going are smartasses.
I have run across 4-5 real® smartasses on my journey through the
internet.
They tried to tell me off, but I quickly showed them that that was an
effort in futility. I'm rated a black belt when it comes to
smartasses.
You wanna be #6 on the list? Go ahead...Make my day....
At this point, I don't consider you a full fledged smartass...
You are like a 9 year old that is hyperactive from gorging on
chocolate, that turned out to actually be coated preluden pills...
So I cut you a bit of slack...So far.....Don't push your luck...

You would have never heard from me, except that your post struck
me as overall BS...As usual....
Note...

This room is less about antennas than it is about egos, hero worship,
and the "good ole boys club."

Only people that are defensive, or are trying to cover something up,
or trying to claim some silly nonsense would feel this way.
All people have ego's to varying degrees.
Best to ignore....
I would never worship a man.
I don't belong to any clubs, but I reckon if I did, I would
prefer a bunch of good ole boys, over a bunch of anal
retentive "girly men" who constantly feel victimized
by the world in general...
MK

Cecil Moore wrote:
Ian White GM3SEK wrote:
Where the RF current meter really helps is if you decide you do have
an RFI problem. Then it lets you *see* how well you're managing to
fix it. Too much of RFI investigation work is like groping in the
dark. The RF current meter is like taking a blindfold off.

And the "RF current meter" doesn't have to be expensive or
complicated. A before/after relative current value may be
all one needs. I use a toroid with ten turns feeding a
1N34A curcuit and a DC voltmeter with very short leads. It
was "calibrated" using forward current into a 50 ohm load.

Simple measurements like this are covered in, "Baluns,
What They Do And How They Do It", by Roy, W7EL, in the
ARRL Antenna Compendium, Vol 1, article available at:

http://www.eznec.com/Amateur/Articles/

There are also full constructional details and examples on my site, at:
http://www.ifwtech.co.uk/g3sek/clip-on/clip-on.htm

--
73 from Ian GM3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek