That's an excellent summary by Dave. However, I didn't see any mention
of common mode current due to mutual coupling between the feedline and
the antenna. Even if you perfectly choke the common mode current at the
feedpoint, considerable current can be induced onto the feedline. This
won't happen with a symmetrical dipole if the feedline is oriented at
right angles to the antenna. but where the antenna and feedline are
collinear, as they are in a J-Pole or ground plane antenna, coupling can
be substantial. The criterion for maximum current in that case is
whether the decoupled section of the feedline is approximately resonant.
For example, a quarter wave feedline grounded at the bottom and
decoupled at the top (i.e., with a current balun - common mode choke -
at the feedpoint) can have considerable current induced.

It turns out that the conditions for maximum induced current can be
opposite those for conducted current. For example, a half wavelength
feedline that's not grounded at the bottom end won't have a great deal
of conducted current. However, it can have quite a bit of induced
current *if decoupled at the feedpoint*. If there's a good balun at the
feedpoint, the isolated feedline becomes approximately self-resonant.

Here's an example, for those who have EZNEC:
Begin with example model VHFGP.EZ. First, in the main window, select
Units, change to any units other than Wavelengths, and click Ok. Then
select Units again, change back to Wavelengths, and click Ok. (This is
necessary because of an EZNEC bug I discovered while preparing this. It
affects only old files with Wavelength units, which include example
files VHFGP and W8JK. When opened, these files will show the wire
diameter units as being in Inches in the Units selection, but
Wavelengths in the Wires Window. This bug will be fixed in the next
update release, v. 4.0.17. In the meantime, you can re-save the files
after changing the units and changing back, and they'll be ok from then on.)

Add a wire with End 1 at 0, 0, 5 and End 2 at 0, 0, 4.5, 10
segments. Make the diameter 0.25 inch. This represents the outside
of a feedline. Click the Currents button at the left of the main window
and look at the current on the new wire, Wire 6. You should see that
it's about 0.19 amp at End 1 (the top). (You'll see a different value if
you've set a power level in the Options menu.) Now add a decent balun by
adding a load with R = 1000 ohms to End 1 of Wire 6 (0% of the way from
End 1). Click Currents again. Note that the current is now maximum at
the middle of Wire 6, and it's more than twice what it was before --
about 0.42 amp.

This is a situation that I'd frankly never considered before -- where a
feedpoint balun can actually *increase* the common mode current! The
current can, of course, be lowered to a small value by adding a second
current balun (common mode choke) about a quarter wavelength down the
wire (Wire 6, 50% from End 1).

To stop the conducted current, you need a choke at the feedpoint or,
alternatively, an even number of half wavelengths from it. To stop the
induced current, you need a break up a resonant line by adding a choke
about a quarter wavelength or an odd number of quarter wavelengths from
an open end, or an even number of half wavelengths from a grounded end.
Of course, you can alter the feedline length in such a way that both are
minimized without needing a choke.

Roy Lewallen, W7EL

In article ,
Roy Lewallen wrote:

This is a situation that I'd frankly never considered before -- where a
feedpoint balun can actually *increase* the common mode current! The
current can, of course, be lowered to a small value by adding a second
current balun (common mode choke) about a quarter wavelength down the
wire (Wire 6, 50% from End 1).

To coin a phrase: "Fascinating!" Thanks for pointing this out, Roy.

This gives an interesting twist to some advice I'd read several
times... that to choke off feedline current one should install two
chokes or ferrite beads, a quarter of a wavelength apart on the feedline.

I had always assumed that this was simply a quickie way of making sure
that a useful amount of choking reactance was sure to be installed
fairly close to a current maximum, where it'd be most effective, and
that if you knew where the current maximum actually was and put a
choke there, you wouldn't really need to install the second choke.

The fact that the conduction and induction currents behave differently
would seem to rule that out - there really _is_ a good reason to have
two chokes, to handle the two modes.

Hmmm... slightly crazed idea... I wonder if there's a market for a
coax with some amount of ferrite dust mixed into the PVC jacket when
it's extruded, so as to create a self-choking distributed-inductance
feedline. If the more rabid audiophiles can be convinced to spend
thousands of dollars for a one-meter RCA-plug-tipped interconnection,
maybe the excessively- well-to-do ham (there must be at least one)
would pay ridiculous sums for a self-baluning RG-8?

Might be a neat income opportunity, if one could sell it at high
prices and still stand to look at ones face in the mirror the next
morning (I couldn't, but I imagine there are people who could).

--
Dave Platt AE6EO
Hosting the Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!

On 16 Jun 2005 15:14:13 -0700, "Al" wrote:

Your saying all antennas are the same is like saying all antennas with
a gamma match have a skewed pattern.
73 Al Lowe N0IMW

Damn it Allen, what I am saying is coax is coax and it will behave the
same regardless of what antenna it is connected to.

I feel you missing the whole point.

I am not attacking you antenna. I just disagree with your advise on
its installation.


73,
Danny, K6MHE

Hi Dan,

My, you are a bit on the arrogant and foul-mouthed side, aren't you!

It would be tedious to try and dissect your attributions of positions
that I did not take, so I'll skip that.

I'll just point out that as an E.E. with three degrees from a top-5
engineering school, and a lifelong specialization in RF engineering,
I don't need any patronizing lectures from you. "Physician," heal
thyself... or at least try to understand the difference between
theoretical niceties and practical effects. Reading Dave's posts
might help. (He's much more patient with twits than I am, and
explains things well.)

BTW, I have nothing against coax "baluns," unless they become
physically awkward, or noticeably increase cable loss.

Ed

"Dan Richardson arrl net" k6mheatdot wrote in message
...
On Mon, 13 Jun 2005 22:54:01 GMT, "Old Ed"
wrote:

Don't let the nay-sayers in this NG bother you, and don't waste
your time arguing with them.

Some of them can't stand the thought that any antenna which differs
from their personal pet(s) might be good. Others can't grasp the
idea that when model predictions and actual results differ, it's the
results that count.

Spoken like a true naif.

I suggest that you would take the time to learn what causes common
mode current to be present on coax transmission line - regardless of
what kind of an antenna it is employed. (For openers I suggest you
start here http://www.w2du.com/r2ch21.pdf )

Then, after getting a grasp on that aspect continue to learn how
common mode current on the transmission line can effect the overall
performance of an antenna's radiation pattern - especially with VHF &
UHF antenna systems then you just might understand what people here
are trying to tell you.

However, I doubt that you would do that as it would appear you've made
up your mind and don't want to be confused with known facts.

Still others can't seem to understand that effects too small
to measure usually do not matter in the real world.

Others have measured this and reported it to you, but for some reason
you chosen to discount anyone who can show that it disagrees with your
"pet" theory. For me, I've place your theory along with the world is
flat, the moon is made of green cheese, and Iraq has weapons of mass
destruction .

But even for a skeptic, I would thing that the thought of coiling up a
few turns of coax costs very little, definitely won't hurt, and can
only help. Would be sufficient.

Danny, K6MHE

On 16 Jun 2005 10:29:02 -0700, "Al" wrote:

[snip
I said I was done with this subject, But ........ I can't take it
anymore.

I thought I was too but I can't take it anymore either.


Again, the Arrow Antenna OSJ Antenna does NOT need a choke balun.

No antenna *needs* a balun. Most will deliver more predictable
performance when they incorporate one however. Your's is not an
exception, in fact, it's a glaring example of where one *should be*
used. Furthermore, a non-conductive support should be used as well.
A balun on a piece of coax that is in parallel with a conductive mast
is worthless.

If these things aren't used, then you are selling only part of the
antenna system; the upper part...with the user is supplying the lower
part. I suppose you could argue that the part you sell, when mounted
directly on an infinite ground plane, doesn't need a balun, but I
don't see that limitation spelled out in your literature.


By that I mean it will make no noticeable difference in performance of
this particular antenna.
(I am not lumping all VHF & UHF antennas together. I am ONLY talking
about the OSJ).

When "performance" is measured by whether or not you can break the
squelch of the local repeater, then you are correct, it makes no
difference as long as the squelch is broken.


If you don't have a physical Arrow Antenna OSJ to test with and without
a balun, you don't know what you are talking about.
That goes for computer modeling also. You can't hook up a radio to
computer model.

Uh huh. So since you don't trust modeling, I guess you have a high
class antenna range that you use to design your antennas. That must be
how you substantiate such claims as, "This antenna has a good
combination of gain, front/back & bandwidth" and where you prove that
the pattern of your J-pole is unaffected by the supporting mast and/or
transmission line.

Whoops---wait a minute---right he

http://arrowantennas.com/inst/ijpole.html

I see 3D radiation plots made from a *COMPUTER MODEL*.

Plus I see SWR plots that (convienently) are 1.0:1 over the whole
2-meter band. That is amazing. My dummy load isn't that good.

Look. I'm sure you're a very nice guy and we would probably enjoy
having a few beers together. I've already commented on your ingenuity
in designing and constructing your production machinery and your
products seem to be very well made.

But your performance claims are over the top and while I know that
almost everybody in the ham antenna business is prone to rub a little
snake oil on their ads, it doesn't mean that I'm going to agree with
it or refrain from skewering those who do it.

On Fri, 17 Jun 2005 14:24:20 GMT, "Old Ed"
wrote:

Hi Dan,

My, you are a bit on the arrogant and foul-mouthed side, aren't you!


Foul-mouthed?

Damn it Allen, what I am saying is coax is coax and it will behave the
same regardless of what antenna it is connected to.

I feel you missing the whole point.

I am not attacking you antenna. I just disagree with your advise on
its installation.

Hello, Danny, K6MHE

I am not missing the point, I just have a different point I have to
deal with.
Check the last few posts, especially the one from Roy.
Can you imagine trying to explain that to some one that don't
understand why
the coax he took off an old computer network don't work with his 2
meter radio.
You have to realize, a lot of times I am dealing with hams that just
got their
license last week. One's that have trouble putting a connector on
coax.
A lot of the time they don't even have an SWR meter, or they are trying
to use one from their old CB.
The ham buying a $39. antenna that don't have to be tuned or adjusted,
is
at a different knowledge level than a ham that can set up an EME
station.
I am not an antenna guru, I know less about coax & baluns than I know.
That's why I have been reading this newsgroup for the last 10 years.
Why heck, I cant even spel.
The OSJ is a good entry level to antennas. It get's them on the air,
so
they can learn more.
I think I am doing a good service. At least I feel good reading the
e-mails I get almost every day from people using my antennas.

73 Al Lowe N0IMW

Al, I have a fair idea of antennas and baluns and how the electrons and
homotrons bounce around these magical devices..

Your antenna works well for it's intended purpose. I have two of your
J-poles, one on the house and one on the roof of the car. (Takes 6 large
rubber bands to remove the mechanical resonance at 70 MPH) They connect me
with the local repeaters, withstand hurricane winds, and are trouble free.
At $39.00 a bargain by any description.

Many here enjoy picking fly crap out of pepper, that is their enjoyment of
ham radio. No different than
contester, DX hounds etc. I enjoy the discussions of folks that will spend
days bloviating on the state of an electron named George at an SWR of
1.000000001:1.

These folks have caused me to think, as well as learn, just to keep up with
the pin dancing. That's a good thing

While some can appear officious and supercillious in the process, once you
get beyond that, pearls of wisdom do appear.

Continue making a great antenna, those that desire to achieve antenna
nirvana may spend time with the tweezers removing the afore mentioned
flyspecks.

"Iligitimus non carborundum"





"Al" wrote in message
oups.com...
Damn it Allen, what I am saying is coax is coax and it will behave the
same regardless of what antenna it is connected to.

I feel you missing the whole point.

I am not attacking you antenna. I just disagree with your advise on
its installation.

Hello, Danny, K6MHE

I am not missing the point, I just have a different point I have to
deal with.
Check the last few posts, especially the one from Roy.
Can you imagine trying to explain that to some one that don't
understand why
the coax he took off an old computer network don't work with his 2
meter radio.
You have to realize, a lot of times I am dealing with hams that just
got their
license last week. One's that have trouble putting a connector on
coax.
A lot of the time they don't even have an SWR meter, or they are trying
to use one from their old CB.
The ham buying a $39. antenna that don't have to be tuned or adjusted,
is
at a different knowledge level than a ham that can set up an EME
station.
I am not an antenna guru, I know less about coax & baluns than I know.
That's why I have been reading this newsgroup for the last 10 years.
Why heck, I cant even spel.
The OSJ is a good entry level to antennas. It get's them on the air,
so
they can learn more.
I think I am doing a good service. At least I feel good reading the
e-mails I get almost every day from people using my antennas.

73 Al Lowe N0IMW

On 17 Jun 2005 08:28:45 -0700, "Al" wrote:

The OSJ is a good entry level to antennas. It get's them on the air,
so they can learn more.

I agree.

very 73,
Danny, K6MHE

On Fri, 17 Jun 2005 12:36:14 -0400, "Fred W4JLE"
wrote:

Many here enjoy picking fly crap out of pepper, that is their enjoyment of
ham radio.

Hi Fred,

I've seen you use this platitude more than once. As much truth as it
may offer, it necessarily presumes there is someone energetically
putting fly crap into the pepper.

I won't tarry to imagine how that is done, however.

73's
Richard Clark, KB7QHC