In article ,
Dan Richardson k6mheat wrote:

Allen,

My comments were directed to Ed not you, however, can you please
explain to me what is the magical ingredient in your OSJ that makes it
different than any other antenna known to man in that there will be no
common mode current on a coax transmission line when directly
connected to the antenna?

May I chime in? Here's my best guess as to the situation with this
antenna:

- Like any other VHF antenna without an effective feedline- decoupling
arrangement and isolation from the mast, it's possible for the
feedline and/or mast to carry some amount of RF current. Quite
simply, there's nothing stopping this from happening.

- The RF impedance of the feedline shield and/or the mast will appear
in parallel with the impedance of the longest (2-meter radiator)
and shortest (440 matching arm) elements on the OSJ.

- What this impedance will be, will be extremely installation-
specific, and so will the currents carried on the feedline braid
and/or mast.

- If the length of these elements to the nearest ground is an odd
multiple of a quarter-wavelength, the low-Z ground will transform
back to a high-Z at the feedpoint, and little unwanted current will
flow. If it's an even number of quarter-wavelengths, the impedance
on the unwanted element will be quite a bit lower - the lower limit
is probably the radiation resistance of a wire of that length. If
it's a nonintegral multiple of a quarter wavelength, the impedance
will be intermediate between these points and will be rather
reactive.

- If the mast is ungrounded at the bottom, the relationships in the
previous paragraph will be reversed - high-Z for even number of
quarter wavelengths, lower (set by radiation resistance) at an odd
number of quarter-wavelengths, reactive in between.

- Since the feedline and mast are likely to be longer than a metre,
any radiation from them is likely to be have strong high-elevation-
angle lobes. Power radiated in these lobes will be less "useful"
in many applications, and since it takes away from the towards-the-
horizon pattern of a theoretically-perfect halfwave radiator it
will reduce the antenna's useful pattern gain by some amount.

So far, I think this is all pretty standard per theory.

Here's where I go out on a limb of speculation:

- In a typical OSJ installation, the feedline and mast are relatively
long, compared to the near-half-wavelength size of the radiator.

- For this reason, if the feedline/mast length happens to be one with
a low and non-reactive RF impedance, its impedance will be a good
deal higher than that of the antenna itself due to the higher
radiation resistance (e.g. 100-200 ohms). Only a relatively small
fraction of the power at the feedpoint will flow into the braid
or mast. As a result, the amount of power "robbed" from the
primary radiator will be small, the high-angle lobes will be weak,
and the reduction in the strength of the towards-the-horizon
primary lobe will be minor.

- If the feedline and/or mast happens to be of a length which results
in a high impedance appearing at the feedpoint, then even less
power will flow on these unwanted elements and the pattern
disruption will be even less.

If the above model and speculation are correct, then two things can
probably be said:

[1] In many installations - perhaps most - the OSJ probably works just
fine without any sort of choking or decoupling arrangement (where I
define "just fine" as "Adding a theoretically-perfect decoupling
arrangement would not result in an improvement in pattern, ERP,
receive sensitivity, etc. which the antenna's owner would notice or
consider worth the trouble."

[2] In some installations, under specific conditions (e.g. short
1/2-wavelength-long feedline) a choke might result in at least
some useful (or at least measureable) improvement in towards-
the-horizon pattern.

I'll finish up by adding a personal observation. As Ed mentioned,
the Arrow OSJ (and/or equivalent antennas made by a local amateur) are
quite popular among members of our city's ARES group. I've measured a
couple of them using an MFJ analyzer, and in my measurements I have
*not* noticed the SWR / measured-impedance to change significantly
when I touch or move the antenna feedline (even when it's a relatively
short 6' piece of RJ58). This suggests to me that (in this case at
least) there's not a lot of RF coming back down the outside of the
feedline to the analyzer case, and that the near-50-ohm impedance
presented by the radiator and matching stubs is the dominant "sink"
for the RF current flow.

Do I think the OSJ is perfect? No - no practical antenna is. If I
were putting one up for a permanent installation, I'd probably
insulate it from the mast, and loop the feedline coax into a choke
balun and/or add a couple of ferrite beads, just because I'm picky and
because the effort to do so is so small.

I would not, however, count on noticing any practical difference in
performance from doing so, and I wouldn't bother doing this in any
sort of temporary or field installation (which is the purpose for
which I keep a break-apart OSJ in my van).

To that extent, I think that Al's statement that the OSJ doesn't need
a choke, is a reasonably fair one. It's not a universal TRVTH but
it's probably a fair, practical rule-of-thumb.

--
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!

I've measured a
couple of them using an MFJ analyzer, and in my measurements I have
*not* noticed the SWR / measured-impedance to change significantly
when I touch or move the antenna feedline (even when it's a relatively
short 6' piece of RJ58). =A0This suggests to me that (in this case at
least) there's not a lot of RF coming back down the outside of the
feedline to the analyzer case, and that the near-50-ohm impedance
Sniped

Funny how much the comments differ from some one who has a
physical OSJ Antenna and those who just try and model it or
or assume they know it won't work well.
73 Al Lowe N0IMW

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!