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
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