On May 5, 7:26*pm, Jim Lux wrote:
noname wrote:
Jim Lux wrote:
Or, easier.. put a choke around the feedline where it comes
into the shack, so that there's no appreciable RF current
through the chassis and green-wire ground.

Okay, I'm adding a balun. Now the question is what and where. The
assembly instructions for the antenna say...

* The antenna is "..designed to be used with
* 50 ohm coax cable. No balun is required for
* proper operation.

* If RF on the coax is a problem, simply wind
* a "choke balun" with the coax that's used for
* your feedline. Wind approx 8 turns, at about
* an 8 inch diameter and tape (with outdoor rated
* black electrical tape), like a donut, and place
* this "donut" near the feedpoint of the antenna.
* You should leave about 2 foot of your coax
* sticking out of this "donut" for connection to
* the antenna. Then attach the end of your
* coax to the antenna connector on the center
* insulator, with the coax connector that is
* already on your feed line."

In other words, a scramble-winding choke near the antenna's feedpoint.
The alternative is an MFJ-915 in-line balun with 50 ferrite core beads
on coax (cheaper than I can buy the ferrites), which MFJ says should
be installed closer to the radio (3 feet) as opposed to the antenna
feedpoint at the other end of the coax as described above.

Which of the two options is most sufficient for the task and which is
the correct placement for this - or are they both right depending on
the type?

Read Jim Brown's RFI-Ham.pdf referenced in an earlier post.

"choke baluns" made by winding coax are pretty much single band devices.

A better approach is a single 2.4" toroid of suitable mix (#31 is good),
and put a dozen turns of your RG-8x on it. * Put one at the feedpoint,
put one where it comes in the shack. *Buy some extra cores because they
make good RF filters for things like power cords (and the hole is big
enough that a standard IEC power connector will fit through them)

The MFJ915 is fine, although I wasn't able to find any actual
performance specifications or details of what mix they're using. (in a
few minutes of casual googling).. MFJs is probably an incarnation of the
"W2DU balun", so the data Jim measured is probably reasonable. (about
1500ohms peaking at 10MHz)

I think you'll get better results (cheaper) with the single big
toroid..5 turns gives you more impedance than the W2DU, and it goes up
from there ( based on the measurements in K9YCs paper.. page 12). The
toroids run about $5. *Most of the gain is from using a better mix (31)

What if you use a coax with two shields, one shield for chassis ground
which is the coax connection and the outer shield for earth/ground?
Yes, there could be a ground loop but the nearest ground to a strike/
antenna is probably the best protection