On Fri, 28 Sep 2007 10:01:13 -0400, "Bill Ogden"
wrote:


" Assuming a good balun, feeder in symmetry plane of beam and the 50"
under ground, I do not expect much off it.

Also when there would be noise climbing up the feeder (to the beam),
the noise could also use the (metal) mast.

Good point and that is why I am a little confused about using chokes to
lower noise --- in any situation where the coax goes up a tower.

A number of very well-written and interesting papers
describe the effective rejection of chokes (over 5000 ohms when looping the
coax
through several #31 toroids), but they do not discuss the effects on the
total antenna system. As you said, it would seem that any pickup by the coax
external shield would be mostly duplicated by the tower itself and this
would bypass the choke if the coax shield is "grounded" to the tower at the
feed point. (Yes, I know "ground" has many meanings.)

I can see where the choke would help with a dipole, where the coax is not
connected to the tower at all.

Hi Bill,

The confusion may arrise from the tower's two uses. For the sloper,
it is active; for the beam it shouldn't be.

For the sloper, choke the line at the bottom of the tower. For the
beam, choke the line at the top and the bottom of the tower.

However, as far as this conventional advice goes, the lower choking
would seem to be redundant to the 50 foot run under ground. That
should choke the line just as well. If the beam comes with a choke
(I'm not sure how much they invest in that technically instead of just
for advertising's sake) then there's no point in more there either.

Take a clamp-on ferrite, build a loop of a dozen turns of fine wire
through it; terminate the wire with an LED. Attach this inside the
shack to any feed line and see if the LED lights up during a QSO. If
it does, you may want to replace the LED with a RF Current meter to
see just how much current there is, but my guess is the LED will be
dim at best. [LED current of 10-20 mA @ 2V will be quite bright.]

73's
Richard Clark, KB7QHC