[TowerTalk] Ground plane feed question

[n0tt1 at juno.com]

Hi Chuck,

Several variables for sure.  I've studied the info on W8JI's
website and also the white papers from N6LF re:  "A closer
look at vertical antennas with elevated ground systems"  Parts
1 and 2 and follow-up notes.  VERY interesting read on all.

Charlie, N0TT

On Thu, 21 May 2015 07:47:51 -0500 Chuck Dietz <w5prchuck at gmail.com>
writes:
There are too many variables with the ground plane.  The length of the
radials needs to be longer than the vertical element and the angle they
slope at all change the feedpoint impedance.  


Chuck W5PR 


On Wed, May 20, 2015 at 9:12 PM, <n0tt1 at juno.com> wrote:

OK, thanks Chuck!  will see how this all plays
out, hopefully soon!
 
On Wed, 20 May 2015 20:57:57 -0500 Chuck Dietz <w5prchuck at gmail.com>
writes:
I would try it with just the coax attached to the antenna, if you have RF
on the shield and in the shack, roll up about 6 - 8 turns of coax the
diameter of an oatmeal box at the feed point.  By the way, your tower is
tall enough that It might be better to have a sloping dipole with a balun
and one end attached as high as possible on the tower.  Move the bottom
end out away from the tower so it fits.


Chuck W5PR


On Tue, May 19, 2015 at 7:18 PM, <n0tt1 at juno.com> wrote:

Hi Gents,

Building a 7mhz ground plane/vertical dipole here....using
a 69ft self-supporting, grounded,  "BX" tower.

I'll have the base of the GP up a little over 1/4-wave, with
9 or 12 radials, sloping downward at ~35 degrees.

Right now, I plan on putting a good bifilar-wound current
balun right at the base.  The antenna side will have about
1-inch extra wire so I can clamp on a current meter right
at the balun.

The radials will be insulated from the tower in groups
of 3 or 4, on each of the 3 tower legs.  The radials will
feed from the center of the tower (inside) and will be
attached to the "ground" side of the balun.  The
other lead of the balun will connect to the aluminum "shelf"
that supports the balun and coax.  The shelf mechanically and
RF-wise connects to the 3 tower legs.  Those connections
will place the whole antenna at DC gound.

My question is what to do with the shack side of the coax
feed that connects to the balun....fasten to a tower leg just
a couple of feet down or what?  I.e, thinking that the
tower might be a bit "hot" with RF there...not sure about that.

Or was my placement of the balun not the best choice?  If
chosing a lower location, like level with the lower tips of
the radials, then the question is how to run the balanced
feed out of balun up to the base of the antenna.  And that
should be 50-ohm line.  Twisted pair (Z is too high) or coax?

Please respond direct with a copy to the reflector.
Steve, K7LXC the TT ADMIN has to figure out how
to get me subscribed again...some kinda problem there that
he's working on.    n0tt1 at juno.com

Thanks guys!

73,
Charlie, N0TT
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