On 03/20/2014 07:23 AM, Ralph Mowery wrote:
"Jon Danniken" wrote in message
...
Hi all, I'm cooking up a discone antenna for receiving, and I have a
question about feedline grounding. From everything I have seen, the
discone has an impedance of about 50 ohms, and everyone seems to just
connect the feedline right to the antenna; center conductor to the disc,
and shield to the cone.

My question is about what happens when I ground the shield at the
arrestor block just before the cable comes into the house; without a
balun, won't I just be turning the cone into a ground plane? I know
that there are ground plane antennas, but even though I am new to all of
this I thought that a discone was not a ground plane.

What am I missing here?

Thanks for any suggestions,

Jon

The discone is not a ground plane antenna.

Often the shield of the coax is connected to the cone part. That part is
also often connected to the mounting point which may go to the earth ground.

The cone part isolates the shield of the coax from being part of the antenna
(in simple terms) so what hapens on the shield does not mater to the
antenna.

Maybe you are confusing what an actual ground plane antenna is ? Being
connected to the earth ground has nothing to do with a ground plane antenna.

Thanks Ralph, I guess that is exactly what is what I do not understand
yet. I was just assuming that connecting the cone/shield to earth
ground would change the characteristics of the cone into (what I had
assumed would be) a ground plane.

Extending this concept outside of discones, and applying it to a dipole,
could you also directly feed a dipole, ground the shield, and still have
it behave as a 1/2 wave dipole?

Additionally, when does the need for a balun to transition between a
balanced line and coax arise?

Jon