Ralph Mowery wrote:
"Jon Danniken" wrote:

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?



For the halfwave antenna I am going to assume that you mean one that is
horizontal and up in the air some distance,then you use coax cable to come
into the shack. You can connect the shield of the coax anywhere from right
at the feed point (which would not be practical) to a point near the
transceiver to the earth.

I was actually thinking of a vertical dipole, as it's what I have on my
roof right now (an old set of rabbit ears, so I can listen to airband
while I get the discone built and figure out a proper mast setup).

A balun is mainly used to connect a ballanced antenna to an unballanced line
like coax cable. The balun is a contraction of BALanced to UNbalance.

Aha, thanks, I've been wondering about that for awhile now.

They are not always needed, but may or may not help. A simple 1/2 wave
dipole is a balanced antenna as each side is the same. Theory says to use a
balun to keep the feed line from becomming a part of the antenna. I and
many others have up dipoles that do not have baluns and they work fine.

Good to know, thanks. I'm using a 300:75 converter up there right now
(twinlead from the rabbit ears to the coax), I think I'll take it off
and see if it makes any difference.

Baluns are often used on beam antennas so the radiation patern will not be
distorted.

Unless using an antenna tuner that has a built in balun or is designed for
the open wire feedlines a balun is used to feed the coax connector of the
transceiver. Most often it will be a 4:1 ratio to change the 300 to 600 ohm
feedline to closer to 50 ohms to match the transceiver.

So they'll mostly be located by the transceiver instead of up on the mast?

A ground plane is unbalanced as the elements are not equal and so a balun
would not do any good. Same as the discone you were asking about , no balun
is needed as this is an unbalanced antenna.

There is another thing that is often referred to as a choke balun, which is
not actually a balun. It can be several turns of coax coiled up or a piece
of coax with some of the ferrite beads over it.

Yeah, I've seen the chokes on some antennas made by forming a coil from
the coax.

Thanks Ralph,

Jon