Open Letter
 

Hi, all concerned:

Is this the balun problem?

For openers, a 40-meter half-wave dipole with trees and 3 dogbone
insulators, center-fed directly across the center insulator with 33 feet of
RG-59/U coax, nothing special,

TREE--o///o--------------o///o--------------o///o--TREE
| |
coax
feeder
drooping
down
33
feet
here
to
the
shack

you know, like that ASCII sortagraphic. Like the one I put up in 1957 or
1958.

The thing has voltage loops at all ends, including the shack-end of the
coax-braid, correct?

73, Dave, N3HE

Hi, all concerned:
Is this the balun problem?

40-meter half-wave dipole with trees and 3 dogbone insulators, center-fed
directly across the center insulator with 33 feet of
RG-59/U coax, nothing special.

The thing has voltage loops at all ends, including the shack-end of the
coax-braid, correct?

Well, it would seem to have voltage loops at each end of the dipole. If the
antenna feedpoint impedance is 75 ohms non reactive and the surge impedance of
the coax is 75 ohms, then you should see 75 ohms at the shack. No voltage loop.
Do you need a balun? I would think that if everything were perfectly
balanced, and you had a perfect 75 ohm match at the shack, then no balun
needed. A perfect match at the shack indicates no current on the outer braid
of the coax because of imbalance, or rf pickup. Everything is radiated at the
antenna. That is one of my goals, along with the direction of radiation.
If changing the length of coax changes the impedance at the shack, that
indicates feedline radiation.


73 Gary N4AST

David J Windisch wrote:

Hi, all concerned:

Is this the balun problem?

For openers, a 40-meter half-wave dipole with trees and 3 dogbone
insulators, center-fed directly across the center insulator with 33 feet of
RG-59/U coax, nothing special,

TREE--o///o--------------o///o--------------o///o--TREE
| |
coax
feeder
drooping
down
33
feet
here
to
the
shack

you know, like that ASCII sortagraphic. Like the one I put up in 1957 or
1958.

The thing has voltage loops at all ends, including the shack-end of the
coax-braid, correct?

73, Dave, N3HE


You will have an imbalance with the antenna as described, not pictured.

The inner surface of the braid and the center conductor contains the rf
from your transmitter. The portion of the wire connected to the braid
does NOT get the same amount of power [current] as the portion connected
to the braid. So, one side of the antenna radiates more power than the
other. The coupled signal onto the external portion of the braid does
NOT cancel and you have RF on your coax.

Solution: Balun!

"David J Windisch" wrote in message
...
Hi, all concerned:

Is this the balun problem?

For openers, a 40-meter half-wave dipole with trees and 3 dogbone
insulators, center-fed directly across the center insulator with 33 feet
of
RG-59/U coax, nothing special,

TREE--o///o--------------o///o--------------o///o--TREE
| |
coax
feeder
drooping
down
33
feet
here
to
the
shack

you know, like that ASCII sortagraphic. Like the one I put up in 1957 or
1958.

The thing has voltage loops at all ends, including the shack-end of the
coax-braid, correct?

73, Dave, N3HE


On my 40M inverted Vee, I put 4-5 turns in teh coax and a couple of
clamp on "beads" several years ago and haven't had any problems. I had just
gotten got back on the air and remember seeing both the "coil-up-your-coax"
and "bead-type" baluns, but didn't remember the specifics, so I took a shot
at it with what I call the K9DCI choke-choke balun. hi hi

--
Steve N, K,9;d, c. i My email has no u's.