RadioBanter - View Single Post - Does a j pole need grounding and/or a balun?

January 8th 05, 03:01 PM

verbositized:

So, if coax is kept inside the tubing, there is no stray rf and
therefore a choke or balun is not needed?

Some stray RF somewhere, but not enough to worry about regarding the
antenna operation.
However, it is still a good idea to coil a few turns of the coax
before entering the shack.

Are you saying the shield must be grounded to a surge supressor to
prevent lightening stikes?
Not at all, the SHEILD should never go through a supressor.
For proper operation of ANY antenna, the sheild should be grounded.
For lightning protection the sheild should be grounded to an 8 foot
stake OUTSIDE and the center conductor run through a gas bottle also
grounded to the 8 foot stake outside. Actually, every piece of
equipment in the shack should be properly grounded.

My ideas was to run the wire inside and even make the coaxle
connections inside the tubing with wire terminals and screws, so as to
keep everything neat and weath proof.
Sounds good on paper! But I have no idea if it would work, never
tried it. I do know this, with the shield grounded and the antenna
grounded, logically it should make no difference WHERE the shield is
grounded to, but from an RF standpoint, the shield connection to the
vertical must be horizontal with the connection to the driven element.
On Multi-Band J-Poles with single coax feed, we place a horizontal
shorting strap between the connect points of the unfed stub and
vertical element to get the antenna to work properly.
It's been too many years ago that I studied this stuff to remember
why.

What if you used a non insulated pvc type pipe to connect the stub and
verticle element?
Wrong Analogy! Although we use the word Stub, a J-Pole consists of a
1/2 wave vertical element, affixed to one leg of a 1/4 wave U-shaped
matching transformer.
By making the horizontal member from PVC and not contiguous with the
two legs of the U, you would not have a U shaped matching transformer.
FWIW: The U does not have to be vertical, it can be horizontal as is
the upper matching transformer on a Stacked-J and BOTH stubs become
affixed to the vertical elements.

I have seen a unique antenna design that looks like a J-Pole, but the
1/2 wave vertical is insulated from the U-Shaped matching section,
just as a ground plane is insulated from its 1/2 wave vertical.
The feed for this type of antenna was quite hairy to get just right!
If I recall, a brass rod extended up into the vertical about 8 inches
to it's connection point and the lower connection point was similar to
a J-Pole.

TTUL
Gary