"Dale Parfitt" wrote in message
...

"John Smith" wrote in message
...

"'Doc" wrote in message
...


Stephen Cowell wrote:

Plumber's Delight J-pole doesn't require
an insulator at all... I'd imagine that it could
take all the power you could pour into it,
probably take way over legal limit to melt
a half-inch copper pipe J-pole.
__
Steve
KI5YG
.

None at all? Are you real sure about that?
Think about it...
'Doc


Yeah, no insulators at all... using plumber's delight.

The bottom of the J is strapped together. The feed point is then moved
up
from the bottom along the 1/4 wave matching section until a match is
obtained.

This approach can be pretty nice. I used three sections of steel mast
for
a
30 feet tall pipe, grounded at the bottom for lightning protection. I
then
added a 1/4 wave piece of conduit 3/4 wave down from the top strapped
with
sheet steel. Made a 1/2 wave balun from coax and found the lowest SWR
feed
point along the matching section. The entire arrangement was at earth
ground, so no floating elements. Some designs found in books and on the
Web
feed them directly without the benefit of a balun. I've done it both
ways
and have had great success.

What keeps the mast below the J pole from radiating?
Dale W4OP


They tell me it's the 1/4-wave matching section which stops the rf from
moving on down the mast. A shorted 1/4 wave piece of transmission line looks
like an open circuit or essentially a choke to the antenna. In fact,
microstrip line construction uses 1/4 wave sections for RF chokes.

73,
John - KD5YI