David Forsyth wrote:

In the interest of my recent tinkerings with radios (both antique and
homebrew), I have put up an 80 foot straight wire antenna above the
house.
It sits about 35 feet up from, and horizontal to, the ground. I am
using a
lead in from one end made of insulated stranded copper wire (basically
old
speaker cable, about 18 guage I think). Right now, the lead-in wire
comes
down from the end of the antenna closest to the sun porch, and in
through
the storm window via a well-insulated bushing.
SNIP

Back considerably before the beginning of time, around 1940/1941, my uncle
bought a fancy Sears Silvertone console with multiple bandspread shortwave
bands and the lot. It came with instructions for using a substantial
center-fed dipole antenna with twisted-pair two-wire feedline. Back in
those days twisted-pair feedlines weren't much different from rubber
insulated two-wire AC cords - ZIP cord hadn't appeared yet. The impedance
probably wasn't far off from 75 ohms.

The important point was that the balanced feed system and antenna reduced
pickup of noise by the feedline. The close-spaced two-wire feed also made
it less affected by routing.

Your 80 feet is a small fraction of a wavelength anywhere in the AM
broadcast band, so the antenna will be a high impedance no matter where you
feed it. 300-ohm twinlead might be a start in today's world. Is shielded
twinlead still available? The other choice for feeding a band dipole might
be plastic ZIP cord. The insulation is not great, but a 1500 kHz, who
cares? The spacing is close, giving less effect of close objects, but the
impedance would thus be low.

Just some thoughts, perhaps useless if you can't center-feed the antenna.

R J Carpenter wrote:
Back considerably before the beginning of time, around 1940/1941, my uncle
bought a fancy Sears Silvertone console with multiple bandspread shortwave
bands and the lot. It came with instructions for using a substantial
center-fed dipole antenna with twisted-pair two-wire feedline. Back in
those days twisted-pair feedlines weren't much different from rubber
insulated two-wire AC cords - ZIP cord hadn't appeared yet. The impedance
probably wasn't far off from 75 ohms.

It's closer to 100 ohms, but the KEY phrase here is twisted.
Zip cord and or twin lead isn't twisted (unless you twist it)
and it's the twist that imparts the self sheilding.

Jeff

--
"They that can give up essential liberty to obtain a little temporary
safety deserve neither liberty nor safety." Benjamin Franklin
"A life lived in fear is a life half lived."
Tara Morice as Fran, from the movie "Strictly Ballroom"

Jeffrey D Angus wrote:

R J Carpenter wrote:
Back considerably before the beginning of time, around 1940/1941, my uncle
bought a fancy Sears Silvertone console with multiple bandspread shortwave
bands and the lot. It came with instructions for using a substantial
center-fed dipole antenna with twisted-pair two-wire feedline. Back in
those days twisted-pair feedlines weren't much different from rubber
insulated two-wire AC cords - ZIP cord hadn't appeared yet. The impedance
probably wasn't far off from 75 ohms.

It's closer to 100 ohms, but the KEY phrase here is twisted.
Zip cord and or twin lead isn't twisted (unless you twist it)
and it's the twist that imparts the self sheilding.

Jeff

And if anyone here knows twisted... ;-)


--


Michael A. Terrell
Central Florida

The twist improves the ability to reject noise and signals picked up
by the balanced feed line but even untwisted, parallel lines do quite
well. It is the fact that any signals are picked up by both lines
equally and then cancel at the receiver input because they are in
phase.

"Bob Lewis (AA4PB)" wrote in message
...
The twist improves the ability to reject noise and signals picked up
by the balanced feed line but even untwisted, parallel lines do quite
well. It is the fact that any signals are picked up by both lines
equally and then cancel at the receiver input because they are in
phase.

And low impedance lines are very close spaced so that the twisting is less
of an advantage.

"Bob Lewis (AA4PB)" wrote in message
...
The twist improves the ability to reject noise and signals picked up
by the balanced feed line but even untwisted, parallel lines do quite
well. It is the fact that any signals are picked up by both lines
equally and then cancel at the receiver input because they are in
phase.

And low impedance lines are very close spaced so that the twisting is less
of an advantage.

Jeffrey D Angus wrote:

R J Carpenter wrote:
Back considerably before the beginning of time, around 1940/1941, my uncle
bought a fancy Sears Silvertone console with multiple bandspread shortwave
bands and the lot. It came with instructions for using a substantial
center-fed dipole antenna with twisted-pair two-wire feedline. Back in
those days twisted-pair feedlines weren't much different from rubber
insulated two-wire AC cords - ZIP cord hadn't appeared yet. The impedance
probably wasn't far off from 75 ohms.

It's closer to 100 ohms, but the KEY phrase here is twisted.
Zip cord and or twin lead isn't twisted (unless you twist it)
and it's the twist that imparts the self sheilding.

Jeff

And if anyone here knows twisted... ;-)


--


Michael A. Terrell
Central Florida

The twist improves the ability to reject noise and signals picked up
by the balanced feed line but even untwisted, parallel lines do quite
well. It is the fact that any signals are picked up by both lines
equally and then cancel at the receiver input because they are in
phase.

R J Carpenter wrote:
Back considerably before the beginning of time, around 1940/1941, my uncle
bought a fancy Sears Silvertone console with multiple bandspread shortwave
bands and the lot. It came with instructions for using a substantial
center-fed dipole antenna with twisted-pair two-wire feedline. Back in
those days twisted-pair feedlines weren't much different from rubber
insulated two-wire AC cords - ZIP cord hadn't appeared yet. The impedance
probably wasn't far off from 75 ohms.

It's closer to 100 ohms, but the KEY phrase here is twisted.
Zip cord and or twin lead isn't twisted (unless you twist it)
and it's the twist that imparts the self sheilding.

Jeff

--
"They that can give up essential liberty to obtain a little temporary
safety deserve neither liberty nor safety." Benjamin Franklin
"A life lived in fear is a life half lived."
Tara Morice as Fran, from the movie "Strictly Ballroom"