I'm currently using 600 ohm ladder line (W7FG) on a 135' foot doublet. I
am bringing it into the shack by soldering each wire to the center
conductor of two separate 7 foot lengths of RG-213 coax. The shields are
soldered together at both ends and connected to the ground on the
tuner. Each center conductor is connected to the feed through on the back
of the Palstar BT1500A balanced tuner. Initially I ran the ladder line
directly to the tuner but it runs over the metal frame of my window. I
experienced tuning problems on several bands where I was unable to achieve
an acceptable SWR (less than 3:1). By using the two pieces of coax I can
tune all bands now. The coax is routed to the tuner maintaining roughly
the space spacing as the 600 ohm ladder line. Originally I tie wrapped the
coax pieces to each other side by side with no spacing. I also
experimented with bonding the shields together at both ends and at only the
tuner end. The system performs best with the coax separated and the braid
soldered at both sides. (Lowest SWR on the most bands.) The purpose of
the coax is to allow the balanced line to come close to the metal window
frame and other wires in the shack without causing too much disruption to
the balanced current flow. I did not want to drill the required 6" through
the concrete block construction of my home and the decorative wood covering
the concrete. I am pleased with the results.
I also tried using the standard 450 ohm "window line". This worked well
when it was dry and handled the transition past the metal window frame much
better than the 600 ohm feeders. The problem was that when it rained it
changed the characteristics of the feed line and required different
settings on the tuner. If I adjusted the SWR to the lowest point when the
window line was dry, as soon as it got wet the SWR shot up through the
roof. This is because the water stayed on the window line and now the
dielectric between the conductors was no longer air but a mixture of air
and water. I have not had this problem with the W7FG ladder line.
This system replaces my 80, 40 and 20 meter fan dipole. It allows me to
tune all bands from 80 to 10 meters. I don't have to worry about cut wires
to length. Performance is equal to the fan dipole on 80 and 40. For some
reason it appears worse than the fan dipole on 20 meters. It is also much
more expensive because of the need for a tuner. I tried using a good
quality tuner with a 4:1 balun (Drake MN-2700 with B1000). The balanced
tuner (Palstar BT1500A) is superior on receive by several S units on all
bands. It did not get signal reports to compare the transmit side.
Hope my non scientific but real world experience is of value to the list.
Neil
K4NJK
|