Wayne,
Actually, parallel coax runs make a good choice for shielded parallel
feedline. The paralleled LMR-400 lines should produce a feedline
characteristic impedance of 100 ohms. It is normal to connect the
shields together at both ends. At the antenna, the connected shields
are left floating, but at the rig end, are connected to chassis common
(your BL2 ground lug) - perhaps "floating" is what you meant by
"unterminated". Are the shields connected at the antenna end?
As I recall, the matched impedance loss would be twice the loss of a
single run of the coax, but at reasonable distances and the low loss of
LMR-400, it should not be a problem at HF.
The advantages of using parallel coax for a balanced feedline is the
same as using coax over open-wire or ladderline. It can be run on or in
the ground, run next to or thorough conducting metallic surfaces, or
coiled up with no ill effects. Open wire and ladderline takes more care
in its installation and must use gentle turns and be spaced away from
nearby conducting objects.
73,
Don W3FPR
On 9/18/2017 6:02 PM, Wayne Burdick wrote:
The experimental feedline, running under the house for half its length, was
more deliberate if not justified in the literature. At first I was using 300
ohm mystery twin-lead scavenged from an installation of yore. But given all the
RFI sources I decided to try two lengths of LMR-400, taped together every few
feet, forming a sortof-balanced-shielded feed. Purists I consulted ahead of
time continue to be shocked at how well this works. At the mast, the grounds
are left unterminated. In the shack, the two center conductors go to an
Elecraft BL2 balun. The shields are soldered together and connected to the
BL2’s ground lug.
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