Yes, I agree the shields should be connected together at both ends.
I believe the loss is the same as a single run of coax. Each coax has
only half the loss since it carries only half the power, but there are
two of them so the total loss is the same.
That assumes the SWR is the same in both cases. If 100 ohms is a better
match than 50 ohms, then the loss will be lower in the parallel
configuration.
Alan N1AL
On 09/18/2017 05:31 PM, Don Wilhelm wrote:
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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