The drop from 200 (with LPF) to 100 watts reflected (w/o) at the bottom of
10m into my TH7 was as measured on the Alpha 87A LED meter. When fed into a
dummy load with a Bird 43 after the LPF, the reflected readings were all
down, more so on the Bird. Without the LPF, the 87A reads 1500w forward and
100w reflected at 28000 while my Bird 43 says 1300w out and 35W reflected.
The Drake LPF is rated for 1KW. And while it has margin (I've never had any
arcs at 1500W), the components have likely aged since it's 30 years old.
Running the "new" legal limit through it may have "helped" that process.
The roll off has likely changed.
".......harmonic interference is probably a lot less significant these days
than fundamental overload, which the LPF helps not at all."
"A low pass filter does not necessarily have a flat 50 ohm response. Especially
when you get to 10 meters. It is already starting to roll off some. This
produces some reactance which will show up as a higher or lower swr when
inserted into the line, depending on how much reactance is already on the
line."
Bottom line: It use to be "good" standard practice to always leave an LPF
in the line. I've taken the LPF out. My Alpha 87A likes the antenna systems
alot better. IE, it "senses" less reflected power. With most households on
cable, I don't anticipate any uprising in the neighborhood.
Thanks for the comments from:
Pete N4ZR
Steve G8GSQ
Colin K7FM
Vic K2VCO
Gary K4FMX
Pat GW0VMR
Bill WA4LAV
73, Steve WB6RSE
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