Fw: Re: [Antennas] 450 Ohm Ladder Line

2002年12月16日 12:04:43 -0600

Correct, but what I wanted to know ....is it better to have a mismatch
at the load and loose some power due to the high SWR on the ladder 
line (should be low because it's ladder line), or is it less of a loss to
add a toroid matching network at the load. 
In my previous note on dB loss I'm amazed at the difference a wet line
makes, regardless of webs removed. For the #3 modified ladder line
DRY, its about 0.7dB/100ft better than RG-8 foam coax. For the #3
modified ladder line WET, its about 2.3dB/100ft worse than RG-8 
foam coax (assuming water on the coax outer jacket has no affect).
Now, both N7WS and I used a "wetting agent" (I used a couple drops
dish detergent in the water) to allow complete coverage on the surface 
of the polyethylene. The losses drop substantially if the water coverage
is not complete or it pulls into droplets. Using a wetting agent is 
probably more representative of weathered window ladder line which
has less of a tendency to "bead". That also points out the benefits of 
waxing the line .....but I don't know what the negative electrical
affects 
of the wax would be.
Just as a sanity check, N7WS measured a drop of 5.5dB/100ft WET
at 50Mhz for the 551 line and I measured a drop of 7.6dB/100ft WET
at 50Mhz for the 551 line. That's a reasonable correlation since the 
amount of water on the surface greatly varies the results. I ought to
go run the test again by sloping water on the line WITHOUT the 
"wetting agent". When you wipe the line, still leaving some wet small
wet areas, it immediately returns to the DRY readings (or as best I can 
see on the scope ...within -0.2dB.
73 Kees K5BCQ
--------- Forwarded message ----------
From: Pat W <[email protected]>
To: [email protected]
Date: 2002年12月16日 08:10:54 -0800 (PST)
Subject: Re: [Antennas] 450 Ohm Ladder Line
Message-ID: <[email protected]>
A point about using transformers(4:1 or 9:1, etc) at
the transmitter end of the feedline is that the losses
on the feedline are not changed. The reason is that the
SWR on the line is dependant on the impedance of the
load(the antenna). It is the impedance at the load
that determines the reflection coefficient and the SWR.
The characteristic impedance of the line determines
the transformation of the load impedance. 
To change the losses on the line, a transformer must
be placed at the load end, not the input end.
Pat W0OPW
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