Mike,
I remodeled the antenna again this time using Real/MININEC ground and 15
ohms of ground resistance per K9YC's suggestion.
For a 92 ft horizontal dipole up 40 ft center fed with 40 ft of 450 ohm
ladderline: (with both "sides" of the ladderline tied together at the base
of the antenna) the feedpoint impedance at 1.825 MHz calculates to be 22.8 -
j171.6 - the antenna is "short" for 160M.
Then connect the feedpoint to 50 ft of TMS LMR400. At the station end of the
coax the impedance is about 3.08 -j34.61
You'll need a "serious" (5000 ohms or more) common mode choke where the coax
connects to the vertical element.
With a high-pass L network at the station end of the coax:
---- 695pF ------------
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50 2.45 uH 3.08 -j34.61
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With 100W input the peak voltage across the capacitor will only be 251V.
0.3 dB (7W) loss in matching network
0.74 dB loss in coax
Total loss 1.04 dB: 100W input, 78.7W at antenna feedpoint
Again, you'll need to build the antenna and measure the actual complex
impedance at the station end of whatever length of coax you use to get the
matching network right.
This still leaves the issue of reconfiguring the antenna for 160M -
connecting both "sides" of the ladderline together at the base of the
antenna and connecting to the radials. One way to do this (as opposed to
manual reconfiguration) is with relays in a weatherproof enclosure at the
base of the antenna.
This all is just one way it can be done. See you on the low end of 160M.
73,
Marsh, KA5M
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