Topband: Laddarline or #12 for Inv.-L

[k6se@juno.com]

On Fri, 4 Feb 2000 16:48:37 GMT David Gordon <dgordon@gb.nrao.edu>
writes:

"I am planning on installing an Inverted-L. I was planning on using #12
strained wire for it but got to thinking about bandwidth.

"Would it help or not/make any difference, if I was to use #12 for the
bottom 1/8 wl and then the top 1/8 wl out of laddar line, or put the
laddar line on the bottom 1/8 wl and the #12 on the top 1/8 wl OR make
the whole thing out of laddar line with both ends shorted?  Or just use
the #12 as planned???"
=====
Interesting question.  I modeled all of the scenarios in both EZNEC and
ELNEC, and they agreed quite closely.

With an inverted L 1/8 wave vertical and 1/8 wave horizontal and resonant
at 1.83 MHz:

1) With all #12 AWG wire, the 2:1 SWR bandwidth is 1.800 to 1.861 MHz if
perfectly matched at resonance (22 ohm feedpoint resistance with a
perfect radial system).

2) With 1 inch ladder line with both ends shorted (i.e., a "fat"
conductor) as the vertical section, the 2:1 SWR bandwidth is 1.791 to
1.872 MHz.

3) With 1 inch ladder line with both ends shorted as the horizontal
section, the 2:1 SWR bandwidth is 1.802 to 1.860 MHz.

4) With all ladder line as the inverted L, it is best to configure it in
a folded unipole fashion, i.e., the far end shorted together, and open at
the bottom of the vertical section with one conductor grounded and the
other conductor connected to the coax (75 ohms) center conductor (and the
coax shield grounded).  The feedpoint resistance in this case is 88 ohms,
which yields about a 1.17:1 SWR on the 75-ohm coax at resonance (with a
perfect ground).  The 2:1 SWR bandwidth is 1.796 to 1.865 MHz.

Scenario #2 has the greatest bandwidth.  Use the ladder line with both
ends shorted as the vertical section and #12 AWG as the horizontal
section.  The total length to be resonant in this case is several feet
longer than if the antenna is all #12 AWG.

73, de Earl, K6SE



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