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[TowerTalk] Matching Sections - Modified Yagi

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Subject: [TowerTalk] Matching Sections - Modified Yagi
From: "Scott MacKenzie" <>
Date: Sun, 11 May 2008 12:20:40 -0400
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I have modified an old CC 215WB 2M beam which was purchased used.
Previously, it used to have a triagonal reflector.  It was previously
modified using a more conventional single reflector.  The reflector length
is 1041 mm long, of 12 mm diameter aluminum tubing.  The driven element is
0.982 M long, and is 12 mm diameter aluminum tubing.  The matching appears
to be in a DELTA-type matching section, but with a coax balun of Belden 8238
(RG-11/U) 609.6 mm (24 inches) long.   The predicted impedance of the
antenna, based on modeling, is 13.122 + j0.122 ohms. My desired frequency of
use is 144.1 MHz.
I have measured SWR (not using an analyzer) at the end of approximately 50
feet of 9913 coax. The SWR is high (approximately 2:1 and higher at the
desired frequency.  I used a dummy load at the end of the coax and got a SWR
of 1:1.  The SWR appears to decrease as the frequency is lower than 144.0.
I can adjust the DELTA-match all I want, and it does not seem to change.
I suspect that I really should be using a half-wave transformer of 50 ohm
coax because the reflector is now a single reflector instead of the
triagonal reflector. Based on this assumption, I should be using a piece of
coax for the balun that is 1/2 wave long, i.e., L=492/f * VF.  If I use
RG-213, and the velocity factor is 0.66, then I calculate the length to be
2.253 feet or 27.04 inches 687 mm.  
Does this make sense, or am I missing something obvious?
Scott, aka KB0FHP

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