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[TowerTalk] ADJUSTING RESONANCE OF NVIS JAMACA ANTENNA

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Subject: [TowerTalk] ADJUSTING RESONANCE OF NVIS JAMACA ANTENNA
From: wa3gin@erols.com (David Jordan)
Date: Mon Jun 16 14:15:02 2003
Hi Folks,

Last year I installed a "Jamac" antenna in NVIS configuration for 40m. 
Just threw up two 140 dipoles, spaced 70ft apart and feed wtih 600ohm 
line. The elements are 15ft above earth (sandy loam to a depth of about 
200ft).

During this monsoon season, or sorry, Spring I thought I'd check the 
resonance of the full wave dipole elements.  By formula the wires are 
long but I've heard that the closer the wires get to earth the shorter 
they appear relative to freq.  I fully expected to have to trim a few 
feet of wire off the 140 dipoles.  To my surprise the dipoles were 
resonant about 7.5Mhz, at least that is where the MFJ analyser showed 
zero inductance.  In fact the MFJ reading was zero inductance from about 
7.5-8.0Mhz.  Net-Net I ended up adding 6ft to the ends of the dipole, so 
total lenght is now 152ft
The MFJ shows a dip and zero inductance at 7.200.

Does this all sound about right?

73,
dave
wa3gin


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