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[Towertalk] Wire antennas

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Subject: [Towertalk] Wire antennas
From: (Chuck Counselman)
Date: Sun, 29 Sep 2002 16:44:08 -0400
At 6:25 PM -0400 9/28/02, David wrote:
>I'd like to add a wire antenna primarily for 75 and 40 for DX.   I'm 
>looking at a Carolina Windom 80 or a G5RV, but I'd like your antenna 

A G5RV is a good antenna, if you don't mind using a tuner.  A G5RV 
works well in an inverted-V configuration, supported by a tower.

A Carolina Windom could be supported by a tower at one end, but not 
near its middle, because much of its radiation is from its vertical 
feedline, which is deliberately unbalanced.

>If I go with a G5RV does the vertical ladder line have to be kept
>away from the metal tower (like the Windom), and if so, how far?

It's sufficient to space the feedline away from the tower by six to 
ten times the spacing between the wires in the feedline.  You should 
also twist the feedline by at least ten complete turns per wavelength 
of the highest frequency to be used.

>How do you have your G5RV or Windom configured on the tower?

I have no tower and my G5RV is supported at its ends by pine trees, 
but my friend W1NU has his G5RV supported by his 55-ft. tower, and it 
does very well for him -- so well that he's at the _top_ of the DXCC 
Honor Roll.

>Do you have problems with RFI when using a tunable multiband wire 
>antenna like these?

Not in the slightest, and I believe that it's because I feed my 
balanced line through a balun, and also have common-mode current 
chokes on my coax.

>On another note, anyone using a Radio Works T-4 or T-4G "Line Isolator" on
>your feedlines?

Yes.  I have one on my coax just before my remote antenna tuner and 
balun, and another on my coax just after the low-pass filter at the 
output of my amp.  I run legal-limit power on every band 80 through 
10 meters.

The best place to put a common-mode current choke such as a Radio 
Works "Line Isolator" is immediately adjacent to a big metal object 
(such as your rig/PA or a remote antenna tuner, so that it forms an 
LC low-pass network with that object's capacitance to ground.  You 
can easily prove this (as I have) with a NEC simulation.

73 de Chuck, W1HIS

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