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Re: [TenTec] Opinions sought re OMNI VI

To: tentec@contesting.com
Subject: Re: [TenTec] Opinions sought re OMNI VI
From: Stuart Rohre <rohre@arlut.utexas.edu>
Reply-to: Discussion of Ten-Tec Equipment <tentec@contesting.com>
Date: Fri, 14 Oct 2016 17:08:48 -0500
List-post: <tentec@contesting.com">mailto:tentec@contesting.com>
I think you are on the correct track to pick a rig like the Omni VI.

What is the construction material of the building where you were going to put the attic antenna?

If it is wood frame construction, I have had many hours of pleasure and some round the world DX when the sunspots are at play in just such an attic. The antenna I put into the attic of my single story house is a 3 band dipole array, with one common coax feeder thru a balun. The 20 meter dipole is along the ridge pole of the roof. Fanned out below it are the 15 meter and 10 meter dipoles. There was plenty of room to string the 20 meter in a straight dipole line. And enough height to angle down the two other dipoles. The balun is also the center point of the antenna, and anchors the six dipole legs at the feed point.

In my case, cutting the 3 dipoles to formula (468/f MHz) resulted in the 20 m attic resonance being below the bottom of the band at 13 Mhz instead of 14.250, for example. You can calculate what percentage you are off, or easier still, just fold back the wire ends onto themselves until you get the resonant point where you wish it.

I used 16 gauge enameled copper for the 20m, and no. 20 wire for the other two dipoles. Thus if you use differing wires, you may be correcting for inside the roof effects as well as wire diameters.

I have metal ducts for air conditioning from a common plenum about five feet from the center of dipoles legs on one side. That may be affecting my length as well. Two ducts run under the dipoles and parallel. It is easy to get closer matched, and then let the internal tuner do the rest. Since my shack is central in the house, it is a direct, short coax drop into a closet thru its ceiling, and then out a baseboard outlet for the coax to the operating desk. The coax is 50 ohm, RG 8x. (RG 58 would do for my 100 watt radios, but 8X is lower loss per foot. I probably have about 15 feet of feed line.

If your attic ridge is higher pitch roof, you should do even better than I have.

-Stuart Rohre
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