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Re: [TenTec] open wire feed/loops

To: Discussion of Ten-Tec Equipment <tentec@contesting.com>
Subject: Re: [TenTec] open wire feed/loops
From: Robert & Linda McGraw K4TAX <RMcGraw@Blomand.Net>
Reply-to: Discussion of Ten-Tec Equipment <tentec@contesting.com>
Date: Thu, 02 Feb 2006 22:03:58 -0600
List-post: <mailto:tentec@contesting.com>
OK so the W7FG is great stuff.  Looks like it is robust.   It is made of 
#16 stranded copper with some type of spacer ever 12" or so.  The center to 
center spacing is about 4".   FYI - A true open wire line will be stretched 
taunt between two supports at the ends only.  No spacers.  True air 
dielectric.  It will have a VF of 0.975 or higher.

As to the vinyl covered stuff, aka window line, it performs just like any 
transmission line with a given % of solid dielectric.  Yes, the build-up of 
snow, ice and moisture will change the dielectric constant and the required 
adjustment of the tuner is necessary.  But I though that is how the knobs on 
the tuner are to be used.  Misinformed again!

For the convenience of usage the window line works well.  As to loss 
figures, true open wire line will have about 0.017 dB per 100 ft @ 30 MHz 
matched while the window type will have a loss of about 0.278 under the same 
conditions.  For a comparison of power at 100 watts and the line matched, 
the true open wire line will deliver 97.6 watts to the load while the window 
line will deliver only 93.8 watts.  That is a loss of 0.2 dB between the two 
lines.  I guess the inconvenience of using, routing and tuning the true open 
wire line vs. the window line is up to the whims of the user.

I still stand pat on the recommendation, if one chooses to use a balanced 
feed line, keep the coax out of the system.  Add 7 ft of coax, matched {to 
which it is not in a balanced feed system} an you add another 0.155 dB loss 
to the system.  At 30 MHz and a typical SWR of 20:1 on 7 ft of RG-214 in a 
balanced feed system and the additional attenuation is up a whopping 1.3 dB!

Your mileage may vary.

Bob, K4TAX

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