FW: Open Wire Feedline

[Dave Van Wallaghen]

I received this response from Russ last night and he requested that I post
it to the list. It stemmed from the "KAT100 with Ladder Line" thread.

73,
Dave W8FGU

________________________________________
From: Russell Sherry [mailto:[hidden email]]
Sent: Friday, November 16, 2007 8:17 PM
To: [hidden email]
Subject: Open Wire Feedline

Hi Dave,
I would like to respond to your comments about building open wire feedline.
 
Several years ago I changed from "450" ohm window line to 600 ohm open wire.
The need to change was based on two things. The first was that when there
was moisture in the air the antenna needed to be retuned depending on the
amount of fog and the rain rate etc. The second was that the loop I was
using when tuned to 75 meters presented a very low impedance (<10 ohms) to
the tuner. The loop is 28' by 56'. One could actually feel the window line
(14 ga) heat up after a transmission at 1500 watts on 75 meters. The tuner
is a modified Johnson KW Matchbox. The modification allows variable link
coupling that increases the range of loads the tuner will accommodate.
 
Since I had measured the feedline length and had the window line
characteristics it was possible to determine the impedance of the loop at
the loop end of the transmission line by measuring the impedance at the
shack end of the feedline and reflecting this back to the loop. This was
done using a transmission line simulation program. The program used is named
"Transmission Line Details".
 
After doing this and determining the loss with the window line I looked at
doing this with 600 ohm home made ladder line. Losses were greatly reduced
on 75 epically when the window line was wet when compared to the 600 ohm
line.
 
I then constructed some 600 ohm line. What I decided to try for spacers was
something called "Pex" tubing. Pex tubing is used for containing a heated
fluid in radiant heating systems used in concrete floors in homes. It is not
rated for UV resistance but is holding up VERY well in CA direct sun after
more than two years exposure. I selected the approximate 1/4 inch inside
diameter tubing for use as insulators. It is light in weight and stiff and
contains no "carbon black". Carbon black is used as a UV inhibitor in
colored plastics such as black drip irrigation tubing. I purchased the
tubing at Lowes and it is inexpensive. It is in the plumbing department.
Depending on the store this tubing is in either 5 foot lengths or about 20
foot lengths. I choose the short length because the tubing was more straight
in the needed six inch lengths used for the spacers. The tubing is easily
cut with a box knife.
 
To make the 600 ohm line I chose to use uninsulated 12 ga soft drawn copper
wire to carry the RF. I did not want to thread the wire through holes in the
insulators so I used a different attachment process. I determined that I
needed slightly less than six inches for each insulator to have a feedline
spaced at six inches. I cross drilled the ends of the tubing (across the
tubing diameter) with a drill size that would allow a small tie wrap to pass
through the drilled hole. This diameter is about 1/8 inch. I made a jig for
the drill press to locate the hole at the proper distance from the end of
the insulator. This was about 5/32 of an inch from the end of the
approximately six inch length of Pex tubing. After drilling a hole at one
end of the insulator I inserted a snug fitting nail into that hole and used
this as an indicator to tell when the insulator was properly oriented in the
drill press to have the holes at either end of the insulator parallel to
each other. I drilled about 50 insulators in about 12 minutes and drilled
the second hole in all the 50 insulators paying attention to the position of
the nail that was inserted into the first drilled hole with the nail acting
as an indicator to allow the two holes to be parallel to each other. This
was also a fast process taking about 15 minutes to complete.
 
The attachment of the 12 ga wire is done by inserting a tie wrap through a
hole that was drilled through the diameter of the insulator and wrapping the
12 ga wire twice with the tie wrap and then closing the tie wrap. Position
the 12 ga wire at the extreme end of the insulator and tighten the tie wrap.
This will very snuggly hold the wire to the insulator end. This type of
attachment will allow an extra insulator to be added at any location along
the feedline with out needing the rethread the new insulator onto the
feedline system. This is important as extra insulators may be needed to make
turns etc. in the feedline.
 
I tried to use three foot insulator spacing but needed to much tension on
the feedline to maintain the proper line spacing. I reduced the insulator
spacing to about 18 inches and this has worked very well.
 
There is now no heating of the feedline at 1500 watts as I adjusted the
feedline length to raise the impedance to the tuner to about 275 ohms on 75
meters. Operation on higher bands was never a "problem" and has no issues at
1500 watts on other bands. I do not operate 60 or 30 meters so I do not know
how the system operates there. Going from dry to heavy rain will raise the
indicated VSWR from unity to less than 1.2:1. The losses associated with the
600 ohm line are very small on 75 meters in comparison to the 450 ohm window
line.
 
Please place this e-mail onto the Elecraft bulletin board as it might be
helpful to others also.
 
I may be reached by e-mail if you have any additional questions. My call is
NM6DX.
 
Russ
 
________________________________________
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