[HCARC] Antenna Coupler vs Antenna Tuner (Gary J's weekly test tosee if the Reflector is still working)

[Kerry Sandstrom]

Gary,

I think I had some info on open wire line in the Static a couple months ago. 
In the days of twin lead feed line for TV antennas, you were supposed to 
twist it a couple twists per foot.  I don.t know how many people did.  The 
purpose of the twists are to improve the 'balance' of the line.  The same 
side is not always cpose to the same object.  Open wire line is a little 
more challenging.  It doesn't have the plastic insulator between the wires. 
I think that twisting open wire line would lead to more trouble than it is 
worth.

With coax, the fields are completely internal to the cable so you don't have 
to keep the cable away from ground or conducting objects.  with open wire 
line on the other hand the fields are around each conductor.  You should 
keep the open wire line 2 or 3 times the spacing between the conductors away 
from any conductors or lossy material.  With 6" wire spacing you need 36" or 
so in diameter to run the line.  If you recall, twin lead used about 1/2 
spacing between the conductors and was supposed to be mounted on standoffs.

The open wire shouldn't have any sharp bends in it and should be as straight 
as possible.  Bends cause the line to radiate and thus increases the loss as 
radiation is a loss.  No you can't run it through conduit and you can't lay 
it on the ground or a roof or anything else.  It has very low loss if 
properly done with heavy wire and good insulators.  There isn't any real 
limit to how long a run you can have.  The OTH-B radar I once worked on used 
open wire line that was several hundred feet long to get to the switchyard 
and then to the stacked rhombics.  It worked well, but it was a professional 
installation.

The most practical way to bring open wire line into the shack is with large 
feedthrough insulators spaced the distance between the wires apart and 
mounted on a large flat panel of good dielectric material.  I think a lot of 
people just use a 4:1 balun to bring the impedance down to 75 Ohm umbalanced 
and just bring the 75 Ohm coax into the shack.  I've never actually used 
open wire line myself.  When i've had a balanced antenna on two meters, I 
used a coaxial balun at the antenna feed point and brought coax into the 
shack.  Open wire line tends to be troublesome if ice gets on it, although 
thats not a concern here, it was when I lived in Mass.

Have fun,

Kerry