Carl: Very good question about the feasibility of bringing balanced line into
the shack. I am aware of two methods to do this.
The first and easiest method is to use two pieces of equal length coaxial
cable, in parallel. Just tie the shields together at each
end. The two center conductors form a shielded balanced line. ( This can be
extended to the center of the dipole for a sheilded
balanced line.) However, in most cases this is not necessary). The twin
shielded balanced line will have a nominal impedance of
twice the impedance of the coax; i.e. two lengths of RG-11 (75 ohms) connected
in parallel will give a 150 ohm balanced line. Also,
this is can be connected directly to 600 or 450 ohm ladder line with no
problems. The two lengths of coax can be "snaked" through
wall sections, etc. just like a single length of coax. The second and more
"graphic" method is to locate the rig with the antenna
tuner in front of a convenient window. Remove the glass window pane an!
d replace it with clear acrylic or equivalent plastic. (This will require new
window putty and possible paint to install the
plastic. ) The plastic should be the same thickness as the glass so it will fit
right in the sash.) Then drill holes in the plasic
acrylic window pane and install grommits in the holes to fit the wire or coax
size. Then just run the open wire line (or twin coax)
through the grommits in the window!! I did this with open wire line through
both a double sash window with an exterior aluminum
storm window and it worked great!! (I did replace the storm window glass with
acrylic sheet). The only draw back(?) is that this
arrangement made my shack look like a shipboard radio room from the 1930's!
(This was amusingly intimidating to visiting CB
operators!) It also interfears with opening the window. Also, contrary to
what most people have the mistaken opinon, if installed
properly, open wire line does NOT radiate. It only will radiate if one has an
i!
mbalance in the antenna, i.e. one side of the antenna longer than
radiating many years ago when hams connected one side of it to the end of a
wire zep antenna, instead of the center, thereby creating
a gross imbalance.) Also, when I was using my open wire system brought into
the house, my tv set was about 15 feet from the rig. The
tv was connected to 450 ohm tv twin lead to an outside antenna. I had no
problems with tvi from my Drake TR4C running 300 watts.
Finally, it is worth stating that I believe it is a mistake to use a small,
cheap, ferrite type balun in an open wire system. This
has been discussed several times in QST. The fact that one is mistakenly
assured by a low vswr reading may be an indication that the
balun is acting like a load resistor and converting some of the transmitted
R.F. into heat. This gives a false sense of security
because the swr meter is reading low. Also, I believe it is a mistake to mount
any balum outside because of the effects of moisture
will cause the balun to also start acting like a dummy load. - !
Sherrill W. k4own.
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