I have used that multiband antenna configuration both at my home station
(140' doublet, 450 ohm ladder line, 1500 watts) and when travelling (70'
doublet, 300 ohm TV twinlead, 100 watts). My experience:
1. Balanced tuners (e.g. the Johnson Matchbox) work better than an
unbalanced tuner and a balun. MFJ recently introduced a balanced tuner; I
haven't examined its schematic, but it sure looks like a Johnson Matchbox on
the outside.
2. If you use an unbalanced tuner and an external balun, minimize the
unbalanced connection length between the tuner and balun. My travel tuner is
a small MFJ unit with a built-in balun, and it works very well
3. I almost always end up iteratively trimming the feedline length to enable
the tuner to find a good match on all bands; I don't know of any effective
formula or rule of thumb that would eliminate this process, so I just start
with a lot more feedline than is physically needed and lop off 3' at a time
until the tuner is happy everywhere.
For RFI with a PC peripheral, I'd start by shortening your tuner-to-balun
connection. If that doesn't help, then I'd wrap the peripheral-to-PC cable
around a split rectangular ferrite. These were available from Radio Shack
and are generally available at ham fests; I'm not sure who sells them now,
but Google should find them.
73,
Dave, AA6YQ
-----Original Message-----
From: digitalradio@yahoogroups.com [mailto:[EMAIL PROTECTED]
Behalf Of w4lde
Sent: Wednesday, July 02, 2008 9:29 AM
To: [EMAIL PROTECTED]; TowerTalk
Subject: [digitalradio] Open wire feed line length
After reading several articles both on and off the WEB regarding a
center fed antenna, only one article mentioned a recommended length for
the feed line.
My shack is located on the second floor of a new home, (24') #8 ground
wire to 8' ground stake with a further connection to the common ground
point for the power and the rebar in the pored walls and flooring of the
basement, telephone and cable. I plan on additional ground rods around
the property and at a new tower. In a new home so progress is on-going.
The 1:1 current balum used to convert the 450 ohm balance line is
external to the tuner, use about two feet of coax from the balum to the
tuner.
Antenna is cut for 80 meters and using about 60-70 ft of line to center
of the antenna. Where the antenna lines leave the shack is about four 4
ft from the location inside of the rig and floor mounted PC.
I use the antenna on all bands until such time I complete a new tower
installation. I want to minimize stray RF around the shack since I rely
on a PC and digital sound card as my primary source for ham
enjoyment. I have already had some problems with a USB keyboard that I
think may have been effected by RF when running over 50 watts on 20
meters only.
One article recommended odd multiples of a wave length is desirable at
the lowest operating frequency while other articles don't address this.
I am having no problem with a match using the tuner on all bands 80-6
meters.
Have used the center fed antenna since 1976 but always had the shack on
the ground level and had a good and effective ground system short runs
with feed lines around the 90 to 100ft in length. never had a RF
problem or surge problem.
Any recommended WEB sites or comments are welcomed.
73 de
Ron W4LDE
