Jim,
I agree!
I've been doing some balance measurements on a 100ft doublet I have fed
with ladderline. Because of its local environment it's a pretty
unbalanced antenna system.
I measured ladderline leg current balance using a couple of clip-on
meters; I also ran the ladderline through a single ferrite core wound
with pick-off turns to overcome the phase imbalance "uncertainty
problem". I tried the following arrangements:
Palstar BT1500 balanced tuner
Ten Tec tuner equipped with internal Ruthroff 4:1 Voltage Balun
Ten Tec tuner equipped with home-made external balun configured as 1:1
Guanella
Ten Tec tuner equipped with home-made external balun configured as a 4:1
Ruthroff.
The Ten Tec tuner plus external 1:1 Guanella produced noticeably better
current balance than any of the other arrangements - particularly at the
lower frequencies. The 4:1 Voltage baluns were especially bad.
I was unimpressed with the Palstar BT1500; the 1:1 balun it uses at its
input has pretty low CM impedance on the lower bands.
73,
Steve G3TXQ
On 14/11/2010 18:30, Jim Brown wrote:
The primary concern with balance in an antenna system, INCLUDING THE
LINE, is minimizing common mode current on the line. We care about this
because we do not want the line to radiate to our neighbor's living room
or receive noise from his computers, and because we do not want that
common mode current in our shack. Indded, the primary function of a GOOD
choke at the feedpoint (or down the line below a matching section) is
to disconnect the feedline from the antenna from a common mode point of
view.
|