[Elecraft] OT: Now Balun effectiveness

[Ron D'Eau Claire]

Mike, VP8NO wrote:=20

I'm surprised the SWR police haven't descended upon this.  Last time I
suggested that the antenna was the best place to put the tuner it really
wound them up.

Try running the feed impedance numbers of the Cebik 44ft doublet through
something like TLA which is bundled with the ARRL Antenna Book, Smith
Chart or even model it in NEC.   You will discover that feeding it =
direct
with 50 ohm cable gives SWR's ranging from 1.45 up to 93.9 =20

-------------------------

I certainly agree about putting the ATU at the feed point. My preferred
installation is to drop open wire line from the center of a doublet =
straight
down to a balanced ATU near the ground where it can be kept out of the
weather and so the weight of the ATU doesn't have to be supported up =
high.
At the moment, when I drop the open wire straight down from my inverted =
V
doublet it is right at the shack window, so guess where my ATU is =
located? A
remote tuning balanced ATU would be very handy  for other situations.=20

As several sharp ops here pointed out when we were discussing open wire
feeders a while back, one of the real advantages of the open wire lines =
in
the range of 450 to 600 ohms is that they show LOWER SWR's in typical
doublet installations. For example, a center fed antenna that is a full
wavelength long might show an impedance of about 2160+j1900 ohms =
resulting
in an SWR of 77:1 when fed with 50 ohm coaxial line. However, the same
antenna with a 450 ohm line would show an SWR of less than 9:1. That's a
huge difference that accounts for much of why open wire line is so =
efficient
with multiband doublets.=20

Indeed, a 66 foot center fed wire 30 feet up will show a maximum SWR on =
450
ohm line of 15:1 on any frequency from 7 to 30 MHz. Over most of that =
range
the SWR is in the range of 5:1 to 8:1.=20

The same antenna with 50 ohm line will show an SWR of 40:1 or worse on =
ALL
frequencies from 7 to 30 MHz except, of course, near 7 and 22 MHz where =
it's
self resonant.=20

Ron AC7AC