Jim,
I'll need to dig back through my lab notebooks to see if I made
measurements on other bands.
A SPICE analysis shows that if you connected that window-line load to
the output of an unbalanced tuner through a 1:1 CM choke balun having a
resistive CM impedance of 5000 Ohms, the dissipation in the choke would
be 6.4W with 100W applied to the system; that's a loss in the choke of
of 0.29dB.
Steve G3TXQ
On 26/06/2016 15:48, Jim Thomson wrote:
Date: Sat, 25 Jun 2016 16:58:54 +0100
From: Steve Hunt <steve@karinya.net>
To: towertalk@contesting.com
Subject: Re: [TowerTalk] To get a truly balanced antenna feed
Some readers may be interested in measurements made on a "real world"
antenna - a 100ft doublet centre-fed with window-line.
At the shack end of the window-line, the equivalent star-network
impedances measured at 3.5MHz were:
Z1 - Window-line Leg 1 to Star Common: 15.1 - j79
Z2 - Window-line Leg 2 to Star Common: 1.6 - j109
Z3 - Star Common to Ground: 30.7 + j110
A perfectly balanced system would show Z1 = Z2
Z3 is related to the common-mode impedance of the system to ground.
Steve G3TXQ
## so I guess we can safely say... its not balanced..at least not on 3.5 mhz.
What happens on the other bands..similar results ?
## What do the SW AM broadcast folks do with their curtain arrays, fed with
300 ohm line, build better CM chokes on the coax side of the mating tuner ?
That degree of unbalance would stress any tuner, plus any CM choke. Toss
snow and or ice into the mix..or even rain or dense fog...and high humidity,
and who knows what you would end up with.
Jim VE7RF
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