[TowerTalk] To get a truly balanced antenna feed

[Steve Hunt]

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 at karinya.net>
> To: towertalk at 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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