[TowerTalk] Force 12 B-1/B-1S Baluns

[Steve Hunt]

Even with a perfect 50 Ohm antenna, there can be significant coax braid 
current and stress on any balun that is installed!

With a perfect 50 Ohm antenna, at the 1500W level the differential-mode 
feedpoint voltage would be 274v. If the antenna was perfectly balanced 
with respect to ground, that could be a CM voltage of 137v across the 
balun (assuming, worst case, the coax CM path impedance is low). If the 
balun CM impedance was 250 Ohms resistive - meeting the often-quoted "5 
times load impedance" recommendation - that would mean a dissipation of 75W!

As you can see, a perfect 50 Ohm, balanced, antenna does *not* prevent 
CM current or balun stress. There is no substitute for high CM balun 
impedance - preferably resistive.

73,
Steve G3TXQ


On 12/05/2013 00:55, Dan Hearn wrote:
> Mark, IMO the power dissipated in the Balun will be related to the coax
> termination impedance at the antenna. If it sees 50 ohms then there will be
> little current coming back down the shield outer . Check your antenna
> feedpoint Z at any frequencies you plan to operate.
>    Of course, the "Balun" is really a line choke.
>
> 73, Dan, N5AR
>
>
> On Sat, May 11, 2013 at 4:07 PM, rfman45 <rfman45 at hotmail.com> wrote:
>
>