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@hotmail.com> wrote:
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|