[TenTec] Problem on 160 meters, Whats up?

[d.e.warnick at comcast.net]

The problem of heating an unmatched balun is one of the reasons that I chose the Palstar AT5K. It has a 1:1 balun at the input side of the tuner, that way it sees a 50-ohm load on each side when the tuner is matched. There's a great writeup on that on page 25-15 of my 2006 ARRL Antenna Book. Dean Straw, N6BV, reasoned that a 4:1 balun at the output would seldom see a matched condition and could overheat or arc at 100 watts, let alone full power. The design in the Antenna Book uses a 1:1 balun on the input side of the tuner also 



Dave 

WA3MKB 




----- Original Message ----- 
From: "Jim Brown K9YC" <k9yc at audiosystemsgroup.com> 
To: "Discussion of Ten-Tec Equipment" <tentec at contesting.com> 
Sent: Wednesday, March 18, 2009 4:53:26 PM GMT -05:00 US/Canada Eastern 
Subject: Re: [TenTec] Problem on 160 meters, Whats up? 

On Wed, 18 Mar 2009 13:19:12 -0600, Dr. Gerald N. Johnson wrote: 

>> Another VERY different thing that is CALLED a balun is really a common mode 
>> choke. Put a bunch of ferrite cores around coax, or wind a bunch of turns of 
>> coax through one or more ferrite cores. Those are common mode chokes, 
>> commonly (wrongly) called "current baluns." But when done well (enough turns 
>> to have a high enough choking impedance), they work VERY well. 

>And if they don't have enough core cross section, they too can saturate. 
>But saturation is more difficult achieve with only one turn per core. 

Saturation is VERY unlikely in a coaxial choke if the choking impedance is high 
enough, because the core sees ONLY the common mode component. It sees NONE of the 
differential field -- it's all confined to the dielectric of the coax. That is 
NOT true in a bifilar winding, which has a lot of leakage flux. So the core in a 
common mode choke that is bifilar wound (that is, a pair of wires) IS likely to 
saturate if you run enough power through it. 

>Their big claim to fame is that the allow the antenna to set the balance 
>where the transformer forces voltage balance. 

That's the 30 year old version of why common mode chokes are much better than 
voltage (transformer) baluns. That's certainly true, but they have another HUGE 
advantage -- they prevent the feedline from becoming part of the antenna. This 
keeps RF out of the shack and your neighbor's living room, and it also prevents 
NOISE received on the feedline from coupling to the antenna and making it harder 
to hear weak signals. That is, the only noise you hear is what's picked up on the 
antenna, which is usually further from the noise source (in your home and your 
neighbor's) than the feedline. 

73, 

Jim Brown K9YC 



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