Re: KX3 - Balun

Posted by Steve KC8QVO on Sep 02, 2012; 11:58am
URL: http://elecraft.85.s1.nabble.com/KX3-Balun-tp7561948p7561974.html

I have never used a BNC-binding post adapter with a balanced fed antenna, always 1:1 baluns. A BNC-binding post adapter is essentially taking the wires of your balanced line and putting a BNC plug on there. You have no transmission line transformer to convert between the balanced antenna and the unbalanced output of your rig.

Impedance matching is another story. I believe it is irrelevant what the impedance is of your antenna and what nominal impedance transmission line you use (300 ohm TV/ribbon wire or 450 ohm ladder line) in the case of a doublet, or similarly non-resonant antenna, because, well, its non-resonant. All the impedance matching is in the tuner. The magic of the balanced transmission line and balanced (albeit non-resonant) antenna is you don't get quite the attenuation that you would with coax, an unbalanced transmission line.

The one conversion I believe necessary in a balanced antenna configuration is converting between the unbalanced output of the rig to the balanced transmission line, without an impedance transformation.

The reason for the above is that tuners match higher impedances better than they do lower impedances. So, for example, if you start with a 200 ohm load and use a 4:1 balun you are left with 50 ohms. This is a great match. If you start with 1000 ohms and use a 4:1 balun you are left with 250 ohms. This is easy for a tuner to match. Yet, if you start with a 50 ohm load and use a 4:1 balun you are left with 12.5 ohms. Now, lets go a bit further. If you have a 25 ohm load and use a 4:1 balun you are left with 6.25 ohms. Yea, your tuner may tune it but it will have a hard time and you will risk burning it up. The lower impedances are always harder to match. In this instance, if you used a 1:1 balun you would see a range of 1000 to 25 ohms which your tuner should handle OK. If you have the option of a 4:1 balun then you have the high range covered, but if you have one balun its better a 1:1 than a 4:1.

I will use a BNC-binding post adapter with a random wire, because it is an unbalanced antenna. Nothing more.

Steve, KC8QVO