It’s a common misconception. The *characteristic* impedance of a feed-line is only meaningful when the line is terminated in a non-reactive (resonant) load with a feed-point impedance the same as the line, i.e., no standing waves. For example, a resonant folded dipole (300 ohm feed-point), fed with 300-ohm LL, and a 6:1 balun at the radio, would be a proper use of a transformer balun. But that’s not frequency agile.

The impedance you’ll have at the LL to coax transition will vary widely (~25-4500 ohms) depending on the length and height of the antenna, the length of the feed-line, and the operating frequency. It’ll zoom up and down as you spin the dial.

In any feed-line with standing waves, the impedance varies from low to high every quarter-wave along the feed-line. That’s what standing waves *are* — impedance variations along the feed-line.

For any non-resonant multi-band antenna, the SWR will be high, the impedance will widely vary, and the *characteristic impedance* of the line is meaningless.

Well, except that a 600-ohm line that varies from 25-4500 ohms is a LOT better than a 50-ohm line that does the same! A 600-ohm line at 4500 is a 7.5:1 mis-match ratio, but a 50-ohm line at 4500 is a 90:1! That’s why 450-600 ohm lines are far better for high SWR applications; 450 is near the geometric mean of the expected impedance variation.

As for burning up the balun, well, that’s why you need a “tuner” or “ATU”-type balun; they are designed to handle the very wide impedance variations seen in non-resonant multi-band ladder line fed doublets and loops. I push 1kW through a Balun Designs 1171 from 80 to 10 meters and it it doesn’t even warm up.

73, –kv5r

]]>I have been building and modeling the T3FD terminated 3 wire folded dipole for the past 18 months. It is a powerful weapon against noise in a congested QTH.

Am I correct assuming this is more like a magnetic loop antenna? The receive noice reduction is very well worth the 1 to 7 dB transmit ineffeciency, as simply adding a linear overcomes the Tx reduction. Reducing the noise floor 2+ s-units com

pared to an equivalent length off-center fed dipole is very useful.

What makes me assume the antenna is magnetic is that the NEC/4 models show the RF current is constant over the length of the antenna, whereas a single wire antenna the current is max in the center, and tapers to zero at the ends. This makes me think that contributes to low Rx noise floor.

A second issue: I am also questioning the model’s assumption of the loss in the load resistor, as it can indicate 6 dB of the power goes in the resistor. Some of my test loads are two series 25W resistors that “supposedly” are taking 1400 watts of the energy. It takes many minutes of sustained “key-down” time to cause them to crack.

]]>Hope it helps!

73, -kv5r ]]>