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True Ladder Line and Wire Antennas

How Should I Implement My New True Ladder Line Antenna System?

Posted by Gary Baker-K7EMF on

How Should I Implement My New True Ladder Line Antenna System?

There are several principles of antenna theory that we need to establish:

1) A resonant antenna is used to provide a matched load for 50 ohm coaxial cable.

2) A wire antenna (with no tuned elements) used on several bands is not designed to be a resonant antenna. Rather it is designed to be a non-resonant antenna.

3) The feedpoint impedance of a dipole antenna varies widely over the wide range of operating frequencies. A dipole shorter than a resonant dipole presents an impedance less than 50 ohms resistive and capacitively reactive (-J Ohms). As it gets shorter, the resistive element decreases and the capacitive reactance of the impedance increases. Conversely, as the dipole increases in length relative to a half wave, the resistive element of the impedance increases and reactive element increases inductively.

4) It is impossible to afford a reasonable match between ladder line and coax with a balun or any kind of a transformer over a wide range of frequencies if the ladder line is terminated into a non-resonant dipole antenna. Also, with moderate to high transmitter power, the balun core will saturate causing distortion in the RF sine wave thus radiating harmonics and heating up the balun.

5) A resonant dipole center fed presents a serious mis-match for Ladder Line.  Low loss feedline like TrueLadderLine can tolerate this high swr. 

6) There are two major losses for any feedline (coaxial and Ladder Line): Matched Loss and Mis-Matched Loss. Ladder Line exhibits extremely low Matched and Mis-Matched Losses; Coax exhibits Moderate Matched Losses and Extremely High Mis-Matched Losses at high SWR.

Here are the possible solutions in feeding ladder line non-resonant antenna system for multiple bands:

A) The most efficient multi band wire antenna system is to feed the dipole with ladder line and feed the ladderline with a balanced tuner. The balanced tuner is one using link coupling such as the Johnson Matchbox or the Palstar BT1500A. The balun within the Palstar is located at a low swr location in the circuit allowing for maximum efficiency and no core saturation. This type of installation works well up to the full legal limit. If the ladder line cannot be brought into the radio room, then the tuner should be placed at a location where the ladder line can be connected directly to it. Then, coax to be used from the tuner to the radio room. Because the ATU presents a 50 ohm non-reactive load to the coax, the only losses incurred by the coax is matched losses (typically very low!). With the ATU located remotely from the radio room, some method of tuning the ATU is needed. One solution is to use a StepperTune controller which controls two stepper motors. More info on this is located at StepperTune.com.

B) A less efficient way to feed ladder line is to use an unbalanced ATU and use a 1:1 current balun (bifilar choke) at the output of the ATU. Most ATU’s on the market use a similar configuration but may use a 4:1 balun (more loss) to afford a wider impedance range in one or the other directions. Caution is required to not run excessive transmitter power as the balun is in a high swr location within the circuit and the ladder line symmetry is not maintained due to the high swr placed on the balun and therefore the possibility of feedline radiation. This configuration is a compromise that many hams have had success with. Here again, how do you define “success”?

C) The most inefficient way to feed ladder line is to place a balun between the ladder line and the coax outside so a significant length of coax is used. An unbalanced ATU is used to “tune out the high swr” in the radio room. Many hams have had success with this type of installation (running 100 watts or less) but be aware that the mis-matched losses on the coax are extreme and basically one is defeating the purpose for ladder line but rather making one “feel good” that ladder line is being used and therefore it must be a “low loss” system when in reality it is not.

If one needs to utilize a balan, by all means use a 1:1 current balun which will display the least amount of loss. Trying to choose a balun to afford a “match” between the ladder line and coax over a wide range of frequencies with a non-resonant multiband antenna is impossible.

ATU Impedance Range: If the ATU is not able to resolve the impedance presented to the balanced terminals, the length of the ladder line and/or antenna length may be changed to resolve the issue. As an example of impedance change (derived from Transmission Line Details):

Ladder Line terminated into a 50 ohm non-reactive antenna(resonant):

50ft—2239 ohms to the ATU

75ft---1351 ohms to the ATU

100ft---331 ohms to the ATU

If one wishes to fabricate an ATU to accommodate the impedance range needed for a particular antenna system, I recommend using EzNec to model the antenna. Perform an SWR sweep to determine the feed point impedance. Load the feed point impedance value into Transmission Line Details to arrive at the final impedance presented to the ATU. Once that is accomplished, I use SimSmith to design an L network ATU and derive values for the inductor and capacitor. By designing the ATU, the antenna and ladder line installation can remain the same with no length adjustments.

Gary Baker, K7EMF