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I was watching a video where ladder line (450-ohm) was directly connected to coax (50-ohm) without a matching transformer.

Mating your Coax to Ladder Line

Wouldn't this mismatch make it impossible to achieve a 1:1 ratio back in the shack? What am I missing here? TNX Dax

Dax
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    Welcome Dax and good question! I'll let others with more experience chime in with actual answers but my guess is the ZS6BKW and G0GSF antennas mentioned in the video description are like the G5RV in that the ladder line is actually part of the antenna. So essentially the 50Ω coax is being connected to the "feed point" rather than using the ladder as a transmission line? – natevw - AF7TB Apr 03 '22 at 03:48
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    “You don’t necessarily see 450 ohms at the end of a 450 ohm line - You probably won’t see 450 ohms at the end of a 450 ohm line - The impedance you see at the end of a 450 ohm line depends on what is connected to the other end and on the length of the line.” - Kurt N. Sterba – hobbs - KC2G Apr 05 '22 at 02:13

1 Answers1

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Connecting ladder line directly to coax normally is not a good idea, for the following reasons.

  1. There will be an impedance mismatch at the junction between the two, for receive this will result in reflections from the join back towards the antenna, for transmit there will be reflections back from the join to the transmitter. The mismatch will result in loss of performance of the system.

  2. Because of the impedance mismatch, for transmit the SWR will bad. If the antenna was matched when using just 50 ohm coax, adding the ladder line between the 50 ohm coax and the antenna will make the SWR jump to about 9:1 depending on the length of the ladder line.

  3. Coax is an unbalanced transmission line with an earth and center (live) conductor, whereas ladder line is balanced with two identical conductors. At the junction, for transmit, the RF current from the transmitter traveling on the inside of the coax, when it reaches the join, will split up between the outside of the coax and the ladder line conductor. This will cause RF current to flow on the outside of the coax and result in the coax becoming part of the radiating component of the system.

There are instances where different impedance transmission lines are connected together to work as an impedance transformer, but when this is done the lines have to be of the same type, connecting unbalanced to balanced doesn't work properly.

Having said all that, there may be special types of antenna where the ladder line is used as part of the antenna matching system, as mentioned by Nate.

Hope that helps !

Andrew
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    Andrew, can you explain a bit more about your comment: "adding the ladder line between the 50 ohm coax and the antenna will make the SWR jump to about 9:1 depending on the length of the ladder line." Are you saying that having a mismatched RF connection in the line that's just an inch long has less of a detrimental effect than one - say - that's several inches long, like maybe when you run the feedline through a wall and the distance between conductors is changed? – Dax Apr 04 '22 at 17:00
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    Hi Dax, yes, if you use two different transmissions lines with different impedances joined together, then the SWR you will see at the radio end will become a function of the lengths of both lines and the impedance of the antenna. When using a single piece of coax, the SWR will be constant along the length of the line. – Andrew Apr 04 '22 at 22:08