How Can a Quarter-Wavelength Cable Transform Impedance?

Comments · 19

• TE

  @TelecomTraining · 2 days ago · pinned

  A quarter-wave transformer is one of those RF concepts that seems almost impossible at first — how can a simple piece of transmission line transform one impedance into another?<br><br>The key is that the ¼λ section has its own characteristic impedance, and its electrical length allows the forward and reflected waves to combine so that the impedance seen at the input is transformed.<br><br>For the example in this video:<br><br>50 Ω system → 70.7 Ω ¼λ section → 100 Ω antenna<br><br>The 70.7 Ω section doesn&apos;t create power — it transforms the impedance so the 50 Ω transmission line sees the load it needs.<br><br>What part of the quarter-wave transformer would you like to see explored in more detail in a future video?<br><br>Thanks for watching Telecom Training! 73!

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• US

  @user-r4d2l · 2 days ago

  I ran two 6 meter beam antennas co-phased and used 2 quarter wave 75 ohm RG11 sections to feed them through a T connector fed by 50 ohm coax from my transceiver and it worked flawlessly.. I also used one inch 75 ohm CATV hardline to feed 2 meter and 79 centimeter beams using multiple wavelength calculated lengths of the coax for a tuned transmission line that had super low loss.. Those were the fun wildcat antenna days from my youth 30 years ago when experimenting to make things work was so much fun..

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• N9

  @N9XRradio · 2 days ago

  Here&apos;s what&apos;s happening. When you use a coiled transformer to transform your impedance, the RF sees the transformed impedance reflected back from the load. The coil transformer should have no characteristic impedance of its own. The RF will transform as we normally think of a transformer transmitting. However the coax (transmission line) does have a characteristic impedance. That characteristic impedance plays a part in the &quot;transformation&quot; that takes place here. Look it up. You&apos;re talking about the &quot;line impedance&quot;. In this case, that&apos;s different from the &quot;characteristic impedance&quot;. The characteristic impedance is an intrinsic value of the transmission line. It&apos;s not going to change in any way you use the line. However the line impedance is changeable as you have shown here. <br><br>By using a transmission line as a transformer, we utilize the characteristic impedance of the transmission line. That&apos;s used at a cost. The differential impedance from the input to the output will drop a voltage (ohm&apos;s law dictates this) and some of the signal is transmitted by the line. That&apos;s how the transformation takes place. This is easily proven by measuring the RF current at the feed point and realizing that you will be short on what you would expect. I only use real transmission line transformers. Stubs and lengths like this will radiate.

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• DE

  @denelson83 · 1 day ago

  By taking advantage of the interaction of the forward and reflected RF waves.

• BE

  @Bedfford · 2 days ago

  this is a must see : &quot;AT&amp;T Archives: Similiarities of Wave Behavior (Bonus Edition)&quot; <a href="https://www.youtube.com/watch?v=DovunOxlY1k&amp;t=490">https://www.youtube.com/watch?v=DovunOxlY1k&amp;t=490s</a>

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• EX

  @exponentmantissa5598 · 1 day ago

  At <a href="https://www.youtube.com/watch?v=mscALMX7AW0&amp;t=441">https://youtu.be/mscALMX7AW0?t=441</a> you show a 5/4 length section of transmission line??

  1