The wonder of a 1/2wl run of transmission line.

In my posts about the theory and creation of my phasing harness, I have focused on the idea and theory behind an odd multiple 1/4 wave length section of transmission line. Now specifically the reason given for using an "odd multiple" 1/4wl was that it's at 90 degrees af the phase and thereby at peak current (or voltage); and when this occurs the transmission line will exhibit maximum impedance.

But then you must ask, what happens if I use an "even multiple" 1/4wl section??

Well, you get a 1/2 wave length multiple. 🙂 But, what is the significance of a 1/2wl multiple?

Well, it's the opposite of a 1/4wl. Rather than being at 90 degrees of the phase (and thereby at peak) it is at 180 degrees multiple and thereby at 0. The result of which is that it will exhibit minimum impedance. I refer again to the ARRL antenna book:

"When the line length is a multiple of 180 degrees (that is, a multiple of λ/2), the input resistance is equal to the load resistance, regardless of the line Z0. As a matter of fact, a line an exact multiple of λ/2 in length (disregarding line losses) simply repeats, at its input or sending end, whatever impedance exists at its output or receiving end."

Now what that says to me, is that if I wanted the best match and performance from my transmission line, I would not only try to minimise it's length – to avoid transmission line loss) but I would also try and make it an exact 1/2wl multiple. Something I'll try and keep in mind going forward, and something I'd say is very important when I'm only using at best 5W on my FT-817ND.

But you know what's really fascinating here!? This says that if you were to use a 1/2wl of RG-6 (which has a characteristic impedance of 75 Ohm) to hook up your amateur transceiver (requiring a 50 Ohm Load) to a perfectly tuned antenna (so with a Zload of 50 Ohm) you would get an SWR or 1:1. Further, this would even work with some 300 Ohm ladder line – as long as you had an exact electrical length of line that was a 1/2wl multiple!! How cool is that!

But cool stuff aside, the reason I posted this here was to show the potential performance benefits. So if you want to try and get the best match you can to your antenna with the lowest loss possible, I think attempting to use an exact 1/2 wl would be in your interest. This wont avoid transmission line loss – so still try to use the minimum length you can – but it will help obtain the best match you can.

(Oh, if you want to see this in action, I've run it through VK1OD's TLLC and the numbers came out as expected so have a play!)

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