'The Doctor Is In' the ARRL Letter:

This week, ARRL Letter readers are in luck! The ARRL's very own Doctor, author of the popular QST column "The Doctor Is IN," answers a question from his mailbag:

Question -- Mel, Snyder, K8MIW, of West Olive, Michigan, asks: I have a question about feeding a folded dipole, spaced about 5 inches between wires. What would be the difference between feeding it with the usual 300 ohm twin lead and using the popular 450 ohm window line as a feed line? I operate mostly on 75 meters. Years ago I used a folded dipole and found it to be an effective and wide band antenna. I have heavy duty window line on hand, but would have to buy the twin lead and the only line I can find is light duty line. Would the mismatch with the 450 ohm line cause any problems?

The Doctor answers -- The most significant difference would be the level of mismatch at the transmitter end. If you are using an antenna tuner, I doubt that you will notice any difference at all.

There will actually be slightly less loss with the 450 ohm (usually closer to 400 ohm in my experience) line. Let's see, if the antenna is about a quarter-wavelength high, the actual feed impedance will be closer to 200 ohms than to the free space value of 300 ohms. With 300 ohm line that results a 1.5:1 SWR at resonance, or 2:1 with 400 ohm line.

I would go ahead and use the window line. It should be less bothered by rain and will probably last longer. If your tuner has a problem at any frequency you operate at, try changing the feed line length by 20 feet or so and see what happens -- you are likely to find a length that works across the band. Just don't roll up any excess. In my cellar shack, I usually put any excess window line between the overhead floor joists, away from other wires and secured with TV standoff insulators.

That should be a great antenna! You may even be able to tune it on other bands, if you have a wide range tuner -- especially 30 meters. On that band, the window line will make even more of an improvement.

Source:

The ARRL Letter Vol. 27, No. 19 May 16, 2008

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