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feed line lengths

Mar 29th 2020, 13:58


Joined: Apr 4th 1998, 00:00
Total Topics: 0
Total Posts: 0
All the articles I can find on the web or have read state feed line lengths for dipoles, are the suggested feed line lengths for a loop the same?
I have a ~265ā€™ loop at about 55ā€™ being fed with 600-ohm open wire feed line, Iā€™m a bit confused as to what would be the proper feed line lengths or lengths to avoid for this multi-band antenna. Can you point me in the right direction?

George N1NAZ
Mar 30th 2020, 07:53


Super Moderator

Joined: Apr 4th 1998, 00:00
Total Topics: 0
Total Posts: 0
Yes, the lengths are different. A dipole presents a high impedance on the even harmonics. It presents a moderately low impedance on the fundamental and odd harmonics.

In contrast, a loop presents a low impedance on both the even and odd harmonics. As this is a mismatch to 600 ohm line, you want the feedline to be a multiple of a half wave on those harmonic resonances. And avoid odd multiples of a quarter wave on harmonic resonances.

Hamonic resonances aren't exactly the same as harmonics, due to the end effect or lack thereoff with dfferent antennas. There is only one set of ends with a dipole and none with a closed loop.

If it doesn't work with open wire on a particular band, I'd suggest changing the length by 1/4 or 1/8 of an electrical wavelength.

Zak W1VT
ARRL Senior Lab Engineer

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