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Technical Topics => Equipment => Topic started by: Erik Mattson on January 10, 2022, 2334 UTC
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I'm putting together a new dipole (RX). I think I now understand the need for a 1:1, but I've seen 4:1s mentioned concerning dipoles as well. Are these specifically for simple folded dipoles or T2FDs?
It'll be cut for 43 meters with slightly less than 100ft run of RG6 to the receiver.
Seems like there should be chart listing the different ratios and when you'd use them. Gotta be somewhere.
Thanks.
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Erik, a 4:1 balun and 1:1 balun (if a transformer device) have two windings. The impedance ratio is a function of how the windings are connected. A half wave dipole in free space (i.e. no ground effects) fed at the center has an impedance of 72 ohms. However most dipoles are close to the ground. This lowers the impedance. The nominal value is close to 50 ohms. So the 1:1 (current balun or common mode choke) is normally recommended. However, if you need to get the dipole up quickly, then a direct coax connection to the center of the dipole is done. There are antenna pattern changes, but, it stills works. I have in past done it both ways, it is hard to tell which is better. You make your choices and take your chances ;D
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4:1 is usually only used on a folded dipole, or any antenna with a higher feedpoint impedance. A 1:1 balun should be used on your garden variety dipole.
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I've always used 9:1s on rando wires, but I've never bothered with anything on my dipoles and thought maybe I was missing something. Oh well, I've got a 1:1 wound up on the bench so I'll stick that in there. Just wanted to be sure.
I'll be gettin' around to a T2FD sooner or later.
Thank you both.
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If you will be using the dipole for the freq of resonance, a 1:1 will divorce the feed line from the reception pattern nicely and may provide best transfer of energy, the z the antenna sees is also nominal to the feed line. In the case where a dipole is used at freqs outside of resonance, where the z seen by the wire is very different from the z of the feed, over or under coupling might transfer more received energy and this will be freq/wire length dependent.
Odd 1/4 frequency multiples of antenna wire length should result in low feedpoint z meaning a good match for direct coax feed, evens over a halfwave should result in much higher z. A halfwave endfed has around 2k ohm z.