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I have made a dipole for 40 m, no swr to speak of on 7040 kHz, but I've been told that it should also work on 20 m as it is 20 meters long (2× 9,92 m) but there I get SWR in the 7-9 range. Will this improve if I add a balun to the center?

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A 1 wavelength dipole will always present a high impedance. Low feedpoint impedance occurs at odd half wave harmonics, so 1 half wave, 3 half waves, etc. A 40m dipole will usually present an acceptable SWR on 15 meters, not 20. A 4:1 balun at the feed point may allow you to tune the antenna on 20 meters, but losses in the coax will be extremely high on both bands then, because of the transformation of the impedance on both 40m and 20m without achieving a perfect match on either. A better bet is to turn it in to a fan dipole by adding a 20 meter element to it, which should give you a nice triband antenna.

That said, dipoles should always have baluns if at all possible, ideally ferrite toroid 1:1 choke baluns, as these have a wide bandwidth and will keep RFI to a minimum. While single band dipoles will often work without one, they're more prone to causing or receiving RFI and noise due to common mode current on the coax shield.

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  • $\begingroup$ Fan dipole! Why didn't I think of that. Thanks, i will look for a balun to add to my antenna. $\endgroup$
    – Hans Neve
    Aug 18, 2016 at 15:46
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    $\begingroup$ They're quite simple to make. Pick up a couple FT240-43 toroids from Mouser (if you're in the US) or other supplier. They're usually less than $7 each, and having a couple on hand is always a good idea. They're great for tamping down RFI with PCs and such. Wrap 12-14 bifilar turns of 14awg THHN or magnet wire around the core like so and put it between the coax connection and the dipole legs. The bits of tape to hold the windings together are optional, just depends if the windings are cooperating and keeping close together. $\endgroup$
    – Hamsterdave
    Aug 18, 2016 at 15:48
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    $\begingroup$ The paper located at this URL is an excellent tutorial on designing and making your own baluns. I have been home-brewing baluns for years. k9yc.com/RFI-Ham.pdf $\endgroup$
    – K7PEH
    Aug 18, 2016 at 17:09
  • $\begingroup$ I concur, K9YC's paper should be considered the RFI and Balun bible. I took it to Kinko's and had it printed and put in a 3 ring binder for easy access. $\endgroup$
    – Hamsterdave
    Aug 19, 2016 at 2:19

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