A Short 40m Folded Dipole

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by W5DXP, May 1, 2011.

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  1. W5DXP

    W5DXP Ham Member QRZ Page

    The G5RV matching section method will work for a reduced length single-band folded dipole. For instance, EZNEC sez a ~54 foot long folded dipole (40 feet high) fed with ~24 feet of 450 ohm ladder-line will yield a 50 ohm resistive impedance on 40m looking into the 24 foot section of ladder-line, i.e. no tuner required.

    This kills two birds with one stone. It shortens the 40m folded dipole from the full-size 66 feet to ~54 feet (somewhat like a G5RV) AND provides the ~300 ohm to 50 ohm impedance transformation needed for a full-size folded dipole fed with ladder-line. Of course, a 1:1 current-choke-balun needs to be installed at the ladder-line to coax junction.

    The theory is to reduce the length of the full size folded folded dipole until the SWR on the 450 ohm ladder-line is 9:1. Since 450/9=50 ohms at a current maximum point, all we have to do is locate the current maximum point, install a 1:1 choke, and run 50 ohm coax directly to the transmitter with no tuner required. This same approach would work with 300 ohm line by causing an SWR of 6:1 on the 300 ohm line, i.e. 300/6=50 ohms. For 300 ohm line, the folded dipole would be ~57.5 feet long and the 300 ohm line would be ~22.4 feet long, according to EZNEC.

    The folded dipole could be made even shorter by using 600 ohm open-wire for the matching section by causing a 12:1 SWR, i.e. 600/12=50 ohms. The 40m folded dipole becomes ~51 feet long (G5RV length) and the 500 ohm matching section is ~26.5 feet long (close to the length of the G5RV matching section). I will turn this posting into an article and post it on my web page.

    For 450 ohm ladder-line on other bands, the formula is dipole length = 390/f, 450 ohm ladder-line matching section length = 172/f. Here is the EZNEC file:

    http://www.w5dxp.com/40mFDt.EZ

    EZNEC sez the short 40m folded dipole will also work on 30m, 12m, and 10m with an antenna tuner.
     
  2. EI4GMB

    EI4GMB Ham Member QRZ Page

    Hi Cecil,

    Thank you for posting your findings. It is certainly valuable information and much appreciated. ;)

    Kind Regards

    Fred EI4GMB
     
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