compare resonant feed-line dipole or half wave dipole is bester.

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by HS8JYX, Jan 4, 2009.

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  1. HS8JYX

    HS8JYX Ham Member QRZ Page

    compare resonant feed-line dipole or half wave dipole with balan 1:1 which is bester ?

    my 20m resonant feed-line dipole

  2. W6EL

    W6EL Ham Member QRZ Page

    The more power delivered to the actual antenna, the better.

    I believe you are comparing the following antennas:

    1) half-wave dipole antenna with a quarter-wave of coax going to an RF choke

    2) half-wave dipole antenna with a balun at the center feed point

    In my opinion, the antenna with the balun (#2) is better. Either way though, it is important to adjust the antenna for lowest possible match at the feed point. If you don't do this,

    A 70 ohm half-wave dipole should be able to achieve 1.5:1 SWR at reasonable height.

    The only similar antenna that might be better is called an "extended double zepp" and you can use either a special length of ladder line feeder to a balun, or you can feed it with ladder line to a balanced tuner. Feeding with ladder line to a tuner will let you use the antenna on lots of bands. If you just use a special length of ladder line to a balun will limit performance on other bands, but you won't have to run ladder line inside your house.

  3. AA5CB

    AA5CB Ham Member QRZ Page

    Replaced 40 meter RFD with 40 meter dipole- worse now.

    I ran a 40 meter RFD and worked the world for years. What a simple antenna and so easy to install being end-fed! I made it entirely from cheap quad-shield RG-6U from Lowes with about 18-feet wrapped on a 4" PVC pipe for the choke. I needed an antenna in a hurry and have used other RFD over the years- including 2-meters and on COM-2 in my small private plane.

    Recently I decided to 'upgrade' my antenna to a more traditional 40 meter dipole and locate it in the same place. Results? The 40 meter dipole is noisier and just doesn't seem as effective overall.

    I gave my 40 meter Resonant Feed-line Dipole to my Dad and now HE'S working the world. I just visited his QTH and found the bands full but QRN low. Furthermore, the 40 meter RFD (tweaked and tuned using a MFJ analyzer for 7.1MHz) easily tunes to 1.3 or less SWR on 80-10 meters (yes, 80 meters!) with the auto tuner.

    Needless to say I'll be reconstructing another 40 meter RFD this weekend.

  4. W8JI

    W8JI Ham Member QRZ Page

    You may be really happy with the antenna, but you can be sure you have significant common mode currents on the feedline after the choke.

    It is electrically impossible to stop the feedline past the choke from having significant common mode currents with those end-fed setups like that.

    The thing that usually saves users from having noticeable problems like RF burns or severe RFI with such poor systems is the users generally run low power.

    That's among the poorest ways to feed a "dipole". I'd opt for a center fed nomal dipole any day of the week.

    73 Tom
  5. W8JI

    W8JI Ham Member QRZ Page

    By far with no question a normal center feed dipole is a much better antenna. It is nearly impossible to end-feed a "dipole" without having the coax be a big part of the antenna system.

    73 Tom
  6. AA5CB

    AA5CB Ham Member QRZ Page

    Resonant Feed-line Dipole vs. End-Fed Dipole

    As the ARRL Antenna Handbook describes, a RFD (Resonant Feed-line Dipole), though fed from one end, is not the same as an 'end-fed dipole'. It is actually a center-fed dipole that uses the shield of the feed line from the choke to the center tap as the second element of the dipole. The feed point impedance is ~50 ohms just as you would expect with a traditional center-fed dipole.

    In order for the choke to be fully effective, it needs to have an impedance of several thousand ohms. 20-feet of coax wound on a 6" PVC pipe is only about 500 ohms. By adjusting the spacing of the turns I was able to induce more capacitance and increase the impedance for 40 meters, but the impedance on other bands is less ideal and does result in more common mode currents on those bands.

    It could be argued that a traditional center-fed dipole, fed with coax and NO balun as so many do, likely has more common mode current issues than an RFD antenna.
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