Center Fed VS. OCF Dipole

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by W9OMA, Dec 1, 2018.

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  1. W9OMA

    W9OMA Ham Member QRZ Page

    So... I currently use a G5RV Jr. It works fine. No complaints. But I stick to one or two bands... mainly 40m and 20m. (More 40m than anything) I'm trying to think of a better antenna to put up. If I had the space and money I would put up a Yagi (but thats impractical at this QTH.) I'm trying to decide between a OCF and CF dipole. Ideas? Thoughts? What are the benefits of both? Downsides? Any input would be appreciated.

    W9OMA
     
  2. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    For 40m only without regard for other bands, I'd stick with the center-fed dipole. It can be almost perfectly balanced, so a simple 1:1 current balun and the feedpoint will make it sing when fed with coax.

    There is zero advantage to a single-band OCF; the whole reason anyone uses an OCF doublet is to find a compromise feedpoint that allows it to be reasonably matched on multiple bands. But it's not balanced and the "ideal" balun is never ideal for all bands.
     
    K7TRF, WB5YUZ, AI3V and 1 other person like this.
  3. W9OMA

    W9OMA Ham Member QRZ Page

    I want to do a lot better on 40m, as it's where you can find me 90% of the time. The G5RV is great and all, like I said I have no complaints... but I suspect that something dedicated to 40m will blow this G5RV out of the water. My concern is that if I put up a 40m wire I will suddenly get an itch to work 20m. Should I just put up two wires? One for 40 and one for 20?
     
  4. N7WR

    N7WR Subscriber QRZ Page

    Make a "fan dipole" One feedline. At the feed point you construct a 40 meter dipole and a 20 meter dipole. Dipole radiation patterns are fairly predictable. OCFD's (and I agree with Steve that unless you want to cover more than 2 bands there is no reason for an OCFD) can have weird radiation patterns and nulls. I have built and been happy with OCFD's but I wanted something usable from 160-6 meters. Recently moving to a new QTH I debated putting up the OCFD but, like you, I currently work mostly 40 and 20---so I built a fan dipole and it works fine.
     
    K7TRF likes this.
  5. WA7ARK

    WA7ARK Ham Member QRZ Page

    ...and the OCF has a lot of common-mode current on the feedline. Some of them are designed such that the feedline has to radiate in order for the antenna to meet the specified SWR specs. Placing the feedline close to other conductive material like a tower or coax cable really screws with them. So does trying to suppress the CM with a Choke.

    I had a bad experience with a commercial OCF where the common-mode-current creates radiation off the feedline causing spurious tripping of Arc-Fault-Circuit-Interrupter breakers in a new house. The problem went away when I replaced the OCF with a symmetric center-fed multiband antenna (ZS6BKW).

    If multiband operation is required, then look at replacing the G5RV with a ZS6BKW, which has slightly better performance on 40meters. Otherwise, a center-fed dipole or inverted-V (coax fed, with a 1:1 current-mode balun right at the feedpoint) is hard to beat.

    As far as a comparison of radiation patterns on 40m from a G5RV, a ZS6BKW, a OCF dipole, a CF dipole or invertedV, you will not be able to detect any difference, and neither will anybody else. The are all basically wire dipoles. Mounting height makes the biggest difference...
     
    Last edited: Dec 2, 2018
  6. KC8VWM

    KC8VWM Ham Member QRZ Page

    The next best thing to installing a Yagi on 40m is going to be...

    [​IMG]

    It also functions as an acceptable multiband antenna from 160-10m with an antenna tuner.
     
    WB5YUZ likes this.
  7. W9OMA

    W9OMA Ham Member QRZ Page

    Sadly, I do not have room for a ZS6BKW or the Extended Double Zepp. A local guy around here recommended a 20m double extended zepp. Thoughts on that? Ill have to look more into a fan dipole
     
  8. WB5YUZ

    WB5YUZ Ham Member QRZ Page

    In that case, leave the G5RV alone for now (unless you can get it higher; in which case, get it higher!!!) and install a good 40m vertical (a trap vertical, such as the BTV family, will get you 20m also), and an A/B switch so you can go back and forth between them.

    If you do this, in a few weeks you will want to thank me, but this thread will be buried. So, you're welcome in advance.
     
    Last edited: Dec 2, 2018
    W9OMA, AI3V and KC8VWM like this.
  9. AI3V

    AI3V Ham Member QRZ Page

    Another vote for some sort of naturally ballanced dipole.

    Wether you pick a fan, trapped, or even tune the antenna system with a particular combination of dipole and feedline length, try and keep it as visually ballanced as possible- both legs about the same height and distance from other things,and bring the feedline down as symmetrical as you can.

    Rege
     
  10. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    I encourage resonant dipoles and all that but honestly if you work 20m and 40m and have the G5RV properly installed, single band dipoles for 20m and 40m are not going to blow a G5RV out of the water.

    In theory (and I've found also in practice) a G5RV on 40m vs. a perfectly tuned 66' center-fed dipole on 40m are about identical, and the G5RV works very well on 20m, which was the primary band it was designed for -- it's 3/2-waves on 20m and has four primary lobes and two secondary lobes as opposed to the two big, fat lobes a 1/2-wave dipole has.

    The four primary lobes of a G5RV on 20m actually have gain over a 20m dipole; the two secondary lobes are a little bit weaker than a 20m dipole; and of course the nulls between the lobes are deeper than a dipole's (of course, all antenna gain comes at the expense of loss in some other directions, no matter what the antenna is); so in some directions, the G5RV will outperform a 1/2-wave dipole and in some it won't. But normally it's a good 20m performer.

    The double Zepp or Extended double Zepp for 20m has even more gain, but of course at the expense of more loss in the nulls.

    No free ride with antennas! On 20m it's pretty easy to install something rotatable (even a single-element 1/2-WL dipole, which is lightweight and only 32-33' long) and eliminate that nasty null problem -- or take advantage of steering the nulls to reduce noise or interference.
     
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