If I cut a 72 ohm dipole to match 50 ohms, do I lose out?

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by KE0EYJ, Jun 28, 2020.

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  1. KE0EYJ

    KE0EYJ Ham Member QRZ Page

    I understand that a flat dipole (height making a difference, also) is theoretically a 72ohm antenna. If I trim it to make it 50 ohms, am I actually reducing the efficiency somewhat? Am I better off leaving it at 72 ohms and matching it at the antenna?
     
  2. AE1N

    AE1N Ham Member QRZ Page

    Efficiency change will be minimal and the transmitter power transfer to the antenna will improve slightly. Rather than chopping off pieces you are better off leaving the dipole alone and using a good tuner which will allow you to load the antenna on more bands.
    ---Layne AE1N
     
    KE0EYJ likes this.
  3. UA3TW

    UA3TW Ham Member QRZ Page

    If you shorten it until the real part of the impedance equals 50 Ohm, the imaginary part will pop up and spoil the SWR. So you will have to insert a coil for compensation. Not worthwhile I believe.
     
    AC0GT, K8PG and KU3X like this.
  4. WA7ARK

    WA7ARK Ham Member QRZ Page

    At any given height, over any given earth, when you trim the length of a horizontal dipole that starts near 1/2wl, you are changing the jX part of its complex feed-point impedance much faster than the R part.

    An example:
    33.5ft dipole, 70ft agl, R=70.5, jX=1, SWR=1.41
    30.0ft dipole, 70ft agl, R=51.3, jX=-153, SWR=11

    You obviously do not want to tune the 30ft dipole from the station end of the coax, because the losses of coax at an SWR of 11 would eat lots of power...

    The only hope is to do the tuning at the feedpoint using a remote tuner or at least a series inductance to cancel the jX=-153 of capacitive reactance.

    The tradeoffs:
    1. 33.5ft dipole, suffer the slightly elevated coax loss when it is operated at a SWR of 1.4
    2. Add tuner or inductor to bring SWR to near 1. The losses in the inductor will likely be higher than the power saved by bringing the coax SWR down from 1.4 to 1.0
     
    Last edited: Jun 28, 2020
    KU3X likes this.
  5. KI8DJ

    KI8DJ Ham Member QRZ Page

    Just trim it for lowest swr, no need to overthink this. It's what almost every ham does.
     
    AK5B, W9WQA, AG5CK and 1 other person like this.
  6. KE0EYJ

    KE0EYJ Ham Member QRZ Page

    It must be my mistake, then.... just oddball happenstance. I use the Antenna Tool app, and the wire dipoles I cut with it end up at about 1.4 to 1 SWR. From that, I've trimmed them. I could almost swear I had better luck when they were 1.4 , than at a near perfect SWR.

    I never did an A/B comparison though, so I'm sure it's just in my brain. Thanks for the comments.
     
  7. KK4OBI

    KK4OBI Ham Member QRZ Page

    There is another way to lower impedance... angle. If one arm of a dipole is elevated or lowered, impedance goes down. Matching 50 ohm coaxial cable is simple.
    Angling an arm causes a slight reduction in overall length so a slight lengthening of that arm may done if exact tuning is desired.
     
  8. WT4W

    WT4W Ham Member QRZ Page


    THIS.

    Simply trim your antenna for minimum SWR at the midpoint of your desired frequency range and if that minimum SWR is acceptable you're done. An SWR of 1.4 is not bad at all and not worth worrying about.
     
  9. KL7AJ

    KL7AJ XML Subscriber QRZ Page

    If you look at a Smith Chart, it becomes very evident that the SWR changes much more rapidly with reactance than with an impedance mismatch. You almost always "win" by aiming for resonance rather than a perfect match. There are exceptions, of course, Such as with very short mobile whips, where the minimum SWR does NOT normally occur at resonance.
     
    AK5B likes this.
  10. N1BCG

    N1BCG Ham Member QRZ Page

    You can derive a 50Z feedpoint using an inductor at the feedpoint. This produced a 50Z j0 match at 52 MHz and less than a 1.7:1 at 50 and 54 MHz:
    6 Meter Dipole.jpg

    Note that one element needs to be shortened since the matching inductor also acts as a slight loading coil for that half of the antenna. In addition to dropping the best match from 1.5:1 (without the inductor) to 1:1, it also reduced the reflected power at the band edges.

    Feedpoint.jpg IMG_3025.JPG
     
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