Tune for min SWR or zero reactance?

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by AB0DB, Oct 19, 2013.

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  1. AB0DB

    AB0DB Ham Member QRZ Page

    New ham here. Just built my first HF antenna, a half-wave dipole for 10m with an MFJ 1:1 current balun in the middle. At the moment it's 212" in total length because I haven't trimmed it yet. I'm checking it out with my new RigExpert AA-30 tuner at the end of 25' of RG-8. The tuner can display either SWR or R and X as a function of frequency. What has me confused is that the minimum SWR and zero reactance don't happen at the same frequency. Zero reactance is at 26.050 MHz, while min SWR (1.8) is at 25.280 MHz, where Z = 50 ohms, R=42 ohms and X=-27 ohms.

    So do I tune it so that SWR is minimized at the desired frequency, or so that reactance is zero?

    It will go into my attic, but since that's a painful place to work in, I'm currently doing my testing in the basement with the antenna only about 2 feet off the floor.

    Thanks!

    - Dave, K0QDB
     
  2. KL7AJ

    KL7AJ Ham Member QRZ Page

    AHHHHH! One of the first INTELLIGENT questions about SWR we've ever had on the Zed. Thanks immensely, and Welcome to ham radio!

    In MOST cases the minimum SWR will coincide with the resonance point...but not always. One will usually find that the SWR will rise more QUICKLY as a result of reactance than it will due to a resistive mismatch. (The beloved Smith Chart usually bears this out).

    The bottom line answer here is that if your antenna (or system) is SHARPLY tuned, you're better off tuning for resonance, even if the SWR is not absolutely flat at that point. On the flip side....if your antenna (system) has any reactance, the SWR can NEVER be absolutely 1:1.

    Eric
     
  3. KH6AQ

    KH6AQ Ham Member QRZ Page

    Adjust the antenna length for minimum VSWR.

    The transmission line is causing the complex impedance (R and X) at the point of minimum VSWR.

    If you were to change the transmission line length from 1/4 to 1/2 wavelength you would find that the VSWR at 25.280 MHz remains at 1.8:1 but that the impedance changes from 28 ohms resistive to 90 ohms resistive. At lengths between 1/4 and 1/2 wavelength the impedance at the point of minimum VSWR would have a reactive component.
     
    Last edited: Oct 19, 2013
  4. G4ALA

    G4ALA Ham Member QRZ Page

    Hi,

    Tune for neither, though both are near to the result. What you are really looking for is maximuim current in the antenna conductor.

    This is not so easy with arrays where antenna conductor access is resiricted, However, a field strength meter helps a lot.

    Cheers

    John G4ALA
     
  5. KE7TRP

    KE7TRP Ham Member QRZ Page

    This is great advice. For a learning exercise, start by using a Barrel connector and some random jumpers in lengths of say 1,3,5,6,9 ft ect. Add in the extra coax and take the readings again. You will see you have effected the readings a lot.
     
  6. WB2WIK

    WB2WIK Premium Subscriber QRZ Page

    I'd also raise it higher before making measurements and adjustments, as with the dipole 2' off the floor (or ground), its characteristics including resonant frequency will be different than with it elevated.

    In an attic, surrounding materials may influence it once again (usually they do). Once in the attic, if the "ends" are difficult to reach for major adjustments, you can cut the horizontal parts of the dipole intentionally a bit "short" and just use clip leads hanging verticall down from those tips (being careful not to let them contact any other materials), and make adjustments pretty fast just by adjusting the lengths of the clip leads. For indoor antennas not exposed to the WX or ever touched by anyone, such attachments usually work fine.
     
  7. KH6AQ

    KH6AQ Ham Member QRZ Page

    The complex impedance will change but the VSWR will not.
     
  8. AA7EJ

    AA7EJ Ham Member QRZ Page

    John has a point, and maybe even remembers using neon lamps or fluorescent tubes as "field strength meters".
    The tubes worked exceptionally well with Windom antenna HI HI HI. So did many other metal objects in the shack. ( Don't try that at home!)


    Since this is your first post , here are my advisories for you:

    1. Do not expect straight answers when asking about SWR, baluns or "balums".
    2. Do not expect correct answers when discussing LENGTH of transmission lines of characteristic impedance of 50 Ohms terminated to a dipole or any other antennas with feed point impedance CLOSE to 50 Ohms.
    3. Do expect many opinions when it comes to "how many radials " are needed for vertical antenna.
    4. Expect " I worked 450 DX countries with my setup , ergo I have a good antenna I bouhgt from ....".

    At SWR 1:1 ( 50 Ohms) the frequencies should match. At least good enough for government work.
    If they do not, as in your case , you have a great antenna for CW portion of 10 meters and so so antenna for phone HI HI HI . Just don't sweat the small stuff (much) , especially when you have a tuner, make some contacts anyway.


    Check out W2DU "Reflections x" for real answers. Pay close attention to Chapter 2 "True or false?"


    And just remind yourself that when you get it in the attic it will probably resonate on 27 MHz , good buddy!!!


    Been there done that , attic is no place to be working on the antennas during TX summer heat, but hurricanes are not much better.



    See ya on the air.
    73 Vaclav
     
  9. W5DXP

    W5DXP Ham Member QRZ Page

    There's another possibility besides lowest SWR or purely resistive impedances. Back in the 50's, my Elmer, W5OLV, tuned for maximum field strength and it was probably neither of those two points. :) Of course, he didn't own an SWR meter or an antenna analyzer. He just played with the dip and load settings on his Harvey-Wells.
     
    Last edited: Oct 19, 2013
  10. KE7TRP

    KE7TRP Ham Member QRZ Page

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