nanovna testing vertical antenna resonance

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by MOBILE_BOB, Nov 21, 2021.

ad: L-HROutlet
ad: l-rl
ad: L-Geochron
ad: Left-2
ad: L-MFJ
ad: abrind-2
ad: Left-3
  1. MOBILE_BOB

    MOBILE_BOB QRZ Member

    ok so i have my vertical up, but have not laid down any radials as of yet

    my question is this prefaced on wanting to get a rough idea where the antenna is resonant on each of the
    switched sections of the antenna

    i am concerned that there might be a large shift in resonant frequency in going from no radials to a set of radials

    what i am trying or would like to determine is "roughly" where the antenna is resonant as i step through the switched sections.

    does going from no radials to a decent amount of radials shift the resonant frequency by a large amount?

    i guess i would like to test the lower section of the antenna for the 20m band and see how close it is to resonance? and then switch up to the 40m section, etc, etc.

    so for instance, if say the 20m band is 14.000mhz to 14.350mhz, and i test without radials and come in at maybe 13.000 mhz (which i would then add loading as expected to raise the resonance up to say the center of the band or about 14.175). or would the lack of radials throw things so far off, like say a reading of 12.000 mhz and with radials maybe 14.500mhz

    does the addition of radials have a huge effect on the resonant frequency? and by huge i realize that is a subjective term.

    would i narrow the gap by adding a temporary counterpoise? enough so to get in the ballpark of where the antenna is electrically?

    hope that makes sense?

    anyone?

    i guess i would just like to get an idea what the antenna is at each switched step in resonance, but if you guys think the readings would be so far off as to be not useful, then maybe i need to lay out a couple dozen radials first?

    thanks
    bob g

    edit: or should i at least drive a ground stake in the dirt, and test with at least that connection to the ground, would that narrow the resonant point between readings of ground stake vs. radials?
     
  2. W9WQA

    W9WQA Ham Member QRZ Page

    i would do somerhing
     
  3. KL7AJ

    KL7AJ Ham Member QRZ Page

    Radials will most certainly affect the resonant frequency some. Since you have a nice VNA, why don't you do a sweep before and after...it should be very instructive.
     
    MOBILE_BOB and WE4E like this.
  4. N8PKN

    N8PKN Ham Member QRZ Page

    I am guessing your vertical is ground-mounted, right ?
    Firstly, lay down a minimum of four radials, bury them just below the surface of the dirt, an inch or so would be fine. Don't be overly concerned with length, though around a 1/4 wavelength is a good start. Because the radials are buried they tend not to be as critical. Put a good current choke at the feed-point, 10-13 turns of RG-58, RG-8X or RG-400 through a FT-240-31 core. The choke will help to isolate your antenna from the coax, making tuning easier. Now you have a good start to check resonance. Adding more radials will actually lower your feedpoint impedance, getting near 35 ohms as more are layed down. This is because of the 90 degree angle of the antenna to ground. If you are mounting the antenna above ground, the radial lengths do become critical, and again , usually a minimum of four will suffice. Same current balun/choke at the feed-point for isolation.

    Good luck.

    N8PKN.
     
    AK5B, MOBILE_BOB and KU3X like this.
  5. W9IQ

    W9IQ Ham Member QRZ Page

    A clarification to the above post. Adding radials will lower the feedpoint real resistance (R). You can ignore the reactance (X) and impedance (Z) while adding radials. Keep adding radials until R no longer decreases appreciably. Once you have reached that point, then proceed with tuning the balance of the antenna.

    With a multiband antenna, it generally works out best to measure the radial effect at the lowest frequency of interest.

    - Glenn W9IQ
     
    MOBILE_BOB likes this.
  6. K7TRF

    K7TRF Premium Subscriber QRZ Page

    Yes, a huge amount.

    The radials are basically half of the antenna. Measuring a quarter wave vertical without some kind of radials or counterpoise is very similar to measuring half of a dipole. Prior to adding radials the shield of any coax you connect, the grounding of the analyzer itself and even your body if you're holding the analyzer become part of the antenna's RF Return via common mode currents.

    As suggested above you might learn a lot by sweeping the antenna before and after adding whatever radials/counterpoise you'll use but you can't really use RF measurements to do a rough trim of element lengths prior to adding your radials.
     
    AK5B and MOBILE_BOB like this.
  7. K8DO

    K8DO Ham Member QRZ Page

    If you think of the vertical as a bent dipole as opposed to something just a "vertical" it gives you a different perspective.
    1. Bent dipole with the bottom leg a few inches above the ground, A right side up L-dipole
    First thing you notice is that the bottom quarter wave leg is going to be pulled down in frequency from what it will be if 30 feet in the air. So the resonance of the entire antenna will shift down,

    2. Now let's raise the antenna 30 feet (resonant frequency goes up) and lengthen the bottom wire enough to bring the resonant frequency back down to match what it was just above the ground, What do we have?
    Well, let's straighten it out (ignoring that will also change the resonant point) and look.
    Voila, we have the beginnings of an off center fed dipole. (Or maybe it's someones aunt named Viola)

    3. Now bend it again and bring it down to the ground. Cut off the bent leg and feed it at the bottom using a metal stake shoved in the dirt as the missing leg . Notice I said dirt, not ground. There is a difference.

    4. Now it gets complicated. Dirt is a poor conductor for 40 meter RF, but it is a conductor of sorts. So what we have in the stake is a big resistor in series with the current flowing on the ground and trying to make up for the amputated leg. So, not only an OCF L-dipole but a really poor OCF L-dipole. Joe Ham might think it is a great antenna because it has a really great SWR (all across the band - dummy loads do) And he has a hard time contacting the stations he does hear.

    5. So what do we tell him to do? Of course, we tell him to put out radials. How does that help? Picture each radial as being connected through a different resister to the dirt. Put out 4 radials and you have 4 resistors in parallel. If you passed the Tech or general exam you know how to calculate 4 resistors in parallel. He thinks, "Gee, if four is good 16 will be great." SO he does - and his SWR goes to pot. Now he is mad at the world and he has to buy a "tuner" But he is contacting a lot more stations now.

    6. Back to your original question - what will the resonant frequency do? (whew, finally o_O )
    The answer is Not Much. It will probably rise some as radials are added, but not as much as you might think.
    The formula for a dipole is 468/F and for a quarter wave vertical is 234/F
    What will change greatly is the X-reactance. And that changes the frequency at which the lowest SWR that you see happens (and maybe interpret as a change in resonant frequency)
     
    Last edited: Nov 21, 2021
    AK5B and MOBILE_BOB like this.
  8. W9XMT

    W9XMT Ham Member QRZ Page

    As a start toward your research, below are the values calculated by NEC4.2 "far-field-only" analyses for the feedpoint impedance of two, base-fed, unloaded, vertical monopoles when driven against of a set of 15 equally-spaced radial wires, each wire 5 meters in length, and buried around the base of the monopole in 5 mS/m, dc 13 soil (average Earth).

    5-meter Vertical at 14.2 MHz
    Feedpoint Z = 42 -j18 Ω
    Gain = -0.6 dBi
    Peak Gain Elevation Angle = 28 °

    10-meter Vertical at 7.2 MHz
    Feedpoint Z = 56 -j25 Ω
    Gain = -2.1 dBi
    Peak Gain Elevation Angle = 26 °
     
    MOBILE_BOB likes this.
  9. MOBILE_BOB

    MOBILE_BOB QRZ Member

    thank you everyone for your thoughtful input, what a wonderful asset this forum is!

    i can spend hours and hours researching, and not find an answer to my question, (which i have), or
    come here and ask?

    now i have some idea's to work with, and your experience(s) are very useful.

    thanks again

    bob g
     
    AK5B likes this.

Share This Page