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BTV vertical- adding coil to counteract capacitance

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by NG9F, Dec 9, 2018.

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  1. NG9F

    NG9F Premium Subscriber QRZ Page

    So I am playing around with a coil which as I understand it will lower the SWR on 40 meters because shortened verticals have too much capacitance, and an inductor will counteract that. But some weird things are happening. Or maybe not- maybe I just don't have a clear picture of what is going on.

    My antenna is basically a 4BTV with an added 30M trap. I don't have the 80M resonator installed right now. Now here is a potential issue. When installing the 80M resonator, you are told to remove the spider assembly so I did. Where it is I can't be sure, but it's not on the antenna.

    So we have a 4BTV +30M trap and no spider assembly. It is resonant at 7.050. Here is what the RigExpert AA-30 tells me about it as it is:

    SWR= 2.0
    Z= 59.6 ohms
    R= 47.7 ohms
    X= -35.5 ohms
    C= 634pF

    I have a coil on 3/4" pvc with 15 turns tightly wound of THHN 14AWG solid wire. Temporarily installed it looks like this:


    Once installed the RigExpert tells me this:

    Frequency: 7.050
    Z= 54.5
    R= 40.2
    X= -37.1
    C= 609pF

    So OK, that tells me that some of my capacitance is gone. But just a tiny bit of it. I will need more turns. Here is the funny thing (to me)- the SWR went up, not down. And this is because the resonance shifted down to 6.965. So here are the numbers there:

    Frequency: 6.965
    SWR= 1.9
    Z= 88.7
    R= 85.2
    X= -25.0
    C= 910pF

    If anyone can fill in the gaps in my knowledge I would be very grateful! Here is another way to view that data:

  2. NG9F

    NG9F Premium Subscriber QRZ Page

    I found an article that was using 5 turns of solid 12AWG wire on a 1" form and I was all like "yeah, i will just use this 3/4" and change the inductance by adding more turns. Is this where everything went haywire, or was I on the right track?
    KA0HCP likes this.
  3. NG9F

    NG9F Premium Subscriber QRZ Page

    I am thinking about shortening that long wire and clamping it directly to the (grounded) bracket of the bottom of the antenna- would that be a good thing to try?
    KA0HCP likes this.
  4. KA0HCP

    KA0HCP XML Subscriber QRZ Page

    -Yes, changing the inductor diameter changes the inductance.
    -Having long leads on a component add inductance and capacitance. Keep leads as short as possible. Keep the inductor away from other components (radials) which will affect inductance and capacitance.

    Once you have added some amount of inductance, you may need to adjust the antenna length.

    Lesson: Coils are not just inductive; they have self capacitance as well, and are affected by surroundings inductively and capacitively.

    "Everything is interactive!". :)
    NG9F likes this.
  5. NG9F

    NG9F Premium Subscriber QRZ Page

    Thank you for that reply!!! Here is the latest incarnation of my beast:


    And now that I've seen your post I think you will tell me to shorten the leads more! At any rate, here are the numbers based on the new resonance (well, old resonance of 6.965. I guess I should check resonance again):

    SWR: 1.9
    Z= 86.9
    R= 82.7
    X= -25.7
    C= 884

    Let me gather more data!
  6. NG9F

    NG9F Premium Subscriber QRZ Page

    OK, yes, resonance did shift again by cutting the leads shorter and re-clamping. It went from 6.965 down to 9.950. I did the numbers at 6.950 and here they are:

    SWR: 1.9
    Z= 91.3
    R= 89.8
    X= -15.6
    C= 1470pF

    The coil is very close to the coax that connects the antenna to the bulkhead on the radial plate so that's probably not good either. I will shorten the leads more and report back.
  7. KA0HCP

    KA0HCP XML Subscriber QRZ Page

    Returning to your original post, and the original configuration....

    1. NO, the antenna is not resonant at 7.050 Mhz. X is -35.5 which is NOT resonant. When resonant, a circuit (antenna) has X = Zero (0).

    GO BACK TO "GO", do not collect $200. Remove your coil, and start over...
    Ignore Z, Ignore SWR, Ignore C, for now.

    1. Adjust your antenna length until X = Zero (0) at your desired frequency of 7.050.

    2. Once you have that, then we can discuss adjusting the antenna to obtain Z = 50 Ohms and minimum SWR.

    Edit: Reminder: When tuning trapped verticals, start by adjusting the length of the highest band (10m) then proceed to 15m, down to the lowest band last. That is "Adjust length for resonance at desired frequency on each band; X = Zero (0). There may be some interaction, so several iterations may be required.

    Once you have X = Zero on each band, then we can discuss getting Z = 50 Ohms, and a minimum SWR.

    **You have gotten ahead of yourself and are fighting interacting factors.** bill
    Last edited: Dec 9, 2018
    WB5YUZ and NG9F like this.
  8. NG9F

    NG9F Premium Subscriber QRZ Page

    Oh, wow. OK. So here is my misconception. I would scan with the RigExpert, and move the tubing until the lowest SWR was where I wanted it. And it looks like you are telling me this was dead wrong. I did start with 10 meters and work my way to the lower frequencies when I tuned it. I just never looked at X and didn't know I wanted X=0. Thank you Bill! Much appreciated.
  9. NG9F

    NG9F Premium Subscriber QRZ Page

    Last pic before I take the coil off and start over:


    lowest SWR: 6.946
    SWR= 1.8
    Z= 90.3
    R= 89.6
    X= -12.6
    C= 1840pF

    My X is so close!!!!!
    Last edited: Dec 9, 2018
  10. NG9F

    NG9F Premium Subscriber QRZ Page

    OK, I took the coil off. I'm running out of weekend at this point and the XYL has one of those lists. You know the type. But I am going to spend some time reading up on X and resonance and see if I can fill in some of the blanks for this next weekend. And in the meantime I can still make contacts with CW on my non-resonant vertical at 7.050! Thanks again Bill.

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