Antenna inductors, coil/choke

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by KE8HAG, Mar 18, 2021.

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  1. KE8HAG

    KE8HAG Ham Member QRZ Page

    I've recently become interested in antennas and how and why they work. I ran across a old, what looks like a marine antenna, in a 15' fiberglass pole. I took it apart and found a very confusing configuration. The feed is 50 ohm. The feed point is a RG-58 A/U coaxial connected to, or looks like, a small J-Pole made from #10 wire abt 20" long. The coaxial is connected the same as a J-Pole and was obvious that connection was used for tuning. Connecting to the drive element of this J-Pole is a series of four 3/16 brass elements separated by three air coils/chokes 3/4"D x 6"L made from #10 wire, with coil spacing of 1/4", coils are spaced, or placed, between the elements, except the junction of the J-Pole to the first element, approximately at 1/3rd intervals, making the a total e.element length of 14', in series.

    Don't know the mfg. Looks like it might be a Shakespeare. It has a 14" long aluminum casing at the bottom of the antenna that is obviously acting as a capacitor of some sort to the antenna inside.

    I'm thinking this antenna is a VHF or UHF antenna so I stood it up on a wooden platform about 30" off the ground in a clear area and used my MFJ 259b to see. Sadly, I didn't get a dip at any frequency, could be the coaxial is shorted

    My question is; How are the air coils being used? Are they traps? Are they loading coils to lengthen the antenna. Are they using the coils to get multi band operation?

    I'm needing some guidance. Any help will be appreciated.
  2. M0AGP

    M0AGP Ham Member QRZ Page

    Might be worth posting a photo? Easiest way is to get it onscreen on your laptop then use "snipping tool" to grab the pic and you can paste it directly into the message box when you're composing a post.
  3. WA7ARK

    WA7ARK Ham Member QRZ Page

    Also, the reason you might not be getting a "dip" is because the antenna expects a "ground plane"...
  4. KE8HAG

    KE8HAG Ham Member QRZ Page

    I can try. I'm not good with camera's. Don't own a laptop but we'll see what we can do.
  5. G3YRO

    G3YRO Ham Member QRZ Page

    I can't exactly picture the configuration of what you are describing . . .

    But given the size of the coils you describe, I imagine this is an HF antenna, not VHF.

    When mounted on a boat, it would also have an excellent ground plane (due to the salt water), so it would expect that to work properly too. (just like an HF Mobile whip expects to see a car body to resonate against)

    Roger G3YRO
  6. KE8HAG

    KE8HAG Ham Member QRZ Page


    I got it working. Found several soldered connections that had broken loose in the elements to inductor connections.

    Connected my NanoVNA to sweep the antenna and found that the antennas resonant frequency is 220 MHz , 1.2 swr @ 50 ohms impedance. Under 1.7 across the 1.25 m band.

    Even knowing this doesn't help me understand how the the inductors play a roll in this antenna design. I've googled the subject of inductors beings used in antenna designs but Ive not found anything explaining several inductors being used in series on a supposedly one band vertical antenna.
  7. M0AGP

    M0AGP Ham Member QRZ Page

    Great you got it working! I wonder if those coils might act as traps (via self-resonant freq)? Could it be a multiband antenna? Maybe not - I expect you probably did a pretty wide freq scan...
  8. KE8HAG

    KE8HAG Ham Member QRZ Page

    Pictures of antenna:

    20210319_102810.jpg 20210319_103123.jpg 20210319_103156.jpg 20210319_103206.jpg 20210319_103219.jpg
  9. K3EO

    K3EO Ham Member QRZ Page

    Sounds like you have a 5 element VHF collinear antenna there. The coils are called phasing coils and their purpose is to insure that all of the radiating element currents are in phase. I suspect that the antenna you have is just for a single band (222 MHz).
    Here is a link to a website where the author constructed his own J pole collinear vertical:
  10. KE8HAG

    KE8HAG Ham Member QRZ Page

    Many thanks K3EO. ...I think your right.

    At least I have something to work with and have some home work to do. If and when I get a better understand of this antenna, I'm going to try to make it into a 2m. At the very least, I'll get a good education or a very bad headache... LOL

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