Shunt coils

Discussion in 'Mobile Radio Systems' started by KI4KGR, Jun 14, 2018.

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  1. KI4KGR

    KI4KGR Ham Member QRZ Page

    I can find plenty of information on how to build and tune shunt coils for a screwdriver antenna. The thing I can't seem to find are diagrams and sample pictures of attaching the shunt coil. I've also noticed that neither the Tarheel II or Diamond SD330 do not mention them at all.

    Are they absolutely needed or is it try it without one first and if there are issues this is a possible solution?
     
  2. K4KWH

    K4KWH Ham Member QRZ Page

    In my experience, yes, they are needed--especially if the antenna has any efficiency at all. That's because the feedpoint impedance of a high Q antenna is quite low--about 18-20 ohms. So that's one of the things the shunt coil does; transform that impedance up to 50 ohms.

    You see, there's 3 impedances in an antenna: also a form of resistance. feedpoint, coil--even the atmosphere (whip) around the thing. This is just a simple way to explain it. There is more to it, but it gets the newbie (if you are one) kinda started on the road to understanding the whole picture. So if we have a natural resistance of 18 ohms at the feedpoint, 3o ohms in the loading coil, and 4 ohms around/in/whatever the whip, (radiation resistance) add it up. Voila! It adds up to 52 ohms! SO! We wind a coil to make up the missing part of the picture.
    You can do this yourself with some soft wire of.............8, maybe 6, ga. Use copper or something that won't rust. Experiment. You'll find that winding about 10 turns around a broomstick;) ought to work fine. Place one end of the coil in series with the base of the antenna, the other goes to nearby ground. Now, don't be fooled if you check the system with a VOM and show a direct short! It is NOT a "short". That coil is a potential (RF ground, not DC ground) to ground equal to whatever resistance the coil happens to possess. This is what makes up for the lower impedance of the high efficiency antenna (Tarheel, Bugcatcher, etc).

    The principal works in reverse for the hamsticks where they usually don't need very much feedpoint matching. That's because most of that resistance is up in the center loading coil itself. The radio doesn't care where it "sees" the 50 ohms; after all, it'll load into a 100 watt resistor (dummy load) all day long. You won't have much of a signal, but the radio will be happy! The hamstick is a trade-off (especially at lower frequencies as efficiency DECREASES, and the natural impedance INCREASES as you go up in frequency. (MORE coil, more windings, more LOSS goin' down---Less coil, fewer windings, HIGHER impedance goin' UP!:) So at low frequencies, a shunt *may* not be needed. YMMV!;)

    This is not intended to get into deeply technical stuff, just to kinda break it down so we can grasp it. It isn't entirely accurate, but close. Hope it helps.

    73
     
    KC8VWM likes this.
  3. KI4KGR

    KI4KGR Ham Member QRZ Page

    That makes sense and building a coil looks pretty straight forward. My plan was to try and wrap one around a piece of 1/2 PVC and heat shrink it in place when I was happy with the results. But..... How do you go about making the physical connections? That's the part I can't seem to find anything on. The images I've found are either way out of focus or their are so close up it's hard to see the context that surrounds the picture.
     
  4. AI3V

    AI3V Ham Member QRZ Page

    may I suggest you read "Navy neets module 10" (section 3).

    I think you will find the explanation of feedline and antenna behavior to be a real "a-ha :)"

    Please pay particular attention to the cyclical nature of impedance vs wavelength/frequency, this in my opinion is the key.

    Rege
     
  5. W5DXP

    W5DXP Ham Member QRZ Page

    The coil is connected electrically from the inner coax conductor to the outside braid of the coax at the antenna feedpoint. How it is connected physically depends on the particular installation. I use alligator clips for my coil.
     
  6. K0BG

    K0BG Platinum Subscriber Platinum Subscriber QRZ Page

    Jerry pretty much nailed it. If you make and mount the coil correctly, you can adjust it so the average screwdriver will present a low impedance (≤1.6:1) from 80 through 10 meters. The highest SWR it typically on 40 meters. A different coil (or none at all) is required for 160 meters.
     
  7. AA7QQ

    AA7QQ Ham Member QRZ Page

    If using 40 & 80 meters, some like to balance the response of the coil, toward their band more used. Some keep it in the middle.
    Some act like this. Not sure of all screwdrivers.

    Ed
     
  8. K0BG

    K0BG Platinum Subscriber Platinum Subscriber QRZ Page

    Yes ed, but the difference is less than 2%, so who cares?
     
  9. VE3CGA

    VE3CGA Ham Member QRZ Page

    I had about 8 turns of solid copper house wire on a hunk of black 2-1/2"pvc
    it attached just like everyone says. I only had to use it on 80M to get a good match.
    The little box that housed the base for the whip also had a binding post for the coil while the
    ground side was a large alligator clip.
    Didnt use that in the winter since we put more salt on the roads than fast food joints use on their fries
     
  10. K4KWH

    K4KWH Ham Member QRZ Page

    Don't know which antenna you are using, but one of the advantages of the screwdriver is that one doesn't have to "fiddle" with the shunt coil (changing taps, etc) much at all. Once its set "right", one can leave it alone. One setting usually does the trick! :) That's the reason I"m so sold on the screwdrivers. They are so flexible and versatile. When I had the old Henry Allen Texas Bugcatcher, I used to have change taps. I never could find a "sweet" spot; one size fits all. Ten meters used no matching at all (naturally). When I got my first screwdriver I quickly discovered I didn't have to do that. I'm gonna leave it to Alan to explain the "whys" of that!;)
     

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