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6BTV 80M broadband idea

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by WA4SIX, Nov 13, 2019.

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  1. WA4SIX

    WA4SIX Ham Member QRZ Page

    I am just going to try broadbanding with what I have in stock at the moment. It will still be several days before I get to it.
    Bought the house in August. Just got here recently. Hot water heater, clothes washer & AC went out. I have to keep the XYL happy & get them all fixed first.

  2. KH6AQ

    KH6AQ Ham Member QRZ Page

    Four whips on one RM-80 will tune it somewhere around 2 MHz. It can then work on 160 meters. The tuned circuit consists of the RM-80 inductance (~150 uH) in series with its loss resistance (~30 ohms) in series with the whip capacitance. Four whips will roughly quadruple the capacitance.

    Replacing the RM-80 with an RM-40 and 4 whips can resonate it in the 80 meter band. With this you have about 1/4 the reactance, which with everything else equal, wouold quadruple the VSWR bandwidth. But, the RM-40 loss resistance is less than that of the RM-80 and you will get about double the VSWR bandwidth. What you give up with the RM-40 arrangement is 40 meters. While the RM-80 isolates the whips at 7 MHz the RM-40 does not. I've done this before and made/sold the TopTenna top hat for BTV verticals. A similar product exists as the DX Engineering HotRodz.

    What should work to broadband the 6BTV on 80 meters is three or four RM-80 resonators. Hustler makes the VP-1 adapter to mount three resonators; $10.99 at DX Engineering. RM-80s are priced at $35.99.
  3. WA4SIX

    WA4SIX Ham Member QRZ Page

    What I intend to do is use 5 whips of varying length. They will be vertically oriented in about a 1 1/2& square. I will be using a slug of 2" aluminum that will clamp on the top of the RM-80, with 4 vertical holes for whips. They will all be in the range of the DXE 6BTV adjustment range.
    To start, I will use the slug clamped on with the factory whip. I will run it through the NANOVNA. This will be a baseline.
    Then, I will add the 4 whips of shorter & longer measurements to see what the NANOVNA says.
    My thoughts are it is like an 80 meter dipole with fan ends.
    It's just scrap & spare whips. I have the drill press, drill bits & 10-24 taps. Just have to get some set screws at Harbour Freight.

    It may work & it may not.

    If it does, I will add some form of plastic spacer towards the midpoint of the whips.
    If I can raise it from a theoretical 80 KHz to 400 KHz over a decent radial plane, that will be pretty good to me. If I want wider, I'll pull some radial, hihi

  4. K7TRF

    K7TRF Premium Subscriber QRZ Page

    If you think about this a bit or model it you'll find it doesn't work.

    A set of parallel (fan) elements work if there's implicit impedance switching between them meaning on the set of frequencies where one is active the others present a relatively high impedance and very little RF current flows into them. This is what happens when you fan multiple element sets from the feed point or from a location close enough to the feed point electrically that this implicit impedance based switching occurs. But if you try to run most of an 80m dipole and then just fan a bunch of different length elements out at the far ends you don't get much impedance differentiation and they all end up carrying substantial RF currents. IOW, they set of fanned element tips just becomes a cap hat that lowers the overall resonant frequency of the dipole but doesn't give different RF current distributions into those different length end whips as a function of frequency.

    Same thing if you run multiple whips above the 80m resonator, there isn't much impedance differentiation between say a 2 foot and a 5 foot whip when it's way out at the antenna end electrically so they'll both carry substantial RF current and one is not implicitly selected vs the other when you change frequencies.
    AK5B likes this.
  5. WA4SIX

    WA4SIX Ham Member QRZ Page

    I always value your input, but just have to try. Who knows, I might broadband it just by caging it a bit.
    We shall see.

  6. K7TRF

    K7TRF Premium Subscriber QRZ Page

    I understand, I build things just to see all the time and believe me I'd love to be wrong about this as it would open up some great possibilities.

    FWIW, here's a model of an 80m dipole with the addition of fanned tips.

    Here's the basic dipole at 120 feet long up at 50 feet:


    Now add a pair of fanned tips that are longer:


    The resonant frequency shifts down but there's little to no change in SWR bandwidth and you certainly don't retain a good match at the original higher frequency while adding the new lower frequency. Notice the currents in those fanned tips is about the same in the shorter and longer tip sections, there's not much implicit element switching going on.

    Attached Files:

    AK5B likes this.
  7. WA4SIX

    WA4SIX Ham Member QRZ Page

    Dang.... Still going to try. I'm just that way.

    K7TRF likes this.

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