Increasing 1/4 wave antenna B/W

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by VK5OM, Jan 15, 2021.

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  1. RW4HFN

    RW4HFN Ham Member QRZ Page

    Last edited: Jan 16, 2021
    AH7I likes this.
  2. AK5B

    AK5B Ham Member QRZ Page

    Yes, that's the idea. I made mine (10-15-17-20m) with a 13' fishing pole + driveway marker (small extension to support the 20m element), wooden dowel standoffs about 4" long hot-glued to the pole and wires, 14 AWG insulated wire (THHN) for the elements and a ring section of 1.5" copper pipe drilled for each element and a small copper radial plate that also fit over the pole/support insulated by another ring of PVC. I took lots of photos of my elevated Frankenstein Fan Tenna but since it was painted and constructed for stealth in the jungle it is harder to see in the photos than described herein.

    I tuned mine in the back yard before installing further beyond in the bush and started with the longest element first, worked my way up to 10 meters as the last to tune.

    Basically, it was a breeze (I love my RigExpert:)) and the elements didn't interact to any noticeable agree---I probably could have spaced them closer without much difference, but I'm glad I went this fan route very much. Verticals work well for me here in the #30 ground zone close to saltwater and I may build another with 40/80m inverted L elements if another antenna planned here doesn't materialize.

    Callum, M0MCX's "DX Commander" antenna is the multi-band commercial equivalent that is worth looking into for purchase or homebrewing ideas---he came up with a very nifty system that is constantly receiving highly positive reviews. But unlike the DXC, my homebrew four-bander was elevated above ground about 8 feet which no doubt helped with ground clutter and getting rid of some ground loss as well.

    73,

    Jeff
     
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  3. VK5OM

    VK5OM Ham Member QRZ Page

    Thanks Zac
    First sensible reply,
    I have 4NEC2 and thats on a steep learning curve at the moment. Everything I have done is with 1.25mm wire and I never thought to change that or the material from copper to aluminium.
    There are a lot of other suggestions however it seems yours takes the cake.
    Cheers
    David VK5OM
     
  4. K0UO

    K0UO Platinum Subscriber Platinum Subscriber QRZ Page

    Going from a number 12 gauge wire to a 18-inch face Tower on a 40 or 80 metet vertical does increase bandwidth.
     
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  5. VK5OM

    VK5OM Ham Member QRZ Page

    Any figures to mention to be able to derive a formula or was it just a noticeable increase?
    Cheers
    David VK5OM
     
  6. K0UO

    K0UO Platinum Subscriber Platinum Subscriber QRZ Page

    Pretty much flat SWR for 200kHz with the tower just 25 to 50 kHz with the wire
     
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  7. KM3F

    KM3F Ham Member QRZ Page

    If your using an antenna that has traps, that is limiting bandwidth, inherently. Traps increase the antenna overall Q or sharpness.
    To reduce Q, the surface Resistance of the element/s has to be reduced in some manner.
    The effect is like paralleling two resistors of the same value. Example 2 100 ohm resistors in Parallel = 50 ohms.
    In the RF world, this reduces the surface loss and reactance as diameter or "surface area" (not cross section) of the Antenna is increased. Called skin effect.
    The main point being the reduction of "reactance" and leaving less Resistance to absorb and radiate the power over a wider bandwidth with out having to account for reactance by using a Tuner.
    .
    If you soldered a bunch of Soda Cans end to end to make a Dipole, it likely would have a wider bandwidth just from the larger diameter of the Cans that are hollow inside.
    A Vertical antenna made from large diameter Rain down spouting would show a wider bandwidth than a Wire , if you looked at both on a VNA .
    With a VNA you can see the Resistance, Reactance and which sign it is, bandwidth and SWR values, all separately for the same antenna installation.
    Quite an educational exercise.
    It just Physics we have to work with.
    I thought you needed to know what's behind all this because it's a bit complex and interrelated.
    I just demonstrated all this on a VNA to another ham using a lumped constant circuit that mimics a real antenna. and how the Smith Chart shows all the same info if one able read the display.
    IMO using a program is a short cut if you don't know what's behind what you see for it's answer providing correct input data is provided..
    Good luck.
     
    Last edited: Jan 17, 2021
  8. W9XMT

    W9XMT Ham Member QRZ Page

    Below using NEC4.2 is an example of the change in SWR with frequency of a fixed length dipole for the 75/80m band.

    The lowest SWR for the 0.375" conductor shifts downward in frequency, but is about the same as with the 0.125" OD conductor at its lowest value. However the SWR of the larger OD dipole is lower toward the ends of this r-f spectrum than that of the smaller OD dipole.
    [​IMG]
     
  9. KM3F

    KM3F Ham Member QRZ Page

    From XMT's plots you can see the difference between 1/8" and 3/8" diameter is not very different for practical advantage at the edges of the band where it changed from SWR 4 to a bit over SWR 3.
    Look at the lowest SWR area in the near center area, is in a bit broader but it's not worth the extra weight of the antenna, if it were tried.
    Just an added practical use point of view that the diameter will have to be made huge in comparison to a wire and still, the larger size can be a problem keeping it in the air.
    Good luck.
     
  10. KK4OBI

    KK4OBI Ham Member QRZ Page

    With an elevated vertical antenna two or more radials of the same length are needed. These are tuned to the desired resonant frequency and will produce a circular radiation pattern. SWR is adjusted by the angle of the radials. If wider band width is desired the most simple method is to shorten one radial and lengthen one radial. (This two-frequency principle is why a Moxon antenna has such wide band width).

    An elevated four-band trap vertical would need at least four sets of radials. The width of each band can be adjusted by detuning two radials to be slightly above and below the desired resonant frequency. For a smaller antenna, radials can be loaded too. Roll your own or buy Hamsticks for 6, 12, 17 and 30 meter bands.
     

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