A Full-Blown 1/2 Wave Dipole?

Discussion in 'The Low Bands - 630/2200 Meters - VLF' started by WA2FXM, Nov 27, 2017.

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  1. W9XMT

    W9XMT Ham Member QRZ Page

    Below is a NEC4.2 plot of the elevation pattern fields at an h-distance of 1 mile from the base of a monopole, based on the same model parameters as in my original post above. A comparison is shown for that sector for the fields at 2 miles, also. These calculations include the surface wave, which is necessary to calculate/plot this scenario accurately.

    Z = elevation in meters above a flat Earth, and E = field intensity in mV/m

    Note that the two fields shown there have a ~1/r relationship, which is characteristic of a space wave. That relationship will be maintained for very low, but not zero-angle propagation paths reaching to, and reflected by the ionosphere.
    _____

    As a workaound, a Ø can be included in posts here by entering Alt+Ø216 using the number keypad.


    [​IMG]
     
  2. K8JD

    K8JD Ham Member QRZ Page

    Does this mean that the closer you get the stronger the field is ?
    What is the size of the monopole ? By "T" that seems to mean it's less than a QW tall vertical with a two wire capacity hat ?
    Not much info on the physical side of the antenna !
    I want to build a real antenna.
     
  3. W9XMT

    W9XMT Ham Member QRZ Page

    The T antenna dimensions are shown in the Comments section on the left side of the graphic in my original post about it.
     
  4. W5DFN

    W5DFN Ham Member QRZ Page

    Okay, an update to the above post. About the time I posted this, we sold our home and moved to the property where we own and operate a private gun club. On October 10, Hurricane Michael paid us a visit setting back getting back on the air. This week I got my HF/VHF antennas back up and today I put up half of my 1/2 wave 630 meter horizontal dipole. Running the "live" leg was fairly easy. Take out four damaged oaks, and lay it out in a zigzag. The shield leg came straight down from the 20' feed point and is grounded to the tower base. Guess what it works!
     
  5. AF7XT

    AF7XT Ham Member QRZ Page

  6. AF7XT

    AF7XT Ham Member QRZ Page

    shunt feed ...
    [​IMG]
     
    WA7PRC likes this.
  7. WA7PRC

    WA7PRC Ham Member QRZ Page

    The Wikipedia article leaves a lot of detail out. I toured NLK in the late 70s. Nominal output is around 1MW but, we were told they've had it cranked up to 2MW. When they do, Jim Creek is 1° warmer downstream. When we toured, the output current was 1296A. Into ZLOAD = 0.456Ω, that's "only" 766 kW. It made the broadcast xmtr (16 kW RFOUT) I worked on look like a QRP rig. :cool:
     
    K3RW likes this.
  8. K3RW

    K3RW Ham Member QRZ Page

    Another solution--build a mag-loop. It really works, but it is obviously quite expensive depending on the HOW. There is a cheap way, and a really expensive way. But you want 5w EIRP. I wanted very-high QRO efficiency. So in some ways this is a much easier problem to solve.

    For example, you can build a 1/4 wave mag-loop and engineer it with an appropriate range vacuum variable capacitor, all-copper tubing (big 2" stuff) and silver solder it. And run decent coax to it and fire away. I did that on 30m. It wasn't hard. I was planning to scale it up to 160m but I needed a different vacuum variable, about $400 in copper tubing, and a trailer. Then I got bit by the VHF contesting bug. But I never gave up on it.

    Since a 1/4 wave loop would be huge, go with a multi-turn loop. Its just like a Slinky but with tube elements (not solid ones).

    [​IMG]

    https://www.radioworld.co.uk/mla-t-portable-magnetic-loop-antenna-160-40m

    Here's a commercial version of a 160m mag-loop that's 100w capable. Its a freaking fortune. But total cost wouldn't be anywhere close to that expense if ham-built. If you don't want the loop to be big in diameter, it just needs to have a lot more turns. So there's the complexity of it. You'd need a vacuum variable with enough tuning range for the bandwidth you'd want. The rest is just aesthetics. I don't pretend that it would be 'easy' to build this.

    A really long wire thrown almost on the ground, and impedance matched to death, gets on the band, but its lousy. But I can't argue that if you want to just get on the band, do it cheap and make the 'other guys' antennas do the heavy lifting. I can do the same thing and work my friend across town on the band. And wires are cheap compared to refrigerator tubing.

    But, if your budget permits, build something effective.

    If a loop is out of consideration, I've been reading about 'trapped radials' for top band antennas. A lot work to build BUT they theoretically function. Main element ideally 43', but go as big as you can if not. Plan on a big capacitance hat. Vertical would NOT be ground-mounted. Get it up about 10 feet off the ground, elevate 3-4 of these matched 'trapped radials', and use the biggest loading coil and capacitance hat you can do, and match to 50ohms. That eliminates the ground-plane-as-big-as-your-neighborhood problem. But those need to be fairly close traps and I am not good at building those. But before I had a 6m beam, I used a 33in whip on my screwdriver antenna, with 1/4wave radials that were elevated. Running 20w I worked a bunch of distant states--it was all their antenna doing the heavy lifting, but my signals were great with just an elevated and resonant radial set.
     

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