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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. WA2FXM

    WA2FXM Ham Member QRZ Page

    Is this something worth considering? 630 meters is brand new territory for me. All I've got to go on is theory, old books, new google searches, and smart people like you out there. I've got room for a half-wave dipole. For 472kHz that's 495 feet for each leg. That would fit in to the far corner of the hayfield to the SW, and down to a tree along the riverbank to the NE with the center up on the house chimney. According to the FCC draft guidelines for determining EIRP the 5W limit is based on transmiter power output, antenna gain, and transmission line loss. According to Wikipedia a half-wave dipole has a gain of 2.15 dBi. Assuming negligible losses in 25 ft. of coax for transmission line and using a calculator I found at EverythingRF.com, an output power of 3 watts with 2.15dBi of gain gives you an EIRP in dBm of 36.92, which converts to 5.01 watts. So, back to the bookshelf; for 3 watts of power output any of the simple hollow or solid state cookbook examples of oscillator\buffer circuits could be built to feed the monster dipole and I'd be up and running. Yes? Maybe? Don't bother? Any ideas or comments?

    Mark WA2FXM
     
  2. NO3M

    NO3M XML Subscriber QRZ Page

    2.15 dBi is freespace gain. Over perfect or real ground, it will be higher, about 9-10 dBi. Assuming an efficiency between 90-98%, you'll only need 0.5 - 0.7 W transmitter output to hit 5W EIRP. Most of the radiated energy will be high angle, half-power points (- 3 dB) at 44 deg and above, so it won't be a very effective antenna beyond a few to several hundred miles.

    The better option for "DX", which includes trans-continental work, would be a top-loaded vertical. More of your radiated power will be low-angle.
     
    K2WH likes this.
  3. KD8DEY

    KD8DEY Ham Member QRZ Page

    just think of how long it would be using "ye olde formula"...
    Yowser :eek:
     
    KC8VWM likes this.
  4. KB4QAA

    KB4QAA XML Subscriber QRZ Page

    Put up a loaded vertical for transmitting.

    For receiving, use those acres to install a set of bi-directional Beverages which will give you coverage in four directions.

    Operators on The Band Formerly Known as "Top Band" (160m) have clearly shown separate vertical Tx and directional Rx antennas are the best performing stations.

    p.s. I probably could not resist the opportunity to try the dipole, anyhow. It's relatively simple and quick to install. It WILL work. It will probably receive far better than a loaded vertical.
     
  5. WA2FXM

    WA2FXM Ham Member QRZ Page

    I know. I've got 2 - 500 ft.spools of #14 THHN sitting under the workbench. It would be an easy test setup anyway to start out with. I haven't got any nearby trees or supports for top-loaded T's, fans or cages. I do have my 40 meter vertical on the ground in the backyard relatively close to the house. Without any kind of top loading would just base loading work on that?
     
  6. KB4QAA

    KB4QAA XML Subscriber QRZ Page

    [Edited after re-reading your question about modifying existing 40m Vertical]

    I have not seen any examples of modifying 40m verticals.

    Certainly there are many stations using base loaded verticals. Examples are posted on various sites.

    If your vertical needs guying, you can make the upper portion of the guys into a capacitive top hat (think of an umbrella). Of course you will still need base loading, but the top hat will increase the current in the vertical element, just as it does on HF antennas.

    We are very much in an experimental stage with equipment for 630m. All that can really be said is look at what others have done, and adapt for your equipment and situation. There are no store-bought solutions, yet.

    There are lots of ways to skin the cat! :)

    [Edited to answer question]
     
    Last edited: Nov 27, 2017
  7. KD8DEY

    KD8DEY Ham Member QRZ Page

    Hmmm, 2 500 foot spools?....
    Sounds like the "Makins" fer a full wave loop cloud warmer....
     
  8. NO3M

    NO3M XML Subscriber QRZ Page

    A 40m (33ft) vertical without top loading and a mediocre radial system will need 1.5kW+ TPO to hit 5W EIRP. There is a modeled example of that very antenna in my PDF on receive antennas (posted in another thread). There will also be a very high RF voltage at the base. Of course, the rules limit TPO to 500W.

    As KB4QAA suggested, you'll want some top loading. 33ft vertical is a very workable height with top loading and a decent radial field. 4-8 top wires will help a lot, just hose clamp them to the top of the vertical. Length all depends on how much slope there will be. Don't let the ends go much below 66% of the total vertical height (ie. keep them above 22ft at the end of the wires).
     
  9. N5WVR

    N5WVR Ham Member QRZ Page

    It's always good to experiment. However, I have read that dipoles do not perform well down at LF (for transmit), and you really need to be using some form of vertical. This is probably because ground wave plays a major role in propagation down at these frequencies, and ground wave requires vertical polarization.
     
    N1SCA likes this.
  10. N0NB

    N0NB Subscriber QRZ Page

    What would the average height of the dipole be? Even if you could average 30 feet off the ground, that is on the order of 0.015 wavelength at 630m. That would be about the equivalent of stringing a 160m dipole about 4 feet off the ground! I think most of us would agree that ground losses would be so high as to negate any advantage stringing out a dipole might deliver, however, I'd be rather interested to hear how it actually works.

    Some stations are reporting very good results using rather modest antennas.
     
    N1SCA likes this.

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