1/2 Wave dipoles on 630 meters do work!

Discussion in 'The Low Bands - 630/2200 Meters - VLF' started by W5DFN, Jul 17, 2018.

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  1. W5DFN

    W5DFN Ham Member QRZ Page

    As regards, 1/2 dipoles on 630 meters, they do work. I have one with legs only 30+ degrees apart that seems to perform as well and often better than other stations, if the wsprnet.org map means anything. This antenna is only 10 -16 feet above ground. I am feeding it about 20 watts from an MF Solutions converter -- directly with 50 ohm coax, no tuner needed. That just beats the hell out of building a huge variometer or trying to get a vertical element with a top load into the air and matching it to transmitter output. I do have highly conductive "soil" here. We are located in the Panhandle of Florida and the sadn here is slightly acidic and NEVER dries up below about one foot deep.
     
  2. W5DFN

    W5DFN Ham Member QRZ Page

    Once the novelty of wspr with its long duty cycles wears off, I find it about as exciting as watching paint dry or grass grow. So I am trying something different. Using the same converter, I set the carrier(suppressed, of course) at 472.5khz and feed a 500 hertz tone into it. I use a tone generator producing a low amplitude 500 hertz tone, which keeps the converter PA transmitting only a watt or less. As FLDIGI is triggered to send code with a much higher level 500 hertz tone, the PA goes to full output. Near as I can tell with the rather primitive monitoring I have available, the signal is clean and narrow. I would appreciate and contacts I can make. The code should be easy enough to decode by ear or machine as there is likely 15db or more difference between the high and low output. If you hear me and can transmit on 473 +/-, I will certainly respond. Any reports sent by e-mail to w5dfn@w5dfn.com will also be appreciated. Thanks and comments pro or con are welcome.
     
    K3RW likes this.
  3. K8JD

    K8JD Ham Member QRZ Page

    A dipole that is only a tiny percentage of a wavelength above ground will resonate WELL BELOW the formula length. I Imagine you cut yards and yards (Metres) of wire off it to get the SWR down.
    I will try listening for you tonight if the QRN is not too bad.
     
  4. W5DFN

    W5DFN Ham Member QRZ Page

    I took off maybe 15 feet off each end and started with 500'. It could stand a little more trimming, but the converter is happy and can run several transmit cycles and barely get warm. The propagation the last few nights has been poor. The last few nights, I get nothing until after 0200 zulu. Well, I get the wa4sze beacon before that. I wonder what that beacon proves that wspr won't. I am trying to take advantage of the band to actually communicate. Thanks for listening.
     
  5. K8JD

    K8JD Ham Member QRZ Page

    Why did you start with only 500 Ft ? I get 985 1/4 feet for a dipole on 475 kHz ? A 500 ft halfwave would be on 940 kHz in the middle of the AMBC band ??
    What do u use to measure SWR? I know the common SWR meters take a lot more power to get a forward power reading on 160M compared to ten M. I would guess that you would need a meter with a really long sensing element for VLF
     
    Last edited: Jul 21, 2018
  6. VK4AQJ

    VK4AQJ XML Subscriber QRZ Page

    I get resonance on a 1/2 wave dipole at 260 metres of XLPE insulated wire, but the resistance on a centre fed dipole is so low (about 2 ohms for 20 metre apex inverted V ) that other solutions work better. (End effects and dielectric effects bring the overall length down significantly). Even a classic OCF at 2:1 feedpoint presents too low a resistance at resonance, so it's better to go end fed. Either way, you'll need a matching network.

    Currently using a Marconi T with a radiating grounded counterpoise. One end is grounded as in Short Beverage configuration. I can match that with 187uH and a shunt capacitor of about 2 nF. I wouldn't use bare wire at my location because of the sea air and the corrosion. All my connections are sealed using liquid insulating tape.

    If you use a half wave end fed dipole, it works much better when the far end is close to ground (2 metres for at least 10m length). The ground capacitive coupling (for me) improves the antenna aperature and works much better on skywave.

    For 630m, ground return current starts to become important. For 2200m, it's even more important. Because of my location, all the stations I work except one are via skywave.
     
  7. W5DFN

    W5DFN Ham Member QRZ Page

    The dipole is 500 ' on each leg. Used a mfj meter and cranked it until it showed a flat match on a dummy load. I only have a general idea what the actual vswr is into the dipole, but the PA stays cool, so it is happy and I hear and am heard by about as many stations as most.
     
  8. W5DFN

    W5DFN Ham Member QRZ Page

    I suspect the wet, slightly acidic sand below the antenna is contributing to the performance. Bare aluminum wire is cheap, easy to twist together and seems to work well for all my dipoles. I am 30 miles north of the Gulf of Mexico.
     
  9. VK4AQJ

    VK4AQJ XML Subscriber QRZ Page

    The sea is only 4km from my location (I can see the sea), and I'm on a hill, so the prevailing wind comes straight off the Pacific Ocean. Even 304 Stainless steel doesn't do well. Bare copper corrodes very quickly and becomes unusable. The soil is ancient volcanic clays, and I have to regularly add Epsom salts and water to improve the earths because the climate is relatively dry, especially in winter.

    Aluminium wire is a good idea. I might do some trials with it.
     
  10. W5DFN

    W5DFN Ham Member QRZ Page

    A 1/4 mile of Fi-Shok 14 gauge electric fence wire costs less than $70 US delivered here. That stuff is strong! Splicing it with double wraps like you would splice barb wire seems to work well. All of my dipoles use it. Frankly, I don't care if the 630 meter dipole is not the most efficient way to go. On a decent night of propagation, I receive lots of stations with wspr and am heard by about as many. I just love the idea of feeding the MF Solutions converter directly to the antenna with coax.
     

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