ground radials

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by KC2ZDE, May 24, 2011.

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  1. M3KXZ

    M3KXZ Ham Member QRZ Page

    Right.... looks like I'm homing in on something.... 40m long radials no good. Just shortened to 30m (8 radials only) and get something close to Tom's results. My results are -0.86dBi at 24 degrees, -4.66dBi at 6 degrees. With radials of 4m length I'm getting -2.04dBi at 24 degrees, -5.95dBi at 6 degrees, av gain 0.203). This is with antenna just over 3m AGL.

    If I lower the antenna to 0.5m above AGL, the the long radials are giving results of -7.07dBi @ 24 degrees and -10.98dBi @ 6 degrees (av gain 0.064). The short radials are giving results of -6.7dBi @ 24 degrees and -10.64dBi @ 6 degrees (av gain 0.07).

    But if I reduce the number of radials to 4, then the long radials again come out a lot worse than the shorties!

    I'll keep fiddling over the w/end.
  2. EI4GMB

    EI4GMB Ham Member QRZ Page

    Best of luck with that Pete! ;)

    Kind Regards

    Fred EI4GMB
  3. W8JI

    W8JI Ham Member QRZ Page

    The model I used was just one foot height for radials. When I changed it to 10 feet not much changed.

    When you are fiddling, remember to test by changing segments and height. Also remember programs like we use treat the earth as a uniform homogeneous media....something the real earth is not. Even if we step the media, it is still treated as a uniform media up to the transition.

    The largest errors would occur with closest to ground radial placement, fewest segments, and fewest radials. At about .005 wavelengths height a dipole departs from NEC-2 by several dB IF fields are concentrated like in a dipole.

    We went through extensive measurements here and at other places, and RCA spent a fortune on studying ground systems. I'd personally rely more on the published measured results by RCA.

    73 Tom
  4. M3KXZ

    M3KXZ Ham Member QRZ Page

    ring ring, ring ring, ring ring....Tom, me again. Thanks for the info. Just wondering, why does fewest radials produce large errors in the model? Is there something wrong in the calcs that causes and over-estimation of gain/efficiency when the number of radials is low?
  5. W0BTU

    W0BTU Ham Member QRZ Page

    So when is somebody (like W7EL?) going to come out with an affordable NEC-4 engine?
  6. WB3BEL

    WB3BEL Ham Member QRZ Page

    The NEC4 engine is not that expensive ~$250 for non-commercial use if I recall correctly. Programs like freeware 4NEC2 and others can use either the NEC4 or NEC2 engine.
  7. N8CMQ

    N8CMQ XML Subscriber QRZ Page

    My opinion.
    Computer models are nice to a point. Being able to input as much real world data helps in better data output.
    However, the real world sometimes kicks the computer model data out the window...
    Going with established data helps understand what the model may be missing.
    Antennas, vertical and horizontal, act differently close to the ground. Get them above a certain height, and they start to stabilize in many ways.
    Then there is the electrical height of a vertical. A quarter wave vertical will have few deep nulls in the pattern, start increasing the electrical length, and deep nulls start to appear. These nulls can cause fading if the sky wave changes angle...
    Horizontals do strange things the closer to the ground too...
    I have built 2 meter and higher frequency models, old school, but fun!
  8. M3KXZ

    M3KXZ Ham Member QRZ Page

    Jeff (CMQ), I agree 100% that the real world sometimes kicks the computer model data out of the window!

    For /p I like to use a wire slung up in a tree as a sloper, 10 or 11 metres long, fed at the bottom against a c/poise only 2 or 3m long, via a 4:1 current balun. I use this on 40 thru 10. On 40 it should be a really crap performer, but it's not. 2.5W SSB and I was good sigs to PA3GGB even with a fair amount of QRM. I know it's not a million miles, but modelling over flat ground suggested I should maybe have an erp of only a fraction of a Watt in his direction. But I was on an east facing hillside, so I'm sure that helped. And I wonder if the model under-estimated the effectiveness of the antenna too.
  9. G7DIE

    G7DIE Ham Member QRZ Page

    The truth is, only a fraction of a watt is all that's needed to establish contact when conditions allow, I recently worked Tasmania one morning from the sea front here in Blackpool with 500mW SSB on 20m, that's over 15,000 miles long path, I doubt very much my antenna was 100% efficient, reducing the erp even further.
  10. M3KXZ

    M3KXZ Ham Member QRZ Page

    You're making me jealous now - ya sod! I'm still working on getting the other side of the world from the beach here near Brighton. I really need to get out the front door earlier - perhaps leaving for work two hours early, but with kids to get ready for ain't happening yet.
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