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#1
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Effectiveness of decoupling radials
"Owen Duffy" wrote in message ... I have been playing with some NEC-2 models of a multiband vertical with radials. The vertical is an unloaded vertical of 13m height, and it is mounted on a 6m high grounded metal mast, and an ATU installed at the feedpoint (base of the radiator). I have fitted a pair of opposed nominal quarter wave radials for each of the 80, 40, 30 and 20m bands. To simulate ground loss, I have modelled a 20 ohm resistance in the bottom of the mast, and used a MININEC ground. An interesting observation is the sensitivity of this model to length of the radials. Properly adjusted, each pair of opposed radials near eliminate current on the mast (more than 20dB below the current into the radiator). The exception to this is the 30m radials which seem to suffer some interaction with the 80m radials (near third harmonic). Without the appropriate radials, current in the mast to ground is large, and losses can be 10dB or more. The ideas I take away from the modelling excercise is that: - inadequate decoupling exacerbates ground loss; - decoupling is very dependent on the length of the radials; - one pair of opposed radials is enough for a narrow band; - the radials for different bands have some interaction; and - the optimum length may be quite a deal longer than the expected length of legs of a half wave dipole in the same place. I am grappling with some other way to optimise such a system, other than measuring the mast current (which often isn't easy). These effects probably also apply to a trapped vertical with similar counterpoise, and the traditional wisdom of tuning either the length of the vertical or radial length to achieve low VSWR is probably less than optimal, there is an optimal length for each of them. The traditional wisdom that elevated radials are generally significantly lower loss than buried radials probably depends on careful "tuning" or isolation of feed point ground to minimise current flowing to the real ground. Comments, thoughts? Owen Owen, It may not be too critical, but would not the Sommerfeld/Norton method improve accuracy? Frank |
#2
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Effectiveness of decoupling radials
"Frank" wrote in news:u9qZh.14500$JF6.4868
@edtnps90: Owen, It may not be too critical, but would not the Sommerfeld/Norton method improve accuracy? Hi Frank My understanding was that the MININEC ground model was the better to use if the model caused current to flow into ground (as mine does). The draft model is at http://www.vk1od.net/multibandunload.../13MVERT01.nec if you want to play with it. Owen |
#3
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Effectiveness of decoupling radials
"Owen Duffy" wrote in message ... "Frank" wrote in news:u9qZh.14500$JF6.4868 @edtnps90: Owen, It may not be too critical, but would not the Sommerfeld/Norton method improve accuracy? Hi Frank My understanding was that the MININEC ground model was the better to use if the model caused current to flow into ground (as mine does). The draft model is at http://www.vk1od.net/multibandunload.../13MVERT01.nec if you want to play with it. Owen I was not thinking Owen. I forgot that some versions of NEC support the MININEC ground. I have loaded your program, but noticed my version of NEC does not support a "GN" entry of "3" in the "I1" field. It thinks it is a Sommerfeld/Norton ground, but does not recognize the conductivity and permittivity fields. About the only way I could get the program to run is to extend "GW 10" below round -- at a guess about 5 segments should be ok. I am also concerned about some discontinuity with the large diameter change from GW 9 to GW 10. Also GW 1 to GW 2. Initially I will set all diameters to 1 mm, and see what I get by running the AVG test. Frank |
#4
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Effectiveness of decoupling radials
"Frank's" wrote in
news:4OxZh.8833$Dq6.8346@edtnps82: "Owen Duffy" wrote in message ... "Frank" wrote in news:u9qZh.14500$JF6.4868 @edtnps90: Owen, It may not be too critical, but would not the Sommerfeld/Norton method improve accuracy? Hi Frank My understanding was that the MININEC ground model was the better to use if the model caused current to flow into ground (as mine does). The draft model is at http://www.vk1od.net/multibandunload.../13MVERT01.nec if you want to play with it. Owen I was not thinking Owen. I forgot that some versions of NEC support the MININEC ground. I have loaded your program, but noticed my version of NEC does not support a "GN" entry of "3" in the "I1" field. It thinks it is a Sommerfeld/Norton ground, but does not recognize the conductivity and permittivity fields. About the only way I could get the program to run is to extend "GW 10" below round -- at a guess about 5 segments should be ok. I am also concerned about some discontinuity with the large diameter change from GW 9 to GW 10. Also GW 1 to GW 2. Initially I will set all diameters to 1 mm, and see what I get by running the AVG test. Hi Frank, I built the models in 4NEC2 and EZNEC, both using the MININEC ground feature. My guess is that the radials are far enough clear of the ground that NEC- 2 should be adequate for modelling, but it would be interested to see what results you get from NEC-4. Re extending wire 10, don't forget it is loaded, so you need to deal with that. Owen |
#5
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Effectiveness of decoupling radials
I built the models in 4NEC2 and EZNEC, both using the MININEC ground
feature. My guess is that the radials are far enough clear of the ground that NEC- 2 should be adequate for modelling, but it would be interested to see what results you get from NEC-4. Re extending wire 10, don't forget it is loaded, so you need to deal with that. Owen Owen, GNEC reports the following. Note I have taken the ratio of the current magnitudes without regard to the phase relationship. I can send you a zipped copy of the NEC output file if you are interested. Freq TAG 10 TAG 1 Ratio ABS SEG 169 SEG 1 (MHz) (mA) (mA) (dB) 3.8 1.43 6.5 13 7.1 0.59 5.4 19.2 10.1 0.25 1.0 12 14.1 0.036 3.4 39.5 Large currents in the 3.8 MHz radials are evident on 30 m. I have removed the loading from TAG 10, and EK is not required in NEC 4. Also I show TAG 10 at extending 3 m below ground. Probably not realistic, but I am always confusing meters with feet! Frank |
#6
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Effectiveness of decoupling radials
"Frank's" wrote in
news:ay2_h.21$Vi6.12@edtnps82: I built the models in 4NEC2 and EZNEC, both using the MININEC ground feature. My guess is that the radials are far enough clear of the ground that NEC- 2 should be adequate for modelling, but it would be interested to see what results you get from NEC-4. Re extending wire 10, don't forget it is loaded, so you need to deal with that. Owen Owen, GNEC reports the following. Note I have taken the ratio of the current magnitudes without regard to the phase relationship. I can send you a zipped copy of the NEC output file if you are interested. Freq TAG 10 TAG 1 Ratio ABS SEG 169 SEG 1 (MHz) (mA) (mA) (dB) 3.8 1.43 6.5 13 7.1 0.59 5.4 19.2 10.1 0.25 1.0 12 14.1 0.036 3.4 39.5 Large currents in the 3.8 MHz radials are evident on 30 m. I have removed the loading from TAG 10, and EK is not required in NEC 4. Also I show TAG 10 at extending 3 m below ground. Probably not realistic, but I am always confusing meters with feet! Hi Frank, A plot of this current ratio shows very steep slope around the design frequencies. A plot of my model results is at http://www.vk1od.net/multibandunload...al/new_pa1.gif . Ratio of mast current to radiator current at junction: (MHz) NEC-4(dB) NEC-2 3.8 -13 -14.6 7.1 -19.2 -17.6 10.1 -12 -9.9 14.1 -39.5 -38.3 Note that for the NEC-2 model, these were not the optimal frequencies. Owen |
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