[TowerTalk] Practical Ground Screens for NVIS antennas

[K4SAV]

Jim

I also get 3.26MHz resonant frequency for the ground reflector.  I don't 
know how I got the number I quoted, must have done something really funky.

Jerry

Jim Lux wrote:

> At 10:24 AM 7/25/2005, K4SAV wrote:
>
>> Interesting question.  I have never reasearched this one, even though I
>> have thought about doing it, so I tried a few quick simulations.  At
>> first the suggestion of making the reflector 5% longer seem to make
>> sense, but ... read on...
>>
>> Using an 80 meter dipole, 133 ft long, at various heights, with a
>> reflector at 1 ft above ground with various lengths.
>>
>> In the table below, first column is the reflector length, then seperate
>> columns for different driven element heights, gain figures are for
>> straight up (zenith).
>>
>> R length   20ft   30ft    40ft   50ft
>>    76       4.79   6.17  6.69  6.77
>>    73       4.94   6.24  6.73  6.78
>>    70       5.15   6.34  6.78  6.8
>>    66       5.6     6.54  6.88  6.84
>>    63       5.77   6.67  6.96  6.87
>>    60       3.51   5.93  6.66  6.84
>>     0        3.67   5.61  6.4    6.59
>>
>> At first it may seem surprising that the peak gain seems to be for a
>> reflector that is a little shorter than the driven element.  This is
>> because the reflector is being tuned to a lower resonant frequency by
>> the ground capacitance (which is higher than that for the driven
>> element). Actually, my simulations show the resonant frequency of this
>> 132ft wire, 1 ft off ground is 2.6 MHz.
>
>
>
> Excellent use of modeling..
> Note that even in the worst case (20 ft up) the difference is only 1.5 
> -2 dB, and the fact that it varies fairly quickly indicates that it's 
> quite sensitive to small changes.
>
> FWIW, it's not only just bulk capacitance to ground, but that the 
> field from the wire is partially immersed in a dielectric.
>
> Running in NEC4, and assuming a copper wire 1mm in diameter and 132 ft 
> long, I got a resonant frequency of about 3.28MHz, with the wire 30cm 
> (1ft) off the ground.  With the wire 100 meters off the ground, I got 
> 3.62 for the resonant frequency, and about 3.63 in free space.
>
> Not sure why there's the difference in resonant frequency from your 
> model. (Running NEC2, I get 3.26 for the resonant frequency of the 1 
> ft high case)  If I drop the wire to 3 cm, the resonance drops to 2.7 
> MHz (in NEC4, with 45 segments or with 101 segments).  If I change the 
> epsilon to 12, the resonance is pretty much the same (within 10 kHz). 
> Change epsilon to 30 (i.e. soak that soil down!) and the resonance 
> moves up to 2.73.  Back to 13, but doubling conductivity to 10 mS/m 
> has about the same effect (fres = 2.73)
>
> So, we're seeing 1% changes in resonant frequency with fairly large 
> changes in soil properties. Jerry's changes in length were on the 
> order of 5% (which would have a similar effect in resonant frequency 
> change).
>
> I suppose I should run a model for 126 ft?  (same 3cm off the ground 
> gives fres=2.84 MHz)
>
>
>> These numbers haven't been checked against any other references.  It
>> represents a quick simulation done with EZNEC, using Real/High Accuracy
>> Ground, Cond = .005 s/m, diel const = 13.   Since we are dealing with a
>> wire only 1 ft above ground, I would expect slightly different numbers
>> to be produced by NEC4, but the trend should be the same.
>>
>> In practice, these simulations are not going to be accurate because of
>> the variations in ground parameters.  You probably couldn't even measure
>> the resonant frequency of your ground reflector and keep it constant
>> because of varying conditions, such as rain.  I simulated only one
>> reflector, but if you build this, you will want more than one.
>>
>> If you are not interested in DX and only in generating maximum straight
>> up signal, the max gain appears around 55ft height for a 3.6MHz antenna.
>>
>> Jerry, K4SAV
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