On 8/13/2010 9:29 AM, Richard (EagleEye) Karlquist wrote:
>> I have the ARRL's ANTENNA BOOK 20th Edition, and on Chapter 3 (The
>> Effects of the Earth), p.310 is Table 1 tht maybe contains all you want
>> about that.
>> If you haven't the book here is the table:
>>
>> Number of radials 16 24 36 60 90 120
>>
>> Length of each radial in 0.10 0.125 0.15 0.20 0.25 0.40
>> wavelengths
>>
>> Spacing of radials in degrees 22.5 15 10 6 4 3
>>
>> Total length of radial wire 1.6 3 5.4 12 22.5 48
>> installed in wavelengths
>>
>> Power loss in dB at low angles 3 2 1.5 1 0.5 0*
>>
>> with a L/4 radiating element
>>
>> Feedpoint impedance in ohms 52 46 43 40 37 35
>> with a L/4 radiating element
>>
>> * compared to a perfectly conducting ground
>>
>> 73 Panos SV1GRD
>>
> The dB loss numbers vs feed point impedance don't make sense to me.
> To get 3 dB loss, the feed point impedance should be 70 ohms, not
> 52 ohms. The dB numbers seem to be off by a factor of 2, IMHO,
> like they used 20 LOG instead of 10 LOG.
>
> Also, if your antenna is less than 1/4 wavelength, then of course
> the dB loss entries will be considerably different, etc.
>
> Rick N6RK
And he is right, iff "power loss in dB at low angles" correlates 1:1
with ground losses. In that case the 16 ohms of equivalent loss
dissipate 31% of the power, or 1.6 dB, not 3dB. But perhaps "power
loss in dB at low angles" was calculated by integrating the radiation
pattern and reflects both ohmic ground loss and the change in
pseudoBrewster angle. How low is "low" is not specified.
Garry, NI6T
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