On 8/13/2010 9:29 AM, Richard (Eagle-Eye) Karlquist wrote:
>> I have the ARRL's ANTENNA BOOK 20th Edition, and on Chapter 3 (The
>> Effects of the Earth), p.3-10 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
>>
>> Feed-point 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
pseudo-Brewster angle. How low is "low" is not specified.
Garry, NI6T
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