On 9/13/17 11:46 AM, V. Sciucka wrote:
I just thought that it might explain why different formulas are used.
Charlie gave me formula which is widely available: 246/f(mhz) = element
(feet), but I also found https://www.dxengineering.com/
techarticles/verticalantennainfo/dx-engineering-comtek-verti
cals-for-phased-arrays (see p.5) where 234/f(mhz) is used.
246 is free space half wavelength
234 is an approximation of resonant half wavelength
If you're building a phased array, the elements should be resonant (i.e.
the shorter length), but the spacing should the free space distance.
And let's be realistic - for a 4square with 1/4 wavelength spacing -
whether you are off by a foot or two in spacing won't make much
difference (certainly not in forward gain). In ultimate Front/Back
maybe - at 3.5 MHz , 1/4 wavelength is 21.43 meters. A 1 meter error in
spacing is 4 degrees of phase. That's enough to make a 20 dB null into
a 10dB null. (i.e. sin(4) = .07 - 20*log10(.07) = -11.5 dB)
In forward gain, you'll lose 0.02 dB.
(BTW, this is why you can really screw up a 4 square and the gain will
be reasonably ok, but getting better than 15-20 dB F/B is tough)
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