> far away. In this case 5/8 wave spacing provides
> almost NO noise rejection at all. The null is up
> around 30 degrees and your noise is arriving at zero
> degrees. Use 1/2 wave spacing or about 270 feet on 160
> for optimum performance in a typical urban
> environment.
>
> I've used both spacings on broadside flags and I can
> state very positively that 5/8 wave spacing is useless
> in town!
Pattern and S/N improvement decreases very rapidly as
spacing is decreased from 1/2 wl. For example, at about 1/4
wl spacing while signals add and increase sensitivity or
gain 3dB, the noise will increase a similar amount. This
results in almost NO change in S/N or directivity! (Same for
in-phase Beverages and other lossy RX antennas.)
At 1/2 wl spacing we have two distinct side nulls at zero
degrees in a line through both antennas. The only condition
where that spacing is optimum is when the noise is
groundwave, a long distance from the antenna compared to a
wavelength, and located almost exactly off the sides in a
line running straight through the two antennas.
Statistically that isn't likely.
At larger spacings that deep single point null on each side
changes into a cone. That cone has zero response areas at
zero degrees elevation in FOUR positions instead of just two
with half wave spacing. It also has zero response at
decreasing azimuth spread relative to the sides, but
increasing elevation. This removes a much wider area of
noise arrival than 1/2 wl spacing...even along the ground.
The odds of noise coming from a null area go up as spacing
increases beyond 1/2 wave, not down.
The wider the spacing the further apart the zero degree
elevation nulls. At very wide spacings (~1 wl) we get new
side nulls!
This is why, despite what we read and hear, wider broadside
spacings are almost always better for S/N ratio. If we have
an error caused by using an antenna on multiple bands it is
much better to make it in a direction of too much broadside
spacing than too little.
Unless we have well defined noise arriving at zero degrees
elevation 90 degrees azimuth from the peak response, some
amount of spacing beyond 1/2 wave is always better. Overall,
the more energy removed from the sides the better S/N
becomes whether we are in a city or country location.
73 Tom
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