-=> Quoting Bill Cheek to Tony Szablowski <=-
 BC> "Use" has nothing to do with it.
 BC> Are you familiar at all with how directional beam antennas are
 BC> designed and constructed?  In general, the principle is one driven
 BC> (used) element and two or more parasitic or nearby elements.
 BC> And so it is that any two antennas in close proximity with each other
 BC> WILL interact to produce some effect, typically undesirable.  Antennas
 BC> in the same horizontal plane (side by side) will "skew" the radiation
 BC> patterns to favor some directions at the expense of others.  Antennas
 BC> in the same vertical plane
 BC> (over & under) will pull the horizontal radiation pattern up and down
 BC> to result in reduce performance out to the horizons.
 BC> Antennas within  1 wavelength of each other WILL affect each other.
 BC> Between 1 and 3 wavelengths, the effects are less.....perhaps tolerable
 BC> by the scannist, and beyond 3 wavelengths, there is no measurable
 BC> effect. 
 BC> I wrote an article for a coming issue of Monitoring Times that
 BC> explains this in more detail and describes how one might take advantage
 BC> of the effect. 
My experience on a 75 Honda Civic: 
1 CB ant., 1 465MHz UHF ant., 1 18" scanner ant., 2 cellular ant.
all in an area 30" square. 
The cellular are more then one wavelength apart and not on the 
quarter wave multiple. 
As long as the antenna are not close to being tuned to the 
same wavelegth they don't interfer. At least before and 
after standing wave checks have led me to believe this. 
Mounted one antenna first, then the other, then both. 
 
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