There are two factors here. If they are sharing the same feed, the effect
and match of the antennas will either sum or go differential, depending on a
lot of complex factors with phase delays. Infact the absolute figures would
be hard to calculate. When antennas are stacked or worked in an array, there
is a lot of careful matching that must be done to make sure that their
impedance is matched, and that the phase track through the spectrum that
they are to be used.
If you put two antennas in approximation to each other, they can certainly
effect each other. This has to do with mutual inductance to some degree.
Each antenna is resonant and is a tuned circuit. One will have an effect on
the other, as they may act as a reflector or diverter to each other. In
most cases they would have to be placed more than four times the wavelength
from each other in order to not have a measurable effect on each other. In
practice, this can also be indecisive. If the signal band to be received is
coming in at 3 meters wavelength, they should be placed at about 12 meters
from each other.
There is also the effect of the local oscillator in the receiver radiating
to some extent back up the antenna. Most of the receiver's local oscillator
works at about 10 to 20 mw power level. It is supposed to be isolated and
shielded. But, in many receivers this is not always perfect. In this case
the local oscillators between two receivers may induce some interference to
each other depending on the channels being watched, or stations listened to.
There are many aspects concerning radiated harmonic frequency generation to
also be considered.
What I would do for two receivers in the same location, is work out a way to
share the same antenna. This can be a bit complicated at times, if it is a
radio, but there will be less problems, than having two antennas in the same
area, and there will be a space savings.
--
I have heard some TV servicemen saying "One antenna is sucking from the
other". There is an essence of truth here...
--
Greetings,
Jerry Greenberg GLG Technologies GLG
=========================================
WebPage
http://www.zoom-one.com
Electronics
http://www.zoom-one.com/electron.htm
=========================================
With the glut of sci-fi shows on at the same time, I've set up
two VCRs and two antennas in a single room. I found that the
two antennas affect each other: by changing the settings on one
antenna it will change the picture quality on the other VCR and
TV.
I always thought antennas were passive devices and didn't
radiate any signals. Does anyone know of a way to reduce the
interference between the two antennas?
Thanks.
