Your last comment is incorrect -- the desired signal is typically not
reduced by the unused antenna. Your first comment is correct -- that's
the whole point ... the other antenna doesn't discriminate spatially
between signals in the desired direction versus noise from the side or back.
Receiving antennas are quite directional for the far field, but not
necessarily for the near field (they can be, though). The receiving
antenna will normally discriminate against far field noise coming from
the back or the side of the desired signal, but a resonant transmitting
antenna will pick up noise from everywhere and reradiate it quite
efficiently (it's a transmitting antenna, after all) to the nearby
receiving antenna. The result is that the receive antenna takes on the
omnidirectional characteristics of the transmit antenna. If anything,
the desired signal will go up, but the unwanted noise will go up a lot
more ... maybe by 20 db or more ... and bury the desired signal.
This condition has been documented by dozens of members of the Topband
reflector (which is where the original comment came from) and is
considerably more than a causal observation.
Dave AB7E
Bill Fuqua wrote:
> If it reradiates noise why not reradiating the signal you want to
> receive as well thus S/N remains constant.
> How does the unused antenna know noise from signal?
> However, It can reradiate everything it receives and change your
> radiation pattern thus reducing the weak signal in some cases. It may also
> change the impedance of the connected antenna and perhaps its resonant
> frequency if it is close enough. I think the noise thing is most likely an
> observation someone made and noticed a reduction in S/N and concluded the
> noise increased while it was really change in radiation pattern reducing
> the received signal.
>
> 73
> Bill wa4lav
>
>
>
> At 06:10 PM 1/19/2007 -0500, you wrote:
>
>> I read this on another list ...
>>
>> "It does reradiate enough noise, if not detuned,
>> to mask weak signals on my receiving antennas ..."
>>
>
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--
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